RestEase 1.6.4

RestEase

RestEase is a little type-safe REST API client library for .NET Framework 4.5.2 and higher and .NET Platform Standard 1.1 and higher, which aims to make interacting with remote REST endpoints easy, without adding unnecessary complexity.

To use it, you define an interface which represents the endpoint you wish to communicate with (more on that in a bit), where methods on that interface correspond to requests that can be made on it. RestEase will then generate an implementation of that interface for you, and by calling the methods you defined, the appropriate requests will be made. See Installation and Quick Start to get up and running!

Almost every aspect of RestEase can be overridden and customized, leading to a large level of flexibility.

It also works on platforms which don't support runtime code generation, such as .NET Native and iOS, if you reference RestEase.SourceGenerator. See Using RestEase.SourceGenerator for more information.

RestEase is built on top of HttpClient and is deliberately a "leaky abstraction": it is easy to gain access to the full capabilities of HttpClient, giving you control and flexibility, when you need it.

RestEase is inspired by Anaïs Betts' Refit, which in turn is inspired by Retrofit.

Table of Contents

1. Installation
2. Quick Start
3. Request Types
4. Return Types
5. Query Parameters
   1. Constant Query Parameters
   2. Variable Query Parameters
      1. Formatting Variable Query Parameters
      2. Serialization of Variable Query Parameters
   3. Query Parameters Map
   4. Raw Query String Parameters
   5. Query Properties
6. Paths
   1. Base Address
   2. Base Path
   3. Path Placeholders
      1. Path Parameters
         1. Formatting Path Parameters
         2. URL Encoding in Path Parameters
         3. Serialization of Path Parameters
      2. Path Properties
         1. Formatting Path Properties
         2. URL Encoding in Path Properties
         3. Serialization of Path Properties
7. Body Content
   1. URL Encoded Bodies
8. Response Status Codes
9. Cancelling Requests
10. Headers
    1. Constant Interface Headers
    2. Variable Interface Headers
       1. Formatting Variable Interface Headers
    3. Constant Method Headers
    4. Variable Method Headers
       1. Formatting Variable Method Headers
    5. Redefining Headers
11. Using RestEase.SourceGenerator
12. Using HttpClientFactory
13. Using RestEase with Polly
    1. Using Polly with RestClient
    2. Using Polly with HttpClientFactory
14. HttpClient and RestEase interface lifetimes
15. Controlling Serialization and Deserialization
    1. Custom JsonSerializerSettings
    2. Custom Serializers and Deserializers
       1. Deserializing responses: ResponseDeserializer
       2. Serializing request bodies: RequestBodySerializer
       3. Serializing request query parameters: RequestQueryParamSerializer
       4. Serializing request path parameters: RequestPathParamSerializer
       5. Controlling query string generation: QueryStringBuilder
16. Controlling the Requests
    1. RequestModifier
    2. Custom HttpClient
    3. Adding to HttpRequestMessage.Properties
17. Customizing RestEase
18. Interface Accessibility
19. Using Generic Interfaces
20. Using Generic Methods
21. Interface Inheritance
    1. Sharing common properties and methods
    2. IDisposable
22. Advanced Functionality Using Extension Methods
    1. Wrapping Other Methods
    2. Using IRequester Directly
23. FAQs

Installation

RestEase is available on NuGet. See that page for installation instructions.

If you're using C# 9 or .NET 5 (or higher), reference RestEase.SourceGenerator as well to get compile-time errors and faster execution. See Using RestEase.SourceGenerator for more information. If you're targetting iOS or .NET Native, you will need to do this, as runtime code generation isn't available.

If you're using ASP.NET Core, take a look at Using HttpClientFactory. For failure handling and retries using Polly, see Using RestEase with Polly.

Quick Start

To start, first create an public interface which represents the endpoint you wish to make requests to. Please note that it does have to be public, or you must add RestEase as a friend assembly, see Interface Accessibility below.

using System;
using System.Threading.Tasks;
using Newtonsoft.Json;
using RestEase;

namespace RestEaseSampleApplication
{
    // We receive a JSON response, so define a class to deserialize the json into
    public class User
    {
        public string Name { get; set; }
        public string Blog { get; set; }

        // This is deserialized using Json.NET, so use attributes as necessary
        [JsonProperty("created_at")]
        public DateTime CreatedAt { get; set; }
    }

    // Define an interface representing the API
    // GitHub requires a User-Agent header, so specify one
    [Header("User-Agent", "RestEase")]
    public interface IGitHubApi
    {
        // The [Get] attribute marks this method as a GET request
        // The "users" is a relative path the a base URL, which we'll provide later
        // "{userId}" is a placeholder in the URL: the value from the "userId" method parameter is used
        [Get("users/{userId}")]
        Task<User> GetUserAsync([Path] string userId);
    }

    public class Program
    {
        public static void Main(string[] args)
        {
            // Create an implementation of that interface
            // We'll pass in the base URL for the API
            IGitHubApi api = RestClient.For<IGitHubApi>("https://api.github.com");

            // Now we can simply call methods on it
            // Normally you'd await the request, but this is a console app
            User user = api.GetUserAsync("canton7").Result;
            Console.WriteLine($"Name: {user.Name}. Blog: {user.Blog}. CreatedAt: {user.CreatedAt}");
            Console.ReadLine();
        }
    }
}

Request Types

See the [Get("path")] attribute used above? That's how you mark that method as being a GET request. There are a number of other attributes you can use here - in fact, there's one for each type of request: [Get("path")], [Post("path")], [Put("path")], [Delete("path")], [Head("path")], [Options("path")], [Trace("path"))], [Patch("path")]. Use whichever one you need to.

The argument to [Get] (or [Post], or whatever) is typically a relative path, and will be relative to the base uri that you provide to RestClient.For<T>. (You can specify an absolute path here if you need to, in which case the base uri will be ignored). Also see the section on Paths.

Return Types

Your interface methods may return one of the following types:

• Task: This method does not return any data, but the task will complete when the request has completed
• Task<T> (where T is not one of the types listed below): This method will deserialize the response into an object of type T, using Json.NET (or a custom deserializer, see Controlling Serialization and Deserialization below).
• Task<string>: This method returns the raw response, as a string (although this can be customised, see here).
• Task<HttpResponseMessage>: This method returns the raw HttpResponseMessage resulting from the request. It does not do any deserialiation. You must dispose this object after use.
• Task<Response<T>>: This method returns a Response<T>. A Response<T> contains both the deserialied response (of type T), but also the HttpResponseMessage. Use this when you want to have both the deserialized response, and access to things like the response headers. You must dispose this object after use.
• Task<Stream>: This method returns a Stream containing the response. Use this to e.g. download a file and stream it to disk. You must dispose this object after use.

Non-async methods are not supported (use .Wait() or .Result as appropriate if you do want to make your request synchronous).

If you return a Task<HttpResponseMessage> or a Task<Stream>, then HttpCompletionOption.ResponseHeadersRead is used, so that you can choose whether or not the response body should be fetched (or report its download progress, etc). If however you return a Task<T>, Task<string>, or Task<Response<T>>, then HttpCompletionOption.ResponseContentRead is used, meaning that any CancellationToken that you pass will cancel the body download. If you return a Task, then the response body isn't fetched, unless an ApiException is thrown.

Query Parameters

It is very common to want to include query parameters in your request (e.g. /foo?key=value), and RestEase makes this easy.

Constant Query Parameters

The most basic type of query parameter is a constant - the value never changes. For these, simply put the query parameter as part of the URL:

public interface IGitHubApi
{
   [Get("users/list?sort=desc")]
   Task<List<User>> GetUsersAsync();
}

Variable Query Parameters

Any parameters to a method which are:

• Decorated with the [Query] attribute, or
• Not decorated at all

will be interpreted as query parameters.

The name of the parameter will be used as the key, unless an argument is passed to [Query("key")], in which case that will be used instead.

