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

	Refit	Refit.HttpClientFactory
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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 and any .NET Standard 1.4 target:

• UWP
• Xamarin.Android
• Xamarin.Mac
• Xamarin.iOS
• Desktop .NET 4.6.1
• .NET Core

Note about .NET Core

For .NET Core build-time support, you must use the .NET Core 2 SDK. You can target any supported platform in your library, long as the 2.0+ SDK is used at build-time.

API Attributes

Every method must have an HTTP attribute that provides the request method and relative URL. There are six built-in annotations: Get, Post, Put, Delete, Patch 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"

Round-tripping route parameter syntax: Forward slashes aren't encoded when using a double-asterisk (**) catch-all parameter syntax.

During link generation, the routing system encodes the value captured in a double-asterisk (**) catch-all parameter (for example, {**myparametername}) except the forward slashes.

The type of round-tripping route parameter must be string.

[Get("/search/{**page}")]
Task<List<Page>> Search(string page);

Search("admin/products");
>>> "/search/admin/products"

Dynamic Querystring Parameters

If you specify an object as a query parameter, all public properties which are not null are used as query parameters. This previously only applied to GET requests, but has now been expanded to all HTTP request methods, partly thanks to Twitter's hybrid API that insists on non-GET requests with querystring parameters. Use the Query attribute the change the behavior to 'flatten' your query parameter object. If using this Attribute you can specify values for the Delimiter and the Prefix which are used to 'flatten' the object.

public class MyQueryParams
{
    [AliasAs("order")]
    public string SortOrder { get; set; }

    public int Limit { get; set; }
}


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

[Get("/group/{id}/users")]
Task<List<User>> GroupListWithAttribute([AliasAs("id")] int groupId, [Query(".","search")] MyQueryParams params);


params.SortOrder = "desc";
params.Limit = 10;

GroupList(4, params)
>>> "/group/4/users?order=desc&Limit=10"

GroupListWithAttribute(4, params)
>>> "/group/4/users?search.order=desc&search.Limit=10"

A similar behavior exists if using a Dictionary, but without the advantages of the AliasAs attributes and of course no intellisense and/or type safety.

You can also specify querystring parameters with [Query] and have them flattened in non-GET requests, similar to:

[Post("/statuses/update.json")]
Task<Tweet> PostTweet([Query]TweetParams params);

Where TweetParams is a POCO, and properties will also support [AliasAs] attributes.

Collections as Querystring parameters

Use the Query attribute to specify format in which collections should be formatted in query string

[Get("/users/list")]
Task Search([Query(CollectionFormat.Multi)]int[] ages);

Search(new [] {10, 20, 30})
>>> "/users/list?ages=10&ages=20&ages=30"

[Get("/users/list")]
Task Search([Query(CollectionFormat.Csv)]int[] ages);

Search(new [] {10, 20, 30})
>>> "/users/list?ages=10%2C20%2C30"

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 unless [Body(BodySerializationMethod.Json)] is set which will send it as a StringContent
• 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 using the content serializer specified in RefitSettings (JSON is the default).

Buffering and the Content-Length header

By default, Refit streams the body content without buffering it. This means you can stream a file from disk, for example, without incurring the overhead of loading the whole file into memory. The downside of this is that no Content-Length header is set on the request. If your API needs you to send a Content-Length header with the request, you can disable this streaming behavior by setting the buffered argument of the [Body] attribute to true:

Task CreateUser([Body(buffered: true)] User user);

JSON content

JSON requests and responses are serialized/deserialized using Json.NET. By default, Refit will use the serializer settings that 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 });

As these are global settings they will affect your entire application. It might be beneficial to isolate the settings for calls to a particular API. When creating a Refit generated live interface, you may optionally pass a RefitSettings that will allow you to specify what serializer settings you would like. This allows you to have different serializer settings for separate APIs:

var gitHubApi = RestService.For<IGitHubApi>("https://api.github.com",
    new RefitSettings {
        ContentSerializer = new JsonContentSerializer( 
            new JsonSerializerSettings {
                ContractResolver = new SnakeCasePropertyNamesContractResolver()
        }
    )});

var otherApi = RestService.For<IOtherApi>("https://api.example.com",
    new RefitSettings {
        ContentSerializer = new JsonContentSerializer( 
            new JsonSerializerSettings {
                ContractResolver = new CamelCasePropertyNamesContractResolver()
        }
    )});

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; }
} 

XML Content

XML requests and responses are serialized/deserialized using System.Xml.Serialization.XmlSerializer. By default, Refit will use JSON content serialization, to use XML content configure the ContentSerializer to use the XmlContentSerializer:

var gitHubApi = RestService.For<IXmlApi>("https://www.w3.org/XML",
    new RefitSettings {
        ContentSerializer = new XmlContentSerializer()
    });

