Refit.HttpClientFactory 7.2.1

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

	Refit	Refit.HttpClientFactory	Refit.Newtonsoft.Json
NuGet

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");

.NET Core supports registering via HttpClientFactory

services
    .AddRefitClient<IGitHubApi>()
    .ConfigureHttpClient(c => c.BaseAddress = new Uri("https://api.github.com"));

Table of Contents

• Where does this work?
  • Breaking changes in 6.x
• API Attributes
• Querystrings
  • Dynamic Querystring Parameters
  • Collections as Querystring parameters
  • Unescape Querystring parameters
  • Custom Querystring Parameter formatting
• Body content
  • Buffering and the Content-Length header
  • JSON content
  • XML Content
  • Form posts
• Setting request headers
  • Static headers
  • Dynamic headers
  • Bearer Authentication
  • Reducing header boilerplate with DelegatingHandlers (Authorization headers worked example)
  • Redefining headers
  • Removing headers
• Passing state into DelegatingHandlers
  • Support for Polly and Polly.Context
  • Target Interface type
  • MethodInfo of the method on the Refit client interface that was invoked
• Multipart uploads
• Retrieving the response
• Using generic interfaces
• Interface inheritance
  • Headers inheritance
• Default Interface Methods
• Using HttpClientFactory
• Providing a custom HttpClient
• Handling exceptions
  • When returning Task<IApiResponse>, Task<IApiResponse<T>>, or Task<ApiResponse<T>>
  • When returning Task<T>
  • Providing a custom ExceptionFactory
  • ApiException deconstruction with Serilog

Where does this work?

Refit currently supports the following platforms and any .NET Standard 2.0 target:

• UWP
• Xamarin.Android
• Xamarin.Mac
• Xamarin.iOS
• Desktop .NET 4.6.1
• .NET 5 / .NET Core
• Blazor
• Uno Platform

SDK Requirements

Refit 6 requires Visual Studio 16.8 or higher, or the .NET SDK 5.0.100 or higher. It can target any .NET Standard 2.0 platform.

Refit 6 does not support the old packages.config format for NuGet references (as they do not support analyzers/source generators). You must migrate to PackageReference to use Refit v6 and later.

Breaking changes in 6.x

Refit 6 makes System.Text.Json the default JSON serializer. If you'd like to continue to use Newtonsoft.Json, add the Refit.Newtonsoft.Json NuGet package and set your ContentSerializer to NewtonsoftJsonContentSerializer on your RefitSettings instance. System.Text.Json is faster and uses less memory, though not all features are supported. The migration guide contains more details.

IContentSerializer was renamed to IHttpContentSerializer to better reflect its purpose. Additionally, two of its methods were renamed, SerializeAsync<T> → ToHttpContent<T> and DeserializeAsync<T> → FromHttpContentAsync<T>. Any existing implementations of these will need to be updated, though the changes should be minor.

Updates in 6.3

Refit 6.3 splits out the XML serialization via XmlContentSerializer into a separate package, Refit.Xml. This is to reduce the dependency size when using Refit with Web Assembly (WASM) applications. If you require XML, add a reference to Refit.Xml.

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);

A request url can also bind replacement blocks to a custom object

[Get("/group/{request.groupId}/users/{request.userId}")]
Task<List<User>> GroupList(UserGroupRequest request);

class UserGroupRequest{
    int groupId { get;set; }
    int userId { get;set; }
}

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"

Querystrings

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 to 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; }

    public KindOptions Kind { get; set; }
}

public enum KindOptions
{
    Foo,

    [EnumMember(Value = "bar")]
    Bar
}


[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;
params.Kind = KindOptions.Bar;

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

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

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"

You can also specify collection format in RefitSettings, that will be used by default, unless explicitly defined in Query attribute.

var gitHubApi = RestService.For<IGitHubApi>("https://api.github.com",
    new RefitSettings {
        CollectionFormat = CollectionFormat.Multi
    });

Unescape Querystring parameters

Use the QueryUriFormat attribute to specify if the query parameters should be url escaped

[Get("/query")]
[QueryUriFormat(UriFormat.Unescaped)]
Task Query(string q);

Query("Select+Id,Name+From+Account")
>>> "/query?q=Select+Id,Name+From+Account"

Custom Querystring parameter formatting

Formatting Keys

To customize the format of query keys, you have two main options:

1. Using the AliasAs Attribute:

   You can use the AliasAs attribute to specify a custom key name for a property. This attribute will always take precedence over any key formatter you specify.