For example:

public interface IGitHubApi
{
    [Get("user")]
    Task<User> FetchUserAsync(int userid);

    // Is the same as:

    [Get("user")]
    Task<User> FetchUserAsync([Query] int userid);

    // Is the same as:
    // (Note the casing of the parameter name)

    [Get("user")]
    Task<User> FetchUserAsync([Query("userid")] int userId);
}

IGithubApi api = RestClient.For<IGithubApi>("http://api.github.com");

// Requests http://api.github.com/user?userid=3
await api.FetchUserAsync(3);

You can have duplicate keys if you want:

public interface ISomeApi
{
    [Get("search")]
    Task<SearchResult> SearchAsync([Query("filter")] string filter1, [Query("filter")] string filter2);
}

ISomeApi api = RestClient.For<ISomeApi>("https://api.example.com");

// Requests http://somenedpoint.com/search?filter=foo&filter=bar
await api.SearchAsync("foo", "bar");

You can also have an array of query parameters:

public interface ISomeApi
{
    // You can use IEnumerable<T>, or any type which implements IEnumerable<T>

    [Get("search")]
    Task<SearchResult> SearchAsync([Query("filter")] IEnumerable<string> filters);
}

ISomeApi api = RestClient.For<ISomeApi>("https://api.example.com");

// Requests http://api.exapmle.com/search?filter=foo&filter=bar&filter=baz
await api.SearchAsync(new[] { "foo", "bar", "baz" });

If you specify a key that is null, i.e. [Query(null)], then the name of the key is not used, and the value is inserted into the query string. If you specify a key that is an empty string "", then then query key will be left empty.

public interface ISomeApi
{
    [Get("foo")]
    Task FooAsync([Query(null)] string nullParam, [Query("")] string emptyParam);
}

ISomeApi api = RestClient.For<ISomeApi>("https://api.example.com");

// Requests https://api.example.com/foo?onitsown&=nokey
await api.FooAsync("onitsown", "nokey");

If you pass a value which is null, then the key is not inserted. If you pass any other value (e.g. emptystring) then the value is left empty.

public interface ISomeApi
{
    [Get("path")]
    Task FooAsync([Query] string foo, [Query] string bar);
}

ISomeApi api = RestClient.For<ISomeApi>("https://api.example.com");

// Requests https://api.example.com/path?bar=
await api.FooAsync(null, "");

Formatting Variable Query Parameters

By default, query parameter values will be serialized by calling ToString() on them. This means that the primitive types most often used as query parameters - string, int, etc - are serialized correctly.

However, you can also specify a string format to use using the Format property of the [Query] attribute, for example:

public interface ISomeApi
{
    [Get("foo")]
    Task FooAsync([Query(Format = "X2")] int param);
}

ISomeApi api = RestClient.For<ISomeApi>("https://api.example.com");

// Requests https://api.example.com/foo?param=FE
await api.FooAsync(254);

1. If you use a custom serializer, then the format is passed to that serializer, and you can use it as you like.
2. Otherwise, if the format looks like it could be passed to string.Format, then this happens with param passed as the first arg, and RestClient.FormatProvider as the IFormatProvider. For example, "{0}" or "{0:X2}" or "hello {0}".
3. Otherwise, if param implements IFormattable, then its ToString(string, IFormatProvider) method is called, with param as the format and RestClient.FormatProvider as the IFormatProvider. For example, "X2".
4. Otherwise, the format is ignored.

Serialization of Variable Query Parameters

Sometimes calling ToString() is not enough: some APIs require that you send e.g. JSON as a query parameter. In this case, you can mark the parameter for custom serialization using QuerySerializationMethod.Serialized, and further control it by using a custom serializer.

For example:

public class SearchParams
{
    public string Term { get; set; }
    public string Mode { get; set; }
}

public interface ISomeApi
{
    [Get("search")]
    Task<SearchResult> SearchAsync([Query(QuerySerializationMethod.Serialized)] SearchParams param);
}

ISomeApi api = RestClient.For<ISomeApi>("https://api.example.com");
// Requests https://api.example.com/search?params={"Term": "foo", "Mode": "basic"}
await api.SearchAsync(new SearchParams() { Term = "foo", Mode = "basic" });

You can also specify the default serialization method for an entire api by specifying [SerializationMethods(Query = QuerySerializationMethod.Serialized)] on the interface, or for all parameters in a given method by specifying it on the method, for example:

[SerializationMethods(Query = QuerySerializationMethod.Serialized)]
public interface ISomeApi
{
    [Get("search")]
    [SerializationMethods(Query = QuerySerializationMethod.ToString)]
    Task<SearchResult> SearchWithToStringAsync([Query] SearchParams param);

    [Get("search")]
    Task<SearchResult> SearchWithSerializedAsync([Query] SearchParams param);
}

Query Parameters Map

Sometimes you have a load of query parameters, or they're generated dynamically, etc. In this case, you may want to supply a dictionary of query parameters, rather than specifying a load of method parameters.

To facilitate this, you may decorate one or more method parameters with [QueryMap]. The parameter type must be an IDictionary<TKey, TValue>.

Query maps are handled the same way as other query parameters: serialization, handling of enumerables, null values, etc, behave the same. You can control whether values are serialized using a custom serializer or ToString() using e.g. [QueryMap(QuerySerializationMethod.Serialized)].

For example:

public interface ISomeApi
{
    [Get("search")]
    // I've used IDictionary<string, string[]> here, but you can use whatever type parameters you like,
    // or any type which implements IDictionary<TKey, TValue>
    Task<SearchResult> SearchBlogPostsAsync([QueryMap] IDictionary<string, string[]> filters);
}

var api = RestClient.For<ISomeApi>("https://api.example.com");
var filters = new Dictionary<string, string[]>()
{
    { "title", new[] { "bobby" } },
    { "tag", new[] { "c#", "programming" } }
};

// Requests https://api.example.com/search?title=bobby&tag=c%23&tag=programming
var searchResults = await api.SearchBlogPostsAsync(filters);

Raw Query String Parameters

In rare cases, you may have generated a query string by other means, and want to give this to RestEase. To do this, provide one or more parameters decorated with [RawQueryString].

This parameter can be of any type, and .ToString() will be called on it to turn it into a string. Its value will be prepended, verbatim, to the query string: you are responsible for any escaping. It must not begin or end with "&" or "?".

For example:

public interface ISomeApi
{
    [Get("search")]
    Task<SearchResult> SearchAsync([RawQueryString] string customFilter);
}

var api = RestClient.For<ISomeApi>("https://api.example.com");
var filter = "filter=foo";
// Requests https://api.example.com?filter=foo
var searchResults = await api.SearchAsync(filter);

Query Properties

If you want to have a query string which is included in all of your requests, you can do this by declaring a [Query] property. These work the same way as query parameters, but they apply to all methods in your interface. If the value of the property is null, then it will not be added to your query. Otherwise, it will always be present, even if you declare a query parameter with the same name.

The property must have both a getter and a setter.

For example:

public interface ISomeApi
{
	[Query("foo")]
	string Foo { get; set; }

	[Get("thing")]
	Task ThingAsync([Query] string foo);
}

var api = RestClient.For<ISomeApi>("https://api.example.com");
api.Foo = "bar";

// Requests https://api.example.com?foo=baz&foo=bar
await api.ThingAsync("baz");

Paths

The path to which a request is sent is constructed from the following 3 parts, concatenated together (and separated with /):

1. The Base Address (e.g. https://api.example.com)
2. The Base Path (optional, e.g. api/v1)
3. The path specified on the method, passed to the [Get("path")], etc, attribute (e.g. users)

The Base Address is the domain at which the API can be found. This is normally specified by passing an address to RestClient.For<T>(...), but you can also use [BaseAddress(...)], see Base Address.

The Base Path is optional, and is a prefix which is common to all of your API's paths, e.g. api/v1. If you like, you can specify this once using the [BasePath(...)] attribute, see Base Path.

Each method also has a path, which is passed to the [Get(...)], etc, attribute on the method.

Ordinarily, these three parts are concatenated together, giving e.g. https://api.example.com/api/v1/users. However, there are a number of extra rules:

1. If the path specified on the method begins with a /, then the Base Path is ignored (but the Base Address is not ignored). So if the method's path was /users instead of users, the final address would be https://api.example.com/users.
2. If the path specified on the method is absolute (e.g. it begins with http://), then both the Base Address and Base Path are ignored.
3. If the Base Path is absolute, then the Base Address is ignored.

These rules are particularly useful when working with an API which returns links to other endpoints, see URL Encoding in Path Parameters for an example.