Property serialization/deserialization can be customised using attributes found in the System.Xml.Serialization namespace:

    public class Foo
    {
        [XmlElement(Namespace = "https://www.w3.org/XML")]
        public string Bar { get; set; }
    }

The System.Xml.Serialization.XmlSerializer provides many options for serializing, those options can be set by providing an XmlContentSerializerSettings to the XmlContentSerializer constructor:

var gitHubApi = RestService.For<IXmlApi>("https://www.w3.org/XML",
    new RefitSettings {
        ContentSerializer = new XmlContentSerializer(
            new XmlContentSerializerSettings
            {
                XmlReaderWriterSettings = new XmlReaderWriterSettings()
                {
                    ReaderSettings = new XmlReaderSettings
                    {
                        IgnoreWhitespace = true
                    }
                }
            }
        )
    });

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);

If you have a type that has [JsonProperty(PropertyName)] attributes setting property aliases, Refit will use those too ([AliasAs] will take precedence where you have both). This means that the following type will serialize as one=value1&two=value2:

public class SomeObject
{
    [JsonProperty(PropertyName = "one")]
    public string FirstProperty { get; set; }

    [JsonProperty(PropertyName = "notTwo")]
    [AliasAs("two")]
    public string SecondProperty { get; set; }
}

NOTE: This use of AliasAs applies to querystring parameters and form body posts, but not to response objects; for aliasing fields on response objects, you'll still need to use [JsonProperty("full-property-name")].

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"); 

Authorization (Dynamic Headers redux)

The most common reason to use headers is for authorization. Today most API's use some flavor of oAuth with access tokens that expire and refresh tokens that are longer lived.

One way to encapsulate these kinds of token usage, a custom HttpClientHandler can be inserted instead.
There are two classes for doing this: one is AuthenticatedHttpClientHandler, which takes a Func<Task<string>> parameter, where a signature can be generated without knowing about the request. The other is AuthenticatedParameterizedHttpClientHandler, which takes a Func<HttpRequestMessage, Task<string>> parameter, where the signature requires information about the request (see earlier notes about Twitter's API)

For example:

class AuthenticatedHttpClientHandler : HttpClientHandler
{
    private readonly Func<Task<string>> getToken;

    public AuthenticatedHttpClientHandler(Func<Task<string>> getToken)
    {
        if (getToken == null) throw new ArgumentNullException(nameof(getToken));
        this.getToken = getToken;
    }

    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        // See if the request has an authorize header
        var auth = request.Headers.Authorization;
        if (auth != null)
        {
            var token = await getToken().ConfigureAwait(false);
            request.Headers.Authorization = new AuthenticationHeaderValue(auth.Scheme, token);
        }

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

Or:

class AuthenticatedParameterizedHttpClientHandler : DelegatingHandler
    {
        readonly Func<HttpRequestMessage, Task<string>> getToken;

        public AuthenticatedParameterizedHttpClientHandler(Func<HttpRequestMessage, Task<string>> getToken, HttpMessageHandler innerHandler = null)
            : base(innerHandler ?? new HttpClientHandler())
        {
            this.getToken = getToken ?? throw new ArgumentNullException(nameof(getToken));
        }

        protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
        {
            // See if the request has an authorize header
            var auth = request.Headers.Authorization;
            if (auth != null)
            {
                var token = await getToken(request).ConfigureAwait(false);
                request.Headers.Authorization = new AuthenticationHeaderValue(auth.Scheme, token);
            }

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

While HttpClient contains a nearly identical method signature, it is used differently. HttpClient.SendAsync is not called by Refit. The HttpClientHandler must be modified instead.

This class is used like so (example uses the ADAL library to manage auto-token refresh but the principal holds for Xamarin.Auth or any other library:

class LoginViewModel
{
    AuthenticationContext context = new AuthenticationContext(...);
    
    private async Task<string> GetToken()
    {
        // The AcquireTokenAsync 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"));
        
        return token;
    }

    public async Task LoginAndCallApi()
    {
        var api = RestService.For<IMyRestService>(new HttpClient(new AuthenticatedHttpClientHandler(GetToken)) { BaseAddress = new Uri("https://the.end.point/") });
        var location = await api.GetLocationOfRebelBase();
    }
}

interface IMyRestService
{
    [Get("/getPublicInfo")]
    Task<Foobar> SomePublicMethod();

    [Get("/secretStuff")]
    [Headers("Authorization: Bearer")]
    Task<Location> GetLocationOfRebelBase();
}

In the above example, any time a method that requires authentication is called, the AuthenticatedHttpClientHandler will try to get a fresh access token. It's up to the app to provide one, checking the expiration time of an existing access token and obtaining a new one if needed.

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, ""); 

Multipart uploads

Methods decorated with Multipart attribute will be submitted with multipart content type. At this time, multipart methods support the following parameter types:

• string (parameter name will be used as name and string value as value)
• byte array
• Stream
• FileInfo

The parameter name will be used as the name of the field in the multipart data. This can be overridden with the AliasAs attribute.

