Pathoschild.FluentHttpClient is a strongly-typed easy-to-use asynchronous REST API client, built on top of the .NET 4.5 HttpClient. The client provides a single fluent interface that lets you create an HTTP request, dispatch and wait for it, and process the response. The client will automatically inject any required HTTP configuration (like User-Agent and Accept headers) and handle the plumbing code.

You start by creating a client, and chain methods to configure your request and response:

     IClient client = new FluentClient("http://example.org/api/");

The most common use is an asynchronous HTTP GET, with the response deserialized into a class instance. Let's say we're fetching an Idea object from a REST API (it could be JSON, XML, or anything else):

     Idea idea = await client
        .GetAsync("ideas/14")
        .As<Idea>();

You can retrieve the content