Base Address

The Base Address can be specified in two ways:

1. When instantiating the API using RestClient, either by passing a URI to RestClient.For<T>(...) or new RestClient(...), or by passing a HttpClient which has its BaseAddress property set.
2. Using a [BaseAddress(...)] attribute on the interface itself.

If it's specified both ways, then the [BaseAddress(...)] attribute is ignored. This means that you can have a default Base Address specified on the interface, but this can be overridden when actually instantiating it using RestClient.

The Base Address can contain {placeholders}. Each placeholder must have a corresponding path property, although this will be overridden by a path parameter if one is present.

The Base Address may contain the start of a path as well, e.g. https://api.example.com/api/v1. This path will not be overridden if the path specified on the method starts with a /, in contrast to the Base Path.

The Base Address must be absolute (i.e. it must start with http:// or https://).

Query strings or fragments are not supported in the Base Address, and their behaviour is undefined and subject to change.

Base Path

The Base Path is specified using a [BasePath(...)] attribute on your interface.

The Base Path can contain {placeholders}. Each placeholder must have a corresponding path property, although this will be overridden by a path parameter if one is present.

Query strings, or other parts of a URI, are not supported in the Base Path, and their behaviour is undefined and subject to change.

Path Placeholders

Sometimes you also want to be able to control some parts of the path itself, rather than just the query parameters.

Path Parameters

Path parameters are the most common means of controlling a part of the path. This is done using placeholders in the path, and corresponding method parameters decorated with [Path].

For example:

public interface ISomeApi
{
    [Get("user/{userId}")]
    Task<User> FetchUserAsync([Path] string userId);
}

ISomeApi api = RestClient.For<ISomeApi>("https://api.example.com");

// Requests https://api.example.com/user/fred
await api.FetchUserAsync("fred");

As with [Query], the name of the placeholder to substitute is determined by the name of the parameter. If you want to override this, you can pass an argument to [Path("placeholder")], e.g.:

public interface ISomeApi
{
    [Get("user/{userId}")]
    Task<User> FetchUserAsync([Path("userId")] string idOfTheUser);
}

Every placeholder must have a corresponding parameter, and every parameter must relate to a placeholder.

Formatting Path Parameters

As with [Query], path parameter values will be serialized by calling ToString() on them. This means that the primitive types most often used as query parameters - string, int, etc - are serialized correctly.

However, you can also specify a format using the Format property of the [Path] attribute, for example:

public interface ISomeApi
{
    [Get("foo/{bar}")]
    Task FooAsync([Path("bar", Format = "D2")] int param);
}

ISomeApi api = RestClient.For<ISomeApi>("https://api.example.com");

// Requests https://api.example.com/foo/01
await api.FooAsync(1);

1. If you use a custom serializer, then the format is passed to that serializer, and you can use it as you like.
2. Otherwise, if the format looks like it could be passed to string.Format, then this happens with param passed as the first arg, and RestClient.FormatProvider as the IFormatProvider. For example, "{0}" or "{0:D2}" or "hello {0}".
3. Otherwise, if param implements IFormattable, then its ToString(string, IFormatProvider) method is called, with param as the format and RestClient.FormatProvider as the IFormatProvider. For example, "D2".
4. Otherwise, the format is ignored.

URL Encoding in Path Parameters

By default, path parameters are URL-encoded, which means things like / are escaped. If you don't want this, for example you want to specify a literal section of the URL, this can be disabled using the UrlEncode property of the [Path] attribute, for example:

public interface ISomeApi
{
    [Get("foo/{bar}")]
    Task FooAsync([Path(UrlEncode = false)] string bar);
}

ISomeApi api = RestClient.For<ISomeApi>("https://api.example.com");

// Requests https://api.example.com/foo/bar/baz
await api.FooAsync("bar/baz");

This can be useful if working with an API which returns raw links to other resources, when combined with the logic specified in Paths. For example, let's say we want to make a request to https://api.example.com/v1/first, and that gives us back:

{
    "Second": "/v1/second"
}

We could write:

[BasePath("v1")]
public interface ISomeApi
{
    [Get("first")]
    Task<FirstResponse> GetFirstAsync();

    [Get("{url}")]
    Task GetSecondAsync([Path] string url);
}

ISomeApi api = RestClient.For<ISomeApi>("https://api.example.com");
var response = await api.GetFirstAsync();
await api.GetSecondAsync(response.Second);

Serialization of Path Parameters

Similar to query parameters, calling ToString() is sometimes not enough: you might want to customize how your path parameters are turned into strings (for example, for enum members). In this case, you can mark the parameter for custom serialization using PathSerializationMethod.Serialized, and specifying a RequestPathParamSerializer.

For example:

public enum MyEnum
{
    [Display(Name = "first")]
    First,

    [Display(Name = "second")]
    Second,
}

public interface ISomeApi
{
    [Get("path/{param}")]
    Task<string> GetAsync([Path(PathSerializationMethod.Serialized)] MyEnum param);
}

ISomeApi api = new RestClient()
{
    RequestPathParamSerializer = new StringEnumRequestPathParamSerializer()
}.For<ISomeApi>("https://api.example.com");

// Requests https://api.example.com/path/first
await api.GetAsync(MyEnum.First);

You can also specify the default serialization method for an entire api by specifying [SerializationMethods(Path = PathSerializationMethod.Serialized)] on the interface, or for all parameters in a given method by specifying it on the method, for example:

[SerializationMethods(Path = PathSerializationMethod.Serialized)]
public interface ISomeApi
{
    [Get("path/{foo}")]
    [SerializationMethods(Path = PathSerializationMethod.ToString)]
    Task<string> GetWithToStringAsync([Path] MyEnum foo);

    [Get("path/{foo}")]
    Task<string> GetWithSerializedAsync([Path] MyEnum foo);
}

Path Properties

Sometimes you've got a placeholder which is present in all (or most) of the paths on the interface, for example an account ID. In this case, you can specify a [Path] property. These work in the same way as path parameters, but they're on the level of the entire API.

Properties must have both a getter and a setter. If the placeholder of the path property isn't given (i.e. you use [Path] instead of [Path("placeholder")]), then the name of the property will be used.

Unlike with path parameters, you don't need to have the placeholder present in every path. If you have both a path parameter and a path property with the same name, the path parameter is used.

For example:

public interface ISomeApi
{
    [Path("accountId")]
    int AccountId { get; set; }

    [Get("{accountId}/profile")]
    Task<Profile> GetProfileAsync();

    [Delete("{accountId}")]
    Task DeleteAsync([Path("accountId")] int accountId);
}

var api = RestClient.For<ISomeApi>("https://api.example.com/user");
api.AccountId = 3;

// Requests https://api.example.com/user/3/profile
var profile = await api.GetProfileAsync();

// Requests https://api.example.com/user/4/profile
await api.DeleteAsync(4);

You can also use BasePath if all of your paths start with {accountId}.

Formatting Path Properties

As with Path Parameters, you can specify a string format to use if the value implements IFormattable.

For example:

public interface ISomeApi
{
    [Path("accountId", Format = "N")]
    Guid AccountId { get; set; }

    [Get("{accountId}/profile")]
    Task<Profile> GetProfileAsync();
}

var api = RestClient.For<ISomeApi>("https://api.example.com/user");
api.AccountId = someGuid;

// Requests e.g. /user/00000000000000000000000000000000 /profile
var profile = await api.GetProfileAsync();

URL Encoding in Path Properties

As with path parameters, you can disable URL encoding for path properties.

For example:

public interface ISomeApi
{
    [Path("pathPart", UrlEncode = false)]
    Guid PathPart { get; set; }

    [Get("{pathPart}/profile")]
    Task GetAsync();
}

var api = RestClient.For<ISomeApi>("https://api.example.com");
api.PathPart = "users/abc";

// Requests https://api.example.com/users/abc/profile
await api.GetAsync();

Serialization of Path Properties

As with path parameters, you can specify PathSerializationMethod.Serialized on a path property to use custom serialization behaviour. You must also supply a RequestPathParamSerializer when creating the RestClient. This can be used for things like controlling how enum members are serialized.

