Refit: The automatic type-safe REST library for Xamarin and .NET

Refit is a library heavily inspired by Square's Retrofit library, and it turns your REST API into a live interface:

public interface IGitHubApi
{
    [Get("/users/{user}")]
    Task<User> GetUser(string user);
}

The RestService class generates an implementation of IGitHubApi that uses HttpClient to make its calls:

var gitHubApi = RestService.For<IGitHubApi>("https://api.github.com");

var octocat = await gitHubApi.GetUser("octocat");

Where does this work?

Refit currently supports the following platforms:

• Xamarin.Android
• Xamarin.Mac
• Xamarin.iOS 64-bit (Unified API)
• Desktop .NET 4.5
• Windows Phone 8
• Windows Store (WinRT) 8.0/8.1
• Windows Phone 8.1 Universal Apps

The following platforms are not supported:

• Xamarin.iOS 32-bit - build system doesn't support targets files

API Attributes

Every method must have an HTTP attribute that provides the request method and relative URL. There are five built-in annotations: Get, Post, Put, Delete, and Head. The relative URL of the resource is specified in the annotation.

[Get("/users/list")]

You can also specify query parameters in the URL:

[Get("/users/list?sort=desc")]

A request URL can be updated dynamically using replacement blocks and parameters on the method. A replacement block is an alphanumeric string surrounded by { and }.

If the name of your parameter doesn't match the name in the URL path, use the AliasAs attribute.

[Get("/group/{id}/users")]
Task<List<User>> GroupList([AliasAs("id")] int groupId);

Parameters that are not specified as a URL substitution will automatically be used as query parameters. This is different than Retrofit, where all parameters must be explicitly specified.

The comparison between parameter name and URL parameter is not case-sensitive, so it will work correctly if you name your parameter groupId in the path /group/{groupid}/show for example.

[Get("/group/{id}/users")]
Task<List<User>> GroupList([AliasAs("id")] int groupId, [AliasAs("sort")] string sortOrder);

GroupList(4, "desc");
>>> "/group/4/users?sort=desc"

Body content

One of the parameters in your method can be used as the body, by using the Body attribute:

[Post("/users/new")]
Task CreateUser([Body] User user);

There are four possibilities for supplying the body data, depending on the type of the parameter:

• If the type is Stream, the content will be streamed via StreamContent
• If the type is string, the string will be used directly as the content
• If the parameter has the attribute [Body(BodySerializationMethod.UrlEncoded)], the content will be URL-encoded (see form posts below)
• For all other types, the object will be serialized as JSON.

JSON content

JSON requests and responses are serialized/deserialized using Json.NET. Default settings for the API can be configured by setting Newtonsoft.Json.JsonConvert.DefaultSettings:

JsonConvert.DefaultSettings = 
    () => new JsonSerializerSettings() { 
        ContractResolver = new CamelCasePropertyNamesContractResolver(),
        Converters = {new StringEnumConverter()}
    };

// Serialized as: {"day":"Saturday"}
await PostSomeStuff(new { Day = DayOfWeek.Saturday });

Property serialization/deserialization can be customised using Json.NET's JsonProperty attribute:

public class Foo 
{
    // Works like [AliasAs("b")] would in form posts (see below)
    [JsonProperty(PropertyName="b")] 
    public string Bar { get; set; }
} 

Form posts

For APIs that take form posts (i.e. serialized as application/x-www-form-urlencoded), initialize the Body attribute with BodySerializationMethod.UrlEncoded.

The parameter can be an IDictionary:

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

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

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

Or you can just pass any object and all public, readable properties will be serialized as form fields in the request. This approach allows you to alias property names using [AliasAs("whatever")] which can help if the API has cryptic field names:

public interface IMeasurementProtocolApi
{
    [Post("/collect")]
    Task Collect([Body(BodySerializationMethod.UrlEncoded)] Measurement measurement);
}

public class Measurement
{
    // Properties can be read-only and [AliasAs] isn't required
    public int v { get { return 1; }
 
    [AliasAs("tid")]
    public string WebPropertyId { get; set; }

    [AliasAs("cid")]
    public Guid ClientId { get;set; }

    [AliasAs("t")] 
    public string Type { get; set; }

    public object IgnoreMe { private get; set; }
}

var measurement = new Measurement { 
    WebPropertyId = "UA-1234-5", 
    ClientId = new Guid("d1e9ea6b-2e8b-4699-93e0-0bcbd26c206c"), 
    Type = "event" 
}; 