   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, [Query] MyQueryParams params);
   
   params.SortOrder = "desc";
   params.Limit = 10;
   
   GroupList(1, params);
   

   This will generate the following request:

   /group/1/users?order=desc&Limit=10
   

2. Using the RefitSettings.UrlParameterKeyFormatter Property:

   By default, Refit uses the property name as the query key without any additional formatting. If you want to apply a custom format across all your query keys, you can use the UrlParameterKeyFormatter property. Remember that if a property has an AliasAs attribute, it will be used regardless of the formatter.

   The following example uses the built-in CamelCaseUrlParameterKeyFormatter:

   public class MyQueryParams
   {
       public string SortOrder { get; set; }
   
       [AliasAs("queryLimit")]
       public int Limit { get; set; }
   }
   
   [Get("/group/users")]
   Task<List<User>> GroupList([Query] MyQueryParams params);
   
   params.SortOrder = "desc";
   params.Limit = 10;
   

   The request will look like:

   /group/users?sortOrder=desc&queryLimit=10
   

Note: The AliasAs attribute always takes the top priority. If both the attribute and a custom key formatter are present, the AliasAs attribute's value will be used.

Formatting URL Parameter Values with the UrlParameterFormatter

In Refit, the UrlParameterFormatter property within RefitSettings allows you to customize how parameter values are formatted in the URL. This can be particularly useful when you need to format dates, numbers, or other types in a specific manner that aligns with your API's expectations.

Using UrlParameterFormatter:

Assign a custom formatter that implements the IUrlParameterFormatter interface to the UrlParameterFormatter property.

public class CustomDateUrlParameterFormatter : IUrlParameterFormatter
{
    public string? Format(object? value, ICustomAttributeProvider attributeProvider, Type type)
    {
        if (value is DateTime dt)
        {
            return dt.ToString("yyyyMMdd");
        }

        return value?.ToString();
    }
}

var settings = new RefitSettings
{
    UrlParameterFormatter = new CustomDateUrlParameterFormatter()
};

In this example, a custom formatter is created for date values. Whenever a DateTime parameter is encountered, it formats the date as yyyyMMdd.

Formatting Dictionary Keys:

When dealing with dictionaries, it's important to note that keys are treated as values. If you need custom formatting for dictionary keys, you should use the UrlParameterFormatter as well.

For instance, if you have a dictionary parameter and you want to format its keys in a specific way, you can handle that in the custom formatter:

public class CustomDictionaryKeyFormatter : IUrlParameterFormatter
{
    public string? Format(object? value, ICustomAttributeProvider attributeProvider, Type type)
    {
        // Handle dictionary keys
        if (attributeProvider is PropertyInfo prop && prop.PropertyType.IsGenericType && prop.PropertyType.GetGenericTypeDefinition() == typeof(Dictionary<,>))
        {
            // Custom formatting logic for dictionary keys
            return value?.ToString().ToUpperInvariant();
        }

        return value?.ToString();
    }
}

var settings = new RefitSettings
{
    UrlParameterFormatter = new CustomDictionaryKeyFormatter()
};

In the above example, the dictionary keys will be converted to uppercase.

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 an instance of the IHttpContentSerializer interface. Refit provides two implementations out of the box: SystemTextJsonContentSerializer (which is the default JSON serializer) and NewtonsoftJsonContentSerializer. The first uses System.Text.Json APIs and is focused on high performance and low memory usage, while the latter uses the known Newtonsoft.Json library and is more versatile and customizable. You can read more about the two serializers and the main differences between the two at this link.