For example:

public enum MyEnum
{
    [Display(Name = "first")]
    First,

    [Display(Name = "second")]
    Second,
}

public interface ISomeApi
{
    [Path("pathPart", PathSerializationMethod.Serialized)]
    MyEnum PathPart { get; set; }

    [Get("path/{pathPart}")]
    Task<string> GetAsync();
}

ISomeApi api = new RestClient()
{
    RequestPathParamSerializer = new StringEnumRequestPathParamSerializer()
}.For<ISomeApi>("https://api.example.com");

api.PathPart = MyEnumSecond;
// Requests https://api.example.com/path/second
await api.GetAsync();

Body Content

If you're sending a request with a body, you can specify that one of the parameters to your method contains the body you want to send, using the [Body] attribute.

public interface ISomeApi
{
    [Post("users/new")]
    Task CreateUserAsync([Body] User user);
}

Exactly how this will be serialized depends on the type of parameters:

• If the type is Stream, then the content will be streamed via StreamContent.
• If the type is String, then the string will be used directly as the content (using StringContent).
• If the type is byte[], then the byte array will be used directory as the content (using ByteArrayContent).
• If the parameter has the attribute [Body(BodySerializationMethod.UrlEncoded)], then the content will be URL-encoded (see below).
• If the type is an HttpContent (or one of its subclasses), then it will be used directly. This is useful for advanced scenarios.
• Otherwise, the parameter will be serialized as JSON (by default, or you can customize this if you want, see Controlling Serialization and Deserialization).

URL Encoded Bodies

For APIs which take form posts (i.e. serialized as application/x-www-form-urlencoded), initialize the [Body] attribute with BodySerializationMethod.UrlEncoded. This parameter must implement IDictionary or IDictionary<TKey, TValue>.

If any of the values implement IEnumerable, then they will be serialized as an array of values.

For example:

public interface IMeasurementProtocolApi
{
    [Post("collect")]
    Task CollectAsync([Body(BodySerializationMethod.UrlEncoded)] Dictionary<string, object> data);
}

var data = new Dictionary<string, object> {
    {"v", 1},
    {"tids", new[] { "UA-1234-5", "UA-1234-6" },
    {"cid", new Guid("d1e9ea6b-2e8b-4699-93e0-0bcbd26c206c")},
    {"t", "event"},
};

// Serialized as: v=1&tids=UA-1234-5&tids=UA-1234-6&cid=d1e9ea6b-2e8b-4699-93e0-0bcbd26c206c&t=event
await api.CollectAsync(data);

You can also control the default body serialization method for an entire API by specifying [SerializationMethods(Body = BodySerializationMethod.UrlEncoded)] on the interface itself:

[SerializationMethods(Body = BodySerializationMethod.UrlEncoded)]
public interface ISomeApi
{
    [Post("collect")]
    Task CollectAsync([Body] Dictionary<string, object> data);
}

Response Status Codes

By default, any response status code which does not indicate success (as indicated by HttpResponseMessage.IsSuccessStatusCode) will cause an ApiException to be thrown.

The ApiException has properties which tell you exactly what happened (such as the HttpStatusCode, the URI which was requested, the string content, and also a method DeserializeContent<T>() to let you attempt to deserialize the content as a particular type). This means that you can write code such as:

try
{
    var response = await client.SayHelloAsync();
}
catch (ApiException e) when (e.StatusCode == HttpStatusCode.NotFound)
{
    var notFoundResponse = e.DeserializeContent<NotFoundRepsonse>();
    // ...
}

This is usually what you want, but sometimes you're expecting failure.

In this case, you can apply [AllowAnyStatusCode] to you method, or indeed to the whole interface, to suppress this behaviour. If you do this, then you probably want to make your method return either a HttpResponseMessage or a Response<T> (see Return Types) so you can examine the response code yourself.

For example:

public interface ISomeApi
{
    [Get("users/{userId}")]
    [AllowAnyStatusCode]
    Task<Response<User>> FetchUserThatMayNotExistAsync([Path] int userId);
}

ISomeApi api = RestClient.For<ISomeApi>("https://api.example.com");

using (var response = await api.FetchUserThatMayNotExistAsync(3));
if (response.ResponseMessage.StatusCode == HttpStatusCode.NotFound)
{
    // User wasn't found
}
else
{
    var user = response.GetContent();
    // ...
}

Cancelling Requests

If you want to be able to cancel a request, pass a CancellationToken as one of the method paramters.

public interface ISomeApi
{
    [Get("very-large-response")]
    Task<LargeResponse> GetVeryLargeResponseAsync(CancellationToken cancellationToken);
}

Note that if your method returns a Task<HttpResponseMessage> or Task<Stream>, then the CancellationToken will not cancel the download of the response body, see Return Types for details.

Headers

Specifying headers is actually a surprisingly large topic, and can be done in several ways, depending on the precise behaviour you want.

Constant Interface Headers

If you want to have a header that applies to every single request, and whose value is fixed, use a constant interface header. These are specified as [Header("Name", "Value")] attributes on the interface.

For example:

[Header("User-Agent", "RestEase")]
[Header("Cache-Control", "no-cache")]
public interface IGitHubApi
{
    [Get("users")]
    Task<List<User>> GetUsersAsync();
}

Variable Interface Headers

If you want to have a header that applies to every single request, and whose value is variable, then use a variable interface header. These are specifed using properties, using a [Header("Name")] attribute on that property.

For example:

public interface ISomeApi
{
    [Header("X-API-Key")]
    string ApiKey { get; set; }

    [Get("users/{userId}")]
    Task<User> FetchUserId([Path] string userId);
}

ISomeApi api = RestClient.For<ISomeApi>("https://api.example.com")
api.ApiKey = "The-API-KEY-value";
// ...

For nullable property types, you can also specify a default (which will be used when the property is null):

public interface ISomeApi
{
    [Header("X-API-Key", "None")]
    string ApiKey { get; set; }

    [Get("users/{userId}")]
    Task<User> FetchUserAsync([Path] string userId);
}

ISomeApi api = RestClient.For<ISomeApi>("https://api.example.com")

// "X-API-Key: None"
var user = await api.FetchUserAsync("bob");

Formatting Variable Interface Headers

By default, variable interface header values will be serialized by calling ToString() on them. This means that the primitive types most often used as query parameters - string, int, etc - are serialized correctly.

However, you can also specify a string format to use using the Format property of the [Query] attribute, for example:

public interface ISomeApi
{
    [Header("SomeHeader", Format = "X2")]
    int SomeHeader { get; set; }

    [Get("foo")]
    Task FooAsync();
}

ISomeApi api = RestClient.For<ISomeApi>("https://api.example.com");
api.SomeHeader = 254;
// SomeHeader: FE
await api.FooAsync();

1. If the format looks like it could be passed to string.Format, then this happens with SomeHeader passed as the first arg, and RestClient.FormatProvider as the IFormatProvider. For example, "{0}" or "{0:X2}" or "hello {0}".
2. Otherwise, if SomeHeader implements IFormattable, then its ToString(string, IFormatProvider) method is called, with SomeHeader as the format and RestClient.FormatProvider as the IFormatProvider. For example, "X2".
3. Otherwise, the format is ignored.

Constant Method Headers

If you want to have a header which only applies to a particular method, and whose value never changes, then use a constant method header. Like constant interface headers, these are defined in their entirety using an attribute. However, instead of applying the attribute to the interface, you apply it to the method.

public interface IGitHubApi
{
    [Header("User-Agent", "RestEase")]
    [Header("Cache-Control", "no-cache")]
    [Get("users")]
    Task<List<User>> GetUsersAsync();

    // This method doesn't have any headers applied
    [Get("users/{userId}")]
    Task<User> GetUserAsync([Path] string userId);
}

Variable Method Headers

Finally, you can have headers which only apply to a single method and whose values are variable. These consist of a [Header("Name")] attribute applied to a method parameter.

public interface ISomeApi
{
    [Get("users")]
    Task<List<User>> GetUsersAsync([Header("Authorization")] string authorization);
}

Formatting Variable Method Headers

By default, variable method header values will be serialized by calling ToString() on them. This means that the primitive types most often used as query parameters - string, int, etc - are serialized correctly.

