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

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

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

[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);
}

[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");

[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

[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);

 //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
    }
}

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

[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:");

[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);

[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, "");

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);
}

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);
    }
}

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

public interface IUsersAPI : IGetAPI<User>
{
}

public interface IOrdersAPI : IGetAPI<Order>
{
}

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);
    }
}

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);
    }
}

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

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

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

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

// 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);

//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;

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);
}

// 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");

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);
}

[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();
}

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

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}";
}

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}";
}

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));

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));

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);
    }
}

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

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

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

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

SomeApi.Client.Timeout = timeout;

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

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

// ...
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
}
// ...

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

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