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

Setting request headers

Static headers

You can set one or more static request headers for a request applying a Headers attribute to the method:

[Headers("User-Agent: Awesome Octocat App")]
[Get("/users/{user}")]
Task<User> GetUser(string user);

Static headers can also be added to every request in the API by applying the Headers attribute to the interface:

[Headers("User-Agent: Awesome Octocat App")]
public interface IGitHubApi
{
    [Get("/users/{user}")]
    Task<User> GetUser(string user);
    
    [Post("/users/new")]
    Task CreateUser([Body] User user);
}

Dynamic headers

If the content of the header needs to be set at runtime, you can add a header with a dynamic value to a request by applying a Header attribute to a parameter:

[Get("/users/{user}")]
Task<User> GetUser(string user, [Header("Authorization")] string authorization);

// Will add the header "Authorization: token OAUTH-TOKEN" to the request
var user = await GetUser("octocat", "token OAUTH-TOKEN"); 

Redefining headers

Unlike Retrofit, where headers do not overwrite each other and are all added to the request regardless of how many times the same header is defined, Refit takes a similar approach to the approach ASP.NET MVC takes with action filters — redefining a header will replace it, in the following order of precedence:

• Headers attribute on the interface (lowest priority)
• Headers attribute on the method
• Header attribute on a method parameter (highest priority)

[Headers("X-Emoji: :rocket:")]
public interface IGitHubApi
{
    [Get("/users/list")]
    Task<List> GetUsers();
    
    [Get("/users/{user}")]
    [Headers("X-Emoji: :smile_cat:")]
    Task<User> GetUser(string user);
    
    [Post("/users/new")]
    [Headers("X-Emoji: :metal:")]
    Task CreateUser([Body] User user, [Header("X-Emoji")] string emoji);
}

// X-Emoji: :rocket:
var users = await GetUsers();

// X-Emoji: :smile_cat:
var user = await GetUser("octocat");

// X-Emoji: :trollface:
await CreateUser(user, ":trollface:"); 

Removing headers

Headers defined on an interface or method can be removed by redefining a static header without a value (i.e. without : <value>) or passing null for a dynamic header. Empty strings will be included as empty headers.

[Headers("X-Emoji: :rocket:")]
public interface IGitHubApi
{
    [Get("/users/list")]
    [Headers("X-Emoji")] // Remove the X-Emoji header
    Task<List> GetUsers();
    
    [Get("/users/{user}")]
    [Headers("X-Emoji:")] // Redefine the X-Emoji header as empty
    Task<User> GetUser(string user);
    
    [Post("/users/new")]
    Task CreateUser([Body] User user, [Header("X-Emoji")] string emoji);
}

// No X-Emoji header
var users = await GetUsers();

// X-Emoji: 
var user = await GetUser("octocat");

// No X-Emoji header
await CreateUser(user, null); 

// X-Emoji: 
await CreateUser(user, ""); 

Retrieving the response

Note that in Refit unlike in Retrofit, there is no option for a synchronous network request - all requests must be async, either via Task or via IObservable. There is also no option to create an async method via a Callback parameter unlike Retrofit, because we live in the async/await future.

Similarly to how body content changes via the parameter type, the return type will determine the content returned.

Returning Task without a type parameter will discard the content and solely tell you whether or not the call succeeded:

[Post("/users/new")]
Task CreateUser([Body] User user);

// This will throw if the network call fails
await CreateUser(someUser);

If the type parameter is 'HttpResponseMessage' or 'string', the raw response message or the content as a string will be returned respectively.

// Returns the content as a string (i.e. the JSON data)
[Get("/users/{user}")]
Task<string> GetUser(string user);

// Returns the raw response, as an IObservable that can be used with the
// Reactive Extensions
[Get("/users/{user}")]
IObservable<HttpResponseMessage> GetUser(string user);

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. Refit now supports these, allowing you to define a single API interface with a generic type:

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

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

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

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

    [Delete("/{key}")]
    Task Delete(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 = RestService.For<IReallyExcitingCrudApi<User, string>>("http://api.example.com/users"); 

What's missing / planned?

Currently Refit is missing the following features from Retrofit that are planned for a future release soon:

• Multipart requests (including file upload)