However, you can also specify a string format to use using the Format property of the [Query] attribute, for example:

public interface ISomeApi
{
    [Get("foo")]
    Task FooAsync([Header("SomeHeader", Format = "X2")] int someHeader);
}

ISomeApi api = RestClient.For<ISomeApi>("https://api.example.com");
// SomeHeader: FE
await api.FooAsync(254);

1. If the format looks like it could be passed to string.Format, then this happens with someHeader passed as the first arg, and RestClient.FormatProvider as the IFormatProvider. For example, "{0}" or "{0:X2}" or "hello {0}".
2. Otherwise, if someHeader implements IFormattable, then its ToString(string, IFormatProvider) method is called, with someHeader as the format and RestClient.FormatProvider as the IFormatProvider. For example, "X2".
3. Otherwise, the format is ignored.

Redefining Headers

You've probably noticed that there are 4 places you can define a header: on the interface, as a property, on a method, and as a parameter (or, Constant Interface Headers, Variable Interface Headers, Constant Method Headers, and Variable Method Headers, respectively). There are rules specifying how headers from different places are merged.

Constant and Variable Interface headers are merged, as are Constant and Variable Method headers. That is, if a header is supplied both as an attribute on the interface, and as a property, that header will have multiple values.

Method headers will replace Interface headers. If you have the same header on a method and on the interface, then the header on the method will replace the one on the interface.

Another rule is that a header with a value of null will not be added, but can still replace a previously-defined header of the same name.

Example time:

[Header("X-InterfaceOnly", "InterfaceValue")]
[Header("X-InterfaceAndParamater", "InterfaceValue")]
[Header("X-InterfaceAndMethod", "InterfaceValue"]
[Header("X-InterfaceAndParameter", "InterfaceValue"]
[Header("X-InterfaceAndMethod-ToBeRemoved", "InterfaceValue")]
public interface ISomeApi
{
    [Header("X-ParameterOnly")]
    string ParameterOnlyHeader { get; set; }

    [Header("X-InterfaceAndParameter")]
    string InterfaceAndParameterHeader { get; set; }

    [Header("X-ParameterAndMethod")]
    string ParameterAndMethodHeader { get; set; }

    [Get("url")]
    [Header("X-MethodOnly", "MethodValue")]
    [Header("X-MethodAndParameter", "MethodValue")]
    [Header("X-ParameterAndMethod", "MethodValue")]
    [Header("X-InterfaceAndMethod-ToBeRemoved", null)]
    Task DoSomethingAsync(
        [Header("X-ParameterOnly")] string parameterOnly,
        [Header("X-MethodAndParameter")] string methodAndParameter,
        [Header("X-InterfaceAndParameter")] string interfaceAndParameter
    );
}

ISomeApi api = RestClient.For<ISomeApi>("https://api.example.com");

api.ParameterOnlyHeader = "ParameterValue";
api.InterfaceAndParameterHeader = "ParameterValue";
api.ParameterAndMethodHeader = "ParameterValue";

await api.DoSomethingAsync("ParameterValue", "ParameterValue", "ParameterValue");

// Has the following headers:
// X-InterfaceOnly: InterfaceValue
// X-InterfaceAndParameter: InterfaceValue, ParameterValue
// X-InterfaceAndMethod: MethodValue
// X-InterfaceAndParameter: ParameterValue

// X-ParameterAndMethod: MethodValue

// X-MethodOnly: MethodValue
// X-MethodAndParameter: MethodValue, ParameterValue

// X-ParameterAndMethod-ToBeRemoved isn't set, because it was removed

Using RestEase.SourceGenerator

Source Generators are a new feature which allows NuGet packages to hook into the compilation of your projects and insert their own code. RestEase uses this to generate implementations of your interfaces at compile-time, rather than run-time. To take advantage of this, you need to install the RestEase.SourceGenerator NuGet package as well as RestEase.

The advantages of using a Source Generator are:

1. Compile-time error checking. Find out if your RestEase interface has an error at compile-time, rather than runtime.
2. Supports platforms which don't support System.Reflection.Emit, such as iOS and .NET Native.
3. Faster: no need to generate implementations at runtime.

You will need to be using the .NET 5 SDK (or higher) to make use of source generators. If you're targetting C# 9 or .NET 5 (or higher), you're all set. If you're targetting an earlier language or runtime version, you can still install the latest .NET SDK (make sure you update your global.json if you have one!): you don't need to be targetting .NET 5, you just need to be building with the .NET 5 SDK.

When you build a project which references RestEase.SourceGenerator, RestEase generates implementations of any RestEase interfaces it finds in that project, and adds those implementations to your project. RestClient.For<T> will look for one of these implementations first, before falling back to the old approach of generating one at runtime. This means that you should reference RestEase.SourceGenerator in all projects which contain RestEase interfaces, but projects which only consume interfaces can just reference the RestEase package.

Authors of libraries which expose RestEase interfaces should also install RestEase.SourceGenerator, and the consumers of your library will use the interface implementations it generates a compile-time without needing to install RestEase.SourceGenerator themselves.

Using HttpClientFactory

If you're using ASP.NET Core 2.1 or higher, you can set up RestEase to use HttpClientFactory. Add a reference to RestEase.HttpClientFactory, and add something similar to the following to your ConfigureServices method:

services.AddRestEaseClient<ISomeApi>("https://api.example.com");

If you want to configure the RestClient instance, for example to set a custom serializer, pass in an options with RestClientConfigurer set to an Action<RestClient>:

services.AddRestEaseClient<ISomeApi>("https://api.example.com", new()
{
    RestClientConfigurer = client => client.RequestPathParamSerializer = new StringEnumRequestPathParamSerializer(),
});

If you want fo configure the ISomeApi instance, for example to assign a value to a property:

services.AddRestEaseClient<ISomeApi>("https://api.example.com", new()
{
    InstanceConfigurer = instance => instance.SomeHeader = "Some Value",
});

An IHttpClientBuilder is returned, which you can call further methods on if needed:

services.AddRestEaseClient<ISomeApi>("https://api.example.com")
    .AddHttpMessageHandler<SomeHandler>()
    .SetHandlerLifetime(TimeSpan.FromMinutes(2));

You can then inject ISomeApi into your controllers:

public class SomeController : ControllerBase
{
    private readonly ISomeApi someApi;
    public SomeController(ISomeApi someApi) => this.someApi = someApi;
}

If you want to configure a HttpClient for use with multiple RestEase interfaces, you can use IHttpClientBuilder.UseWithRestEaseClient. This returns the IHttpClientBuilder it was called on, so you can call it multiple times.

Make sure that you use one of the AddHttpClient overloads which takes a name, and also that you configure the BaseAddress on the HttpClient.

services.AddHttpClient("example")
    .ConfigureHttpClient(x => x.BaseAddress = new Uri("https://api.example.com"))
    .UseWithRestEaseClient<ISomeApi>()
    .UseWithRestEaseClient<ISomeOtherApi>();

Using RestEase with Polly

Sometimes request fail, and you want to retry them.

Polly is the industry-standard framework for defining retry/failure/etc policies, and it's easy to integrate with RestEase.

Using Polly with RestClient

If you're working with RestEase using new RestClient(...).For<T>() or RestClient.For<T>(...), then you'll need to install Microsoft.Extensions.Http.Polly. Create your IAsyncPolicy<HttpResponseMessage> following the Polly documentation, and then tell RestEase to use it using a PolicyHttpMessageHandler:

// Define your policy however you want
var policy = Policy
    .HandleResult<HttpResponseMessage>(r => r.StatusCode == HttpStatusCode.NotFound)
    .RetryAsync();

// Tell RestEase to use it
var api = RestClient.For<ISomeApi>("https://api.example.com", new PolicyHttpMessageHandler(policy));
// ... or ...
var api = new RestClient("https://api.example.com", new PolicyHttpMessageHandler(policy)).For<ISomeApi>();

Using Polly with HttpClientFactory

If you're using HttpClientFactory, then you'll need to follow the instructions on using HttpClientFactory, and then install Microsoft.Extensions.Http.Polly.

You can then follow these Polly instructions, but use AddRestEaseClient instead of AddHttpClient, for example:

// Define your policy however you want
var policy = Policy
    .HandleResult<HttpResponseMessage>(r => r.StatusCode == HttpStatusCode.NotFound)
    .RetryAsync();

services.AddRestEaseClient<ISomeApi>("https://api.example.com").AddPolicyHandler(policy);

// ... or perhaps ...

services
    .AddRestEaseClient<ISomeApi>("https://api.example.com")
    .AddTransientHttpErrorPolicy(builder => builder.WaitAndRetryAsync(new[]
    {
        TimeSpan.FromSeconds(1),
        TimeSpan.FromSeconds(5),
        TimeSpan.FromSeconds(10)
    }));

HttpClient and RestEase interface lifetimes

Each instance of the interface which you define will create its own HttpClient instance.