For instance, here is how to create a new RefitSettings instance using the Newtonsoft.Json-based serializer (you'll also need to add a PackageReference to Refit.Newtonsoft.Json):

var settings = new RefitSettings(new NewtonsoftJsonContentSerializer());

If you're using Newtonsoft.Json APIs, you can customize their behavior by setting the Newtonsoft.Json.JsonConvert.DefaultSettings property:

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 NewtonsoftJsonContentSerializer(
            new JsonSerializerSettings {
                ContractResolver = new SnakeCasePropertyNamesContractResolver()
        }
    )});

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

JSON source generator

To apply the benefits of the new JSON source generator for System.Text.Json added in .NET 6, you can use SystemTextJsonContentSerializer with a custom instance of RefitSettings and JsonSerializerOptions:

var options = new JsonSerializerOptions() {
    TypeInfoResolver = MyJsonSerializerContext.Default
};

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

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
                    }
                }
            }
        )
    });

<a name="form-posts"></a>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");

Adding an Authorization header is such a common use case that you can add an access token to a request by applying an Authorize attribute to a parameter and optionally specifying the scheme:

[Get("/users/{user}")]
Task<User> GetUser(string user, [Authorize("Bearer")] string token);

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

//note: the scheme defaults to Bearer if none provided

If you need to set multiple headers at runtime, you can add a IDictionary<string, string> and apply a HeaderCollection attribute to the parameter and it will inject the headers into the request:

[Get("/users/{user}")]
Task<User> GetUser(string user, [HeaderCollection] IDictionary<string, string> headers);

var headers = new Dictionary<string, string> {{"Authorization","Bearer tokenGoesHere"}, {"X-Tenant-Id","123"}};
var user = await GetUser("octocat", headers);

Bearer Authentication

Most APIs need some sort of Authentication. The most common is OAuth Bearer authentication. A header is added to each request of the form: Authorization: Bearer <token>. Refit makes it easy to insert your logic to get the token however your app needs, so you don't have to pass a token into each method.

1. Add [Headers("Authorization: Bearer")] to the interface or methods which need the token.
2. Set AuthorizationHeaderValueGetter in the RefitSettings instance. Refit will call your delegate each time it needs to obtain the token, so it's a good idea for your mechanism to cache the token value for some period within the token lifetime.

Reducing header boilerplate with DelegatingHandlers (Authorization headers worked example)

Although we make provisions for adding dynamic headers at runtime directly in Refit, most use-cases would likely benefit from registering a custom DelegatingHandler in order to inject the headers as part of the HttpClient middleware pipeline thus removing the need to add lots of [Header] or [HeaderCollection] attributes.

In the example above we are leveraging a [HeaderCollection] parameter to inject an Authorization and X-Tenant-Id header. This is quite a common scenario if you are integrating with a 3rd party that uses OAuth2. While it's ok for the occasional endpoint, it would be quite cumbersome if we had to add that boilerplate to every method in our interface.

In this example we will assume our application is a multi-tenant application that is able to pull information about a tenant through some interface ITenantProvider and has a data store IAuthTokenStore that can be used to retrieve an auth token to attach to the outbound request.

 //Custom delegating handler for adding Auth headers to outbound requests
 class AuthHeaderHandler : DelegatingHandler
 {
     private readonly ITenantProvider tenantProvider;
     private readonly IAuthTokenStore authTokenStore;

    public AuthHeaderHandler(ITenantProvider tenantProvider, IAuthTokenStore authTokenStore)
    {
         this.tenantProvider = tenantProvider ?? throw new ArgumentNullException(nameof(tenantProvider));
         this.authTokenStore = authTokenStore ?? throw new ArgumentNullException(nameof(authTokenStore));
         // InnerHandler must be left as null when using DI, but must be assigned a value when
         // using RestService.For<IMyApi>
         // InnerHandler = new HttpClientHandler();
    }

    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        var token = await authTokenStore.GetToken();

        //potentially refresh token here if it has expired etc.

        request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", token);
        request.Headers.Add("X-Tenant-Id", tenantProvider.GetTenantId());

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

//Startup.cs
public void ConfigureServices(IServiceCollection services)
{
    services.AddTransient<ITenantProvider, TenantProvider>();
    services.AddTransient<IAuthTokenStore, AuthTokenStore>();
    services.AddTransient<AuthHeaderHandler>();

    //this will add our refit api implementation with an HttpClient
    //that is configured to add auth headers to all requests

    //note: AddRefitClient<T> requires a reference to Refit.HttpClientFactory
    //note: the order of delegating handlers is important and they run in the order they are added!