To specify the file name and content type for byte array (byte[]), Stream and FileInfo parameters, use of a wrapper class is required. The wrapper classes for these types are ByteArrayPart, StreamPart and FileInfoPart.

public interface ISomeApi
{
    [Multipart]
    [Post("/users/{id}/photo")]
    Task UploadPhoto(int id, [AliasAs("myPhoto")] StreamPart stream);
}

To pass a Stream to this method, construct a StreamPart object like so:

someApiInstance.UploadPhoto(id, new StreamPart(myPhotoStream, "photo.jpg", "image/jpeg"));

Note: The AttachmentName attribute that was previously described in this section has been deprecated and its use is not recommended.

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 payload);

    [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"); 

Interface inheritance

When multiple services that need to be kept separate share a number of APIs, it is possible to leverage interface inheritance to avoid having to define the same Refit methods multiple times in different services:

public interface IBaseService
{
    [Get("/resources")]
    Task<Resource> GetResource(string id);
}

public interface IDerivedServiceA : IBaseService
{
    [Delete("/resources")]
    Task DeleteResource(string id);
}

public interface IDerivedServiceB : IBaseService
{
    [Post("/resources")]
    Task<string> AddResource([Body] Resource resource);
}

In this example, the IDerivedServiceA interface will expose both the GetResource and DeleteResource APIs, while IDerivedServiceB will expose GetResource and AddResource.

Headers inheritance

When using inheritance, existing header attributes will passed along as well, and the inner-most ones will have precedence:

[Headers("User-Agent: AAA")]
public interface IAmInterfaceA
{
    [Get("/get?result=Ping")]
    Task<string> Ping();
}

[Headers("User-Agent: BBB")]
public interface IAmInterfaceB : IAmInterfaceA
{
    [Get("/get?result=Pang")]
    [Headers("User-Agent: PANG")]
    Task<string> Pang();

    [Get("/get?result=Foo")]
    Task<string> Foo();
}

Here, IAmInterfaceB.Pang() will use PANG as its user agent, while IAmInterfaceB.Foo and IAmInterfaceB.Ping will use BBB. Note that if IAmInterfaceB didn't have a header attribute, Foo would then use the AAA value inherited from IAmInterfaceA. If an interface is inheriting more than one interface, the order of precedence is the same as the one in which the inherited interfaces are declared:

public interface IAmInterfaceC : IAmInterfaceA, IAmInterfaceB
{
    [Get("/get?result=Foo")]
    Task<string> Foo();
}

Here IAmInterfaceC.Foo would use the header attribute inherited from IAmInterfaceA, if present, or the one inherited from IAmInterfaceB, and so on for all the declared interfaces.

Using HttpClientFactory

Refit has first class support for the ASP.Net Core 2.1 HttpClientFactory. Add a reference to Refit.HttpClientFactory and call the provided extension method in your ConfigureServices method to configure your Refit interface:

services.AddRefitClient<IWebApi>()
        .ConfigureHttpClient(c => c.BaseAddress = new Uri("https://api.example.com"));
        // Add additional IHttpClientBuilder chained methods as required here:
        // .AddHttpMessageHandler<MyHandler>()
        // .SetHandlerLifetime(TimeSpan.FromMinutes(2));

Optionally, a RefitSettings object can be included:

var settings = new RefitSettings(); 
// Configure refit settings here

services.AddRefitClient<IWebApi>(settings)
        .ConfigureHttpClient(c => c.BaseAddress = new Uri("https://api.example.com"));
        // Add additional IHttpClientBuilder chained methods as required here:
        // .AddHttpMessageHandler<MyHandler>()
        // .SetHandlerLifetime(TimeSpan.FromMinutes(2));

Note that some of the properties of RefitSettings will be ignored because the HttpClient and HttpClientHandlers will be managed by the HttpClientFactory instead of Refit.

You can then get the api interface using constructor injection:

public class HomeController : Controller
{
    public HomeController(IWebApi webApi)
    {
        _webApi = webApi;
    }

    private readonly IWebApi _webApi;

    public async Task<IActionResult> Index(CancellationToken cancellationToken)
    {
        var thing = await _webApi.GetSomethingWeNeed(cancellationToken);
        return View(thing);
    }
}

Handling exceptions

To encapsulate any exceptions that may come from a service, you can catch an ApiException which contains request- and response information. Refit also supports the catching of validation exceptions that are thrown by a service implementing the RFC 7807 specification for problem details due to bad requests. For specific information on the problem details of the validation exception, simply catch ValidationApiException:

// ...
try
{
   var result = await awesomeApi.GetFooAsync("bar");
}
catch (ValidationApiException validationException)
{
   // handle validation here by using validationException.Content, 
   // which is type of ProblemDetails according to RFC 7807
}
catch (ApiException exception)
{
   // other exception handling
}
// ...