Prior to .NET Core 2.1, you should avoid creating and destroying many HttpClient instances (e.g. one per client request in a web app): instead create a single instance and keep using it (see here).

When using RestEase, this means that you should create a single instance of your interface and reuse it. If you use properties (e.g. Path Properties or Header Properties) which are set more than once, you could create a singleton HttpClient, and pass it to RestClient.For<T> to create many instances of your interface which share the same HttpClient.

If you're using .NET Core 2.1+, don't worry: HttpClient works as expected.

Controlling Serialization and Deserialization

By default, RestEase will use Json.NET to deserialize responses, and serialize request bodies and query parameters. However, you can change this, either by specifying custom JsonSerializerSettings, or by providing your own serializers / deserializers

Custom JsonSerializerSettings

If you want to specify your own JsonSerializerSettings, you can do this by constructing a new RestClient, assigning JsonSerializerSettings, then calling For<T>() to obtain an implementation of your interface, for example:

var settings = new JsonSerializerSettings()
{
    ContractResolver = new CamelCasePropertyNamesContractResolver(),
    Converters = { new StringEnumConverter() }
};

var api = new RestClient("https://api.example.com")
{
    JsonSerializerSettings = settings
}.For<ISomeApi>();

Custom Serializers and Deserializers

You can completely customize how requests are serialized, and responses deserialized, by providing your own serializer/deserializer implementations:

• To control how responses are deserialized, subclass ResponseDeserializer
• To control how request bodies are serialized, subclass RequestBodySerializer
• To control how request query parameters are serialized, subclass RequestQueryParamSerializer
• To control how request path parameters are serialized, subclass RequestPathParamSerializer

You can, of course, provide a custom implementation of only one of these, or all of them, or any number in between.

Deserializing responses: ResponseDeserializer

This class has a single method, which is called whenever a response is received which needs deserializing. It is passed the HttpResponseMessage (so you can read headers, etc, if you want) and its string content which has already been asynchronously read.

For an example, see JsonResponseDeserializer.

To tell RestEase to use it, you must create a new RestClient, assign its ResponseDeserializer property, then call For<T>() to get an implementation of your interface.

// This API returns XML

public class XmlResponseDeserializer : ResponseDeserializer
{
    public override T Deserialize<T>(string content, HttpResponseMessage response, ResponseDeserializerInfo info)
    {
        // Consider caching generated XmlSerializers
        var serializer = new XmlSerializer(typeof(T));

        using (var stringReader = new StringReader(content))
        {
            return (T)serializer.Deserialize(stringReader);
        }
    }
}

// ...

var api = new RestClient("https://api.example.com")
{
    ResponseDeserializer = new XmlResponseDeserializer()
}.For<ISomeApi>();

If you want your deserializer to receive strings (which lets you change the default behaviour, where an interface method which returns a Task<string> will return the raw response body), set the property ResponseDeserializer.HandlesStrings to true.

Serializing request bodies: RequestBodySerializer

This class has a single method, which is called whenever a request body requires serialization (i.e. is decorated with [Body(BodySerializationMethod.Serialized)]). It returns any HttpContent subclass you like, although StringContent is likely to be a common choice.

When writing an RequestBodySerializer's SerializeBody implementation, you may choose to provide some default headers, such as Content-Type. These will be overidden by any [Header] attributes.

For an example, see JsonRequestBodySerializer.

To tell RestEase to use it, you must create a new RestClient, assign its RequestBodySerializer property, then call For<T>() to get an implementation of your interface.

For example:

public class XmlRequestBodySerializer : RequestBodySerializer
{
    public override HttpContent SerializeBody<T>(T body, RequestBodySerializerInfo info)
    {
        if (body == null)
            return null;

        // Consider caching generated XmlSerializers
        var serializer = new XmlSerializer(typeof(T));

        using (var stringWriter = new StringWriter())
        {
            serializer.Serialize(stringWriter, body);
            var content = new StringContent(stringWriter.ToString());
            // Set the default Content-Type header to application/xml
            content.Headers.ContentType.MediaType = "application/xml";
            return content;
        }
    }
}

// ...

var api = new RestClient("https://api.example.com")
{
    RequestBodySerializer = new XmlRequestBodySerializer()
}.For<ISomeApi>();

Serializing request query parameters: RequestQueryParamSerializer

This class has two methods: one is called whenever a scalar query parameter requires serialization (i.e. is decorated with [Query(QuerySerializationMethod.Serialized)]); the other is called whenever a collection of query parameters (that is, the query parameter has type IEnumerable<T> for some T) requires serialization.

Both of these methods want you to return an IEnumerable<KeyValuePair<string, string>>, where each key corresponds to the name of a query name/value pair, and each value corresponds to the value. For example:

return new[]
{
    new KeyValuePair<string, string>("foo", "bar"),
    new KeyValuePair<string, string>("foo", "baz"),
    new KeyValuePair<string, string>("yay", "woo")
}

// Will get serialized to '...?foo=bar&foo=baz&yay=woo'

It is unlikely that you will return more than one KeyValuePair from the method which serializes scalar query parameters, but the flexibility is there.

For an example, see JsonRequestQueryParamSerializer.

To tell RestEase to use it, you must create a new RestClient, assign its RequestQueryParamSerializer property, then call For<T>() to get an implementation of your interface.

For example:

// It's highly unlikely that you'll get an API which requires xml-encoded query
// parameters, but for the sake of an example:

public class XmlRequestQueryParamSerializer : RequestQueryParamSerializer
{
    public override IEnumerable<KeyValuePair<string, string>> SerializeQueryParam<T>(string name, T value, RequestQueryParamSerializerInfo info)
    {
        if (value == null)
            yield break;

        // Consider caching generated XmlSerializers
        var serializer = new XmlSerializer(typeof(T));

        using (var stringWriter = new StringWriter())
        {
            serializer.Serialize(stringWriter, value);
            yield return new KeyValuePair<string, string>(name, stringWriter.ToString()));
        }
    }

    public override IEnumerable<KeyValuePair<string, string>> SerializeQueryCollectionParam<T>(string name, IEnumerable<T> values, RequestQueryParamSerializerInfo info)
    {
        if (values == null)
            yield break;

        // Consider caching generated XmlSerializers
        var serializer = new XmlSerializer(typeof(T));

        foreach (var value in values)
        {
            if (value != null)
            {
                using (var stringWriter = new StringWriter())
                {
                    serializer.Serialize(stringWriter, value);
                    yield return new KeyValuePair<string, string>(name, stringWriter.ToString()));
                }
            }
        }
    }
}

var api = new RestClient("https://api.example.com")
{
    RequestQueryParamSerializer = new XmlRequestQueryParamSerializer()
}.For<ISomeApi>();

If you specified a Format property on the [Query] attribute, this will be available as info.Format. By default, this is null.

Serializing request path parameters: RequestPathParamSerializer

This class has one method, called whenever a path parameter requires serialization (i.e. is decorated with [Path(PathSerializationMethod.Serialized)]).

This method wants you to return a string, which is the value that will be inserted in place of the placeholder in the path string.

There is no default path serializer, as its usage is often very specific. In order to use PathSerializationMethod.Serialized, you must set RestClient.RequestPathParamSerializer.

There is a StringEnumRequestPathParamSerializer provided with RestEase designed for serializing enums that have EnumMember, DisplayName or Display attributes specified on their members (evaluated in that order). This can be used as-is or as a reference for your own implementation.

To tell RestEase to use a path serializer, you must create a new RestClient, assign its RequestPathParamSerializer property, then call For<T>() to get an implementation of your interface.

For example:

var api = new RestClient("https://api.example.com")
{
    RequestPathParamSerializer = new StringEnumRequestPathParamSerializer(),
}.For<ISomeApi>();

If you specified a Format property on the [Path] attribute, this will be available as info.Format. By default, this is null.

Controlling query string generation: QueryStringBuilder

RestEase has logic to turn a collection of query parameters into a single suitably-encoded query string. However, some servers don't correctly decode query strings, and so users may want to control how query strings are encoded.

To do this, subclass QueryStringBuilder and assign it to the RestClient.QueryStringBuilder property. See the method BuildQueryParam in Requester for the default implementation.

Controlling the Requests

RestEase provides two ways for you to manipulate how exactly requests are made, before you need to resort to Customizing RestEase.

RequestModifier

The first is a RestClient.For<T> overload which lets you specify a delegate which is invoked whenever a request is made. This allows you to inspect and alter the request in any way you want: changing the content, changing the headers, make your own requests in the meantime, etc.

For example, if you need to refresh an oAuth access token occasionally (using the ADAL library as an example):

public interface IMyRestService
{
    [Get("getPublicInfo")]
    Task<Foobar> SomePublicMethodAsync();

    [Get("secretStuff")]
    [Header("Authorization", "Bearer")]
    Task<Location> GetLocationOfRebelBaseAsync();
}

AuthenticationContext context = new AuthenticationContext(...);
IGitHubApi api = RestClient.For<IGitHubApi>("http://api.github.com", async (request, cancellationToken) =>
{
    // See if the request has an authorize header
    var auth = request.Headers.Authorization;
    if (auth != null)
    {
        // The AquireTokenAsync call will prompt with a UI if necessary
        // Or otherwise silently use a refresh token to return a valid access token
        var token = await context.AcquireTokenAsync("http://my.service.uri/app", "clientId", new Uri("callback://complete")).ConfigureAwait(false);
        request.Headers.Authorization = new AuthenticationHeaderValue(auth.Scheme, token);
  }
});

If you need, you can get the IRequestInfo for the current request using request.Options.TryGetValue(RestClient.HttpRequestMessageRequestInfoOptionsKey, out var requestInfo) (for .NET 5+), or (IRequestInfo)request.Properties[RestClient.HttpRequestMessageRequestInfoPropertyKey] (pre .NET 5).

Custom HttpClient

The second is a RestClient.For<T> overload which lets you specify a custom HttpClient to use. This lets you customize the HttpClient, e.g. to set the request timeout. It also lets you specify a custom HttpMessageHandler subclass, which allows you to control all sorts of things.

For example, if you wanted to 1) adjust the request timeout, and 2) allow invalid certificates (although the same approach would apply if you wanted to customize how certificates are validated), you could do something like this. Note that WebRequestHandler is a HttpMessageHandler subclass which allows you to specify things like ServerCertificateValidationCallback.

public class CustomHttpClientHandler : WebRequestHandler
{
    // Let's log all of our requests!
    private static readonly Logger logger = LogManager.GetCurrentClassLogger();

    public CustomHttpClientHandler()
    {
        // Allow any cert, valid or invalid
        this.ServerCertificateValidationCallback = (sender, certificate, chain, sslPolicyErrors) => true;
    }

    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        if (logger.IsTraceEnabled)
        {
            var response = await base.SendAsync(request, cancellationToken);
            logger.Trace((await response.Content.ReadAsStringAsync()).Trim());
            return response;
        }
        else
        {
            return await base.SendAsync(request, cancellationToken);
        }
    }
}

var httpClient = new HttpClient(new CustomHttpClientHandler())
{
    BaseAddress = new Uri("https://secure-api.example.com"),
    Timeout = TimeSpan.FromSeconds(3), // Very slow to respond, this server
};

ISomeApi api = RestClient.For<ISomeApi>(httpClient);

If you need, you can get the IRequestInfo for the current request using (IRequestInfo)request.Properties[RestClient.HttpRequestMessageRequestInfoPropertyKey].

Adding to HttpRequestMessage.Properties

In very specific cases (i.e. you use a custom HttpMessageHandler), it might be useful to pass an object reference into the handler. In such case HttpRequestMessage.Properties can be used. This is done by decorating method parameters with [HttpRequestMessageProperty]. If key parameter is not specified then the name of the parameter will be used.

If all (or most) of the methods on the interface pass such object you can specify a [HttpRequestMessageProperty] property. These work in the same way as path parameters, but they're on the level of the entire API. Properties must have both a getter and a setter.

Property keys used at interface method level must be unique: a parameter key must not be same as a property key.

For example:

public interface ISomeApi
{
    [HttpRequestMessageProperty]
    CommonData AlwaysIncluded { get; set; }

    [Get]
    Task Get([HttpRequestMessageProperty("myKey")] string myValueIWantToAccessFromHttpMessageHandler);
}

In your HttpMessageHandler subclass:

protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
{
    var alwaysIncluded = (CommonData)request.Properties["AlwaysIncluded"];
    var myValueIWantToAccessFromHttpMessageHandler = (string)request.Properties["myKey"];

    // Let's use the properties!

    return await base.SendAsync(request, cancellationToken).ConfigureAwait(false);
}

Customizing RestEase

You've already seen how to specify custom Serializers and Deserializers, and control requests.

RestEase has been written in a way which makes it very easy to customize exactly how it works. In order to describe this, I'm first going to have to outline its architecture.

Given an API like:

public interface ISomeApi
{
    [Get("users/{userId}")]
    Task GetUserAsync([Path] string userId);
}

Calling RestClient.For<ISomeApi>(...) will cause a class like this to be generated:

namespace RestEase.AutoGenerated
{
    public class ISomeApi
    {
        private readonly IRequester requester;

        public ISomeApi(IRequester requester)
        {
            this.requester = requester;
        }

        public Task GetUserAsync(string userId)
        {
            var requestInfo = new RequestInfo(HttpMethod.Get, "users/{userId}");
            requestInfo.AddPathParameter<string>("userId", userId);
            return this.requester.RequestVoidAsync(requestInfo);
        }
    }
}

Now, you cannot customize what this generated class looks like, but you can see it doesn't actually do very much: it just builds up a RequestInfo object, then sends it off to the IRequester (which does all of the hard work). What you can do however is to provide your own IRequester implementation, and pass that to an appropriate overload of RestClient.For<T>. In fact, the default implementation of IRequester, Requester, has been carefully written so that it's easy to extend: each little bit of functionality is broken out into its own virtual method, so it's easy to replace just the behaviour you need.

Have a read through Requester, figure out what you want to change, subclass it, and provide an instance of that subclass to RestClient.For<T>.

Interface Accessibility

Since RestEase generates an interface implementation in a separate assembly, the interface ideally needs to be public.

If you don't want to do this, you'll need to mark RestEase as being a 'friend' assembly, which allows RestEase to see your internal types. Add the following line to your AssemblyInfo.cs:

[assembly: InternalsVisibleTo(RestEase.RestClient.FactoryAssemblyName)]

You can place the interface inside any namespace, or nest the interface inside another public type if you wish.

Using Generic Interfaces

When using something like ASP.NET Web API, it's a fairly common pattern to have a whole stack of CRUD REST services. RestEase supports these, allowing you to define a single API interface with a generic type:

public interface IReallyExcitingCrudApi<T, TKey>
{
    [Post("")]
    Task<T> Create([Body] T paylod);

    [Get("")]
    Task<List<T>> ReadAll();

    [Get("{key}")]
    Task<T> ReadOne([Path] TKey key);

    [Put("{key}")]
    Task Update([Path] TKey key, [Body] T payload);

    [Delete("{key}")]
    Task Delete([Path] TKey key);
}

Which can be used like this:

// The "/users" part here is kind of important if you want it to work for more
// than one type (unless you have a different domain for each type)
var api = RestClient.For<IReallyExcitingCrudApi<User, string>>("https://api.example.com/users");

Note that RestEase makes certain choices about how parameters and the return type are processed when the implementation of the interface is generated, and not when it is known (and the exact parameter types are known). This means that, for example, if you declare a return type of Task<T>, then call with T set to String, then you will not get a stream back - the response will be deserialized as a stream, which will almost certainly fail. Likewise if you declare a query parameter of type T, then set T to IEnumerable<string>, then your query will contain something like String[], instead of a collection of query parameters.

Using Generic Methods

You can define generic methods, if you wish. These have all of the same caveats as generic interfaces.

public interface IReallyExcitingCrudApi
{
    [Post("")]
    Task<T> Create<T>([Body] T paylod);

    [Get("")]
    Task<List<T>> ReadAll<T>();

    [Get("{key}")]
    Task<T> ReadOne<T, TKey>(TKey key);

    [Put("{key}")]
    Task Update<T, TKey>(TKey key, [Body] T payload);

    [Delete("{key}")]
    Task Delete<TKey>(TKey key);
}

Interface Inheritance

Sharing common properties and methods

You're allowed to use interface inheritance to share common properties and methods between different APIs.