    services.AddRefitClient<ISomeThirdPartyApi>()
        .ConfigureHttpClient(c => c.BaseAddress = new Uri("https://api.example.com"))
        .AddHttpMessageHandler<AuthHeaderHandler>();
        //you could add Polly here to handle HTTP 429 / HTTP 503 etc
}

//Your application code
public class SomeImportantBusinessLogic
{
    private ISomeThirdPartyApi thirdPartyApi;

    public SomeImportantBusinessLogic(ISomeThirdPartyApi thirdPartyApi)
    {
        this.thirdPartyApi = thirdPartyApi;
    }

    public async Task DoStuffWithUser(string username)
    {
        var user = await thirdPartyApi.GetUser(username);
        //do your thing
    }
}

If you aren't using dependency injection then you could achieve the same thing by doing something like this:

var api = RestService.For<ISomeThirdPartyApi>(new HttpClient(new AuthHeaderHandler(tenantProvider, authTokenStore))
    {
        BaseAddress = new Uri("https://api.example.com")
    }
);

var user = await thirdPartyApi.GetUser(username);
//do your thing

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 or HeaderCollection 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:");

Note: This redefining behavior only applies to headers with the same name. Headers with different names are not replaced. The following code will result in all headers being included:

[Headers("Header-A: 1")]
public interface ISomeApi
{
    [Headers("Header-B: 2")]
    [Post("/post")]
    Task PostTheThing([Header("Header-C")] int c);
}

// Header-A: 1
// Header-B: 2
// Header-C: 3
var user = await api.PostTheThing(3);

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, "");

Passing state into DelegatingHandlers

If there is runtime state that you need to pass to a DelegatingHandler you can add a property with a dynamic value to the underlying HttpRequestMessage.Properties by applying a Property attribute to a parameter:

public interface IGitHubApi
{
    [Post("/users/new")]
    Task CreateUser([Body] User user, [Property("SomeKey")] string someValue);

    [Post("/users/new")]
    Task CreateUser([Body] User user, [Property] string someOtherKey);
}

The attribute constructor optionally takes a string which becomes the key in the HttpRequestMessage.Properties dictionary. If no key is explicitly defined then the name of the parameter becomes the key. If a key is defined multiple times the value in HttpRequestMessage.Properties will be overwritten. The parameter itself can be any object. Properties can be accessed inside a DelegatingHandler as follows:

class RequestPropertyHandler : DelegatingHandler
{
    public RequestPropertyHandler(HttpMessageHandler innerHandler = null) : base(innerHandler ?? new HttpClientHandler()) {}

    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        // See if the request has a the property
        if(request.Properties.ContainsKey("SomeKey"))
        {
            var someProperty = request.Properties["SomeKey"];
            //do stuff
        }

        if(request.Properties.ContainsKey("someOtherKey"))
        {
            var someOtherProperty = request.Properties["someOtherKey"];
            //do stuff
        }

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

Note: in .NET 5 HttpRequestMessage.Properties has been marked Obsolete and Refit will instead populate the value into the new HttpRequestMessage.Options.

Support for Polly and Polly.Context

Because Refit supports HttpClientFactory it is possible to configure Polly policies on your HttpClient. If your policy makes use of Polly.Context this can be passed via Refit by adding [Property("PolicyExecutionContext")] Polly.Context context as behind the scenes Polly.Context is simply stored in HttpRequestMessage.Properties under the key PolicyExecutionContext and is of type Polly.Context. It's only recommended to pass the Polly.Context this way if your use case requires that the Polly.Context be initialized with dynamic content only known at runtime. If your Polly.Context only requires the same content every time (e.g an ILogger that you want to use to log from inside your policies) a cleaner approach is to inject the Polly.Context via a DelegatingHandler as described in #801

Target Interface Type and method info

There may be times when you want to know what the target interface type is of the Refit instance. An example is where you have a derived interface that implements a common base like this:

public interface IGetAPI<TEntity>
{
    [Get("/{key}")]
    Task<TEntity> Get(long key);
}

public interface IUsersAPI : IGetAPI<User>
{
}

public interface IOrdersAPI : IGetAPI<Order>
{
}

You can access the concrete type of the interface for use in a handler, such as to alter the URL of the request:

class RequestPropertyHandler : DelegatingHandler
{
    public RequestPropertyHandler(HttpMessageHandler innerHandler = null) : base(innerHandler ?? new HttpClientHandler()) {}