You can only put an [AllowAnyStatusCode] attribute on the derived interface, and not on any parent interfaces. An [AllowAnyStatusCode] attribute on the derived interface also applies to all methods on all parent interfaces.

For example:

public interface IAuthenticatedEndpoint
{
    [Header("X-Api-Token")]
    string ApiToken { get; set; }

    [Header("X-Api-Username")]
    string ApiUsername { get; set; }

    [Path("userId")]
    int UserId { get; set; }
}

public interface IDevicesEndpoint : IAuthenticatedEndpoint
{
    [Get("/devices")]
    Task<IList<Device>> GetAllDevices([QueryMap] IDictionary<string, string> filters);
}

public interface IUsersEndpoint : IAuthenticatedEndpoint
{
    [Get("/user/{userId}")]
    Task<User> FetchUserAsync();
}

IDisposable

If your interface implements IDisposable, then RestEase will generate a Dispose() method which disposes the underlying HttpClient. Do this if you want to be able to dispose the HttpClient.

Advanced Functionality Using Extension Methods

Sometimes you'll have cases where you want to do something that's more complex than can be achieved using RestEase alone (e.g. uploading multipart form data), but you still want to provide a nice interface to consumers. One option is to write extension methods on your interface. There are two ways of doing this.

Wrapping other methods

The easiest thing to do is to put a method on your interface which won't be called by your code, but which can be wrapped by your extension method. This approach is unit testable.

public interface ISomeApi
{
    // Method which is only used by SomeApiExtensions
    [Post("upload")]
    Task UploadAsync([Body] HttpContent content);
}

public static class SomeApiExtensions
{
    public static Task UploadAsync(this ISomeApi api, byte[] imageData, string token)
    {
        var content = new MultipartFormDataContent();

        var imageContent = new ByteArrayContent(imageData);
        imageContent.Headers.ContentType = new MediaTypeHeaderValue("image/jpeg");
        content.Add(imageContent);

        var tokenContent = new FormUrlEncodedContent(new[]
        {
            new KeyValuePair<string, string>("token", token)
        });
        content.Add(tokenContent);

        return api.UploadAsync(content);
    }
}

Using IRequester directly

Alternatively, you can put a property of type IRequester on your interface, then write an extension method which uses the IRequester. Note that the attributes or properties you put on your interface (ISomeApi in the example below) will not be added to the RequestInfo, since you are not invoking any code which does this. Note also that this approach is not unit testable.

public interface ISomeApi
{
    IRequester Requester { get; }
}

public static class SomeApiExtensions
{
    public static Task UploadAsync(this ISomeApi api, byte[] imageData, string token)
    {
        var content = new MultipartFormDataContent();

        var imageContent = new ByteArrayContent(imageData);
        imageContent.Headers.ContentType = new MediaTypeHeaderValue("image/jpeg");
        content.Add(imageContent);

        var tokenContent = new FormUrlEncodedContent(new[]
        {
            new KeyValuePair<string, string>("token", token)
        });
        content.Add(tokenContent);

        var requestInfo = new RequestInfo(HttpMethod.Post, "upload");
        requestInfo.SetBodyParameterInfo(BodySerializationMethod.Default, content);
        return api.Requester.RequestVoidAsync(requestInfo);
    }
}

IRequester and RequestInfo are not documented in this README. Read the doc comments.

FAQs

I want to use Basic Authentication

Something like this...

public interface ISomeApi
{
    [Header("Authorization")]
    AuthenticationHeaderValue Authorization { get; set; }

    [Get("foo")]
    Task DoSomethingAsync();
}

ISomeApi api = RestClient.For<ISomeApi>("https://api.example.com");
var value = Convert.ToBase64String(Encoding.ASCII.GetBytes("username:password1234"));
api.Authorization = new AuthenticationHeaderValue("Basic", value);

await api.DoSomethingAsync();

I need to request an absolute path

Sometimes your API responses will contain absolute URLs, for example a "next page" link. Therefore you'll want a way to request a resource using an absolute URL which overrides the base URL you specified.

Thankfully this is easy: if you give an absolute URL to e.g. [Get("https://api.example.com/foo")], then the base URL will be ignored. You will also need to disable URL encoding.

public interface ISomeApi
{
    [Get("users")]
    Task<UsersResponse> FetchUsersAsync();

    [Get("{url}")]
    Task<UsersResponse> FetchUsersByUrlAsync([Path(UrlEncode = false)] string url);
}

ISomeApi api = RestClient.For<ISomeApi>("https://api.example.com");

var firstPage = await api.FetchUsersAsync();
// Actually put decent logic here...
var secondPage = await api.FetchUsersByUrlAsync(firstPage.NextPage);

I may get responses in both XML and JSON, and want to deserialize both

Occasionally you get an API which can return both JSON and XML (apparently...). In this case, you'll want to auto-detect what sort of response you got, and deserialize with an appropriate deserializer.

To do this, use a custom deserializer, which can do this detection.

public class HybridResponseDeserializer : ResponseDeserializer
{
    private T DeserializeXml<T>(string content)
    {
        // Consider caching generated XmlSerializers
        var serializer = new XmlSerializer(typeof(T));

        using (var stringReader = new StringReader(content))
        {
            return (T)serializer.Deserialize(stringReader);
        }
    }

    private T DeserializeJson<T>(string content)
    {
        return JsonConvert.Deserialize<T>(content);
    }

    public override T Deserialize<T>(string content, HttpResponseMessage response)
    {
        switch (response.Content.Headers.ContentType.MediaType)
        {
            case "application/json":
                return this.DeserializeJson<T>(content);
            case "application/xml":
                return this.DeserializeXml<T>(content);
        }

        throw new ArgumentException("Response was not JSON or XML");
    }
}

var api = RestClient.For<ISomeApi>("https://api.example.com", new HybridResponseDeserializer());

Is RestEase thread safe?

Yes. It is safe to create implementations of interfaces from multiple threads at the same time (and to create multiple implementations of the same interface), and it is safe to use an implementation from multiple threads at the same time.

I want to upload a file

Let's assume you want to upload a file (from a stream), setting its name and content-type manually (skip these bits of not). There are a couple of ways of doing this, depending on your needs:

public interface ISomeApi
{
    [Header("Content-Disposition", "form-data; filename=\"somefile.txt\"")]
    [Header("Content-Type", "text/plain")]
    [Post("upload")]
    Task UploadFileVersionOneAsync([Body] Stream file);

    [Post("upload")]
    // You can use strings instead of strongly-typed header values, if you want
    Task UploadFileVersionTwoAsync(
        [Header("Content-Disposition")] ContentDispositionHeaderValue contentDisposition,
        [Header("Content-Type")] MediaTypeHeaderValue contentType,
        [Body] Stream file);

    [Post("upload")]
    Task UploadFileVersionThreeAsync([Body] HttpContent content);
}

ISomeApi api = RestClient.For<ISomeApi>("https://api.example.com");

// Version one (constant headers)
using (var fileStream = File.OpenRead("somefile.txt"))
{
    await api.UploadFileVersionOneAsync(fileStream);
}

// Version two (variable headers)
using (var fileStream = File.OpenRead("somefile.txt"))
{
    var contentDisposition = new ContentDispositionHeaderValue("form-header") { FileName = "\"somefile.txt\"" };
    var contentType = new MediaTypeHeaderValue("text/plain");
    await api.UploadFileVersionTwoAsync(contentDisposition, contentType, fileStream);
}

// Version three (precise control over HttpContent)
using (var fileStream = File.OpenRead("somefile.txt"))
{
    var fileContent = new StreamContent(fileStream);
    fileContent.Headers.ContentDisposition = new ContentDispositionHeaderValue("form-header") { FileName = "\"somefile.txt\"" };
    fileContent.Headers.ContentType = new MediaTypeHeaderValue("text/plain");
    await api.UploadFileVersionThreeAsync(fileContent);
}

Obviously, set the headers you need - don't just copy me blindly.

You can use extension methods to make this more palatable for consumers.

I want to ensure that all of the required properties on my request body are set

User @netclectic has a solution, see this issue.