    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        // Get the type of the target interface
        Type interfaceType = (Type)request.Properties[HttpMessageRequestOptions.InterfaceType];

        var builder = new UriBuilder(request.RequestUri);
        // Alter the Path in some way based on the interface or an attribute on it
        builder.Path = $"/{interfaceType.Name}{builder.Path}";
        // Set the new Uri on the outgoing message
        request.RequestUri = builder.Uri;

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

The full method information (RestMethodInfo) is also always available in the request options. The RestMethodInfo contains more information about the method being called such as the full MethodInfo when using reflection is needed:

class RequestPropertyHandler : DelegatingHandler
{
    public RequestPropertyHandler(HttpMessageHandler innerHandler = null) : base(innerHandler ?? new HttpClientHandler()) {}

    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        // Get the method info
        if (request.Options.TryGetValue(HttpRequestMessageOptions.RestMethodInfoKey, out RestMethodInfo restMethodInfo))
        {
            var builder = new UriBuilder(request.RequestUri);
            // Alter the Path in some way based on the method info or an attribute on it
            builder.Path = $"/{restMethodInfo.MethodInfo.Name}{builder.Path}";
            // Set the new Uri on the outgoing message
            request.RequestUri = builder.Uri;
        }

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

Note: in .NET 5 HttpRequestMessage.Properties has been marked Obsolete and Refit will instead populate the value into the new HttpRequestMessage.Options. Refit provides HttpRequestMessageOptions.InterfaceTypeKey and HttpRequestMessageOptions.RestMethodInfoKey to respectively access the interface type and REST method info from the options.

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

Name of the field in the multipart data priority precedence:

• multipartItem.Name if specified and not null (optional); dynamic, allows naming form data part at execution time.
• [AliasAs] attribute (optional) that decorate the streamPart parameter in the method signature (see below); static, defined in code.
• MultipartItem parameter name (default) as defined in the method signature; static, defined in code.

A custom boundary can be specified with an optional string parameter to the Multipart attribute. If left empty, this defaults to ----MyGreatBoundary.

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);

There is also a generic wrapper class called ApiResponse<T> that can be used as a return type. Using this class as a return type allows you to retrieve not just the content as an object, but also any metadata associated with the request/response. This includes information such as response headers, the http status code and reason phrase (e.g. 404 Not Found), the response version, the original request message that was sent and in the case of an error, an ApiException object containing details of the error. Following are some examples of how you can retrieve the response metadata.

//Returns the content within a wrapper class containing metadata about the request/response
[Get("/users/{user}")]
Task<ApiResponse<User>> GetUser(string user);

//Calling the API
var response = await gitHubApi.GetUser("octocat");

//Getting the status code (returns a value from the System.Net.HttpStatusCode enumeration)
var httpStatus = response.StatusCode;

//Determining if a success status code was received
if(response.IsSuccessStatusCode)
{
    //YAY! Do the thing...
}

//Retrieving a well-known header value (e.g. "Server" header)
var serverHeaderValue = response.Headers.Server != null ? response.Headers.Server.ToString() : string.Empty;

//Retrieving a custom header value
var customHeaderValue = string.Join(',', response.Headers.GetValues("A-Custom-Header"));

//Looping through all the headers
foreach(var header in response.Headers)
{
    var headerName = header.Key;
    var headerValue = string.Join(',', header.Value);
}

//Finally, retrieving the content in the response body as a strongly-typed object
var user = response.Content;

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 be 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.

Default Interface Methods

Starting with C# 8.0, default interface methods (a.k.a. DIMs) can be defined on interfaces. Refit interfaces can provide additional logic using DIMs, optionally combined with private and/or static helper methods:

public interface IApiClient
{
    // implemented by Refit but not exposed publicly
    [Get("/get")]
    internal Task<string> GetInternal();
    // Publicly available with added logic applied to the result from the API call
    public async Task<string> Get()
        => FormatResponse(await GetInternal());
    private static String FormatResponse(string response)
        => $"The response is: {response}";
}

The type generated by Refit will implement the method IApiClient.GetInternal. If additional logic is required immediately before or after its invocation, it shouldn't be exposed directly and can thus be hidden from consumers by being marked as internal. The default interface method IApiClient.Get will be inherited by all types implementing IApiClient, including - of course - the type generated by Refit. Consumers of the IApiClient will call the public Get method and profit from the additional logic provided in its implementation (optionally, in this case, with the help of the private static helper FormatResponse). To support runtimes without DIM-support (.NET Core 2.x and below or .NET Standard 2.0 and below), two additional types would be required for the same solution.

internal interface IApiClientInternal
{
    [Get("/get")]
    Task<string> Get();
}
public interface IApiClient
{
    public Task<string> Get();
}
internal class ApiClient : IApiClient
{
    private readonly IApiClientInternal client;
    public ApiClient(IApiClientInternal client) => this.client = client;
    public async Task<string> Get()
        => FormatResponse(await client.Get());
    private static String FormatResponse(string response)
        => $"The response is: {response}";
}

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));

// or injected from the container
services.AddRefitClient<IWebApi>(provider => new RefitSettings() { /* configure 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);
    }
}

Providing a custom HttpClient

You can supply a custom HttpClient instance by simply passing it as a parameter to the RestService.For<T> method:

RestService.For<ISomeApi>(new HttpClient()
{
    BaseAddress = new Uri("https://www.someapi.com/api/")
});

However, when supplying a custom HttpClient instance the following RefitSettings properties will not work:

• AuthorizationHeaderValueGetter
• HttpMessageHandlerFactory

If you still want to be able to configure the HtttpClient instance that Refit provides while still making use of the above settings, simply expose the HttpClient on the API interface:

interface ISomeApi
{
    // This will automagically be populated by Refit if the property exists
    HttpClient Client { get; }

    [Headers("Authorization: Bearer")]
    [Get("/endpoint")]
    Task<string> SomeApiEndpoint();
}

Then, after creating the REST service, you can set any HttpClient property you want, e.g. Timeout:

SomeApi = RestService.For<ISomeApi>("https://www.someapi.com/api/", new RefitSettings()
{
    AuthorizationHeaderValueGetter = (rq, ct) => GetTokenAsync()
});

SomeApi.Client.Timeout = timeout;

Handling exceptions

Refit has different exception handling behavior depending on if your Refit interface methods return Task<T> or if they return Task<IApiResponse>, Task<IApiResponse<T>>, or Task<ApiResponse<T>>.

<a id="when-returning-taskapiresponset"></a>When returning Task<IApiResponse>, Task<IApiResponse<T>>, or Task<ApiResponse<T>>

Refit traps any ApiException raised by the ExceptionFactory when processing the response, and any errors that occur when attempting to deserialize the response to ApiResponse<T>, and populates the exception into the Error property on ApiResponse<T> without throwing the exception.

You can then decide what to do like so:

var response = await _myRefitClient.GetSomeStuff();
if(response.IsSuccessStatusCode)
{
   //do your thing
}
else
{
   _logger.LogError(response.Error, response.Error.Content);
}

When returning Task<T>

Refit throws any ApiException raised by the ExceptionFactory when processing the response and any errors that occur when attempting to deserialize the response to Task<T>.

// ...
try
{
   var result = await awesomeApi.GetFooAsync("bar");
}
catch (ApiException exception)
{
   //exception handling
}
// ...

Refit can also throw ValidationApiException instead which in addition to the information present on ApiException also contains ProblemDetails when the service implements the RFC 7807 specification for problem details and the response content type is application/problem+json

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

   // If the response contains additional properties on the problem details,
   // they will be added to the validationException.Content.Extensions collection.
}
catch (ApiException exception)
{
   // other exception handling
}
// ...

Providing a custom ExceptionFactory

You can also override default exceptions behavior that are raised by the ExceptionFactory when processing the result by providing a custom exception factory in RefitSettings. For example, you can suppress all exceptions with the following:

var nullTask = Task.FromResult<Exception>(null);

var gitHubApi = RestService.For<IGitHubApi>("https://api.github.com",
    new RefitSettings {
        ExceptionFactory = httpResponse => nullTask;
    });

Note that exceptions raised when attempting to deserialize the response are not affected by this.

ApiException deconstruction with Serilog

For users of Serilog, you can enrich the logging of ApiException using the Serilog.Exceptions.Refit NuGet package. Details of how to integrate this package into your applications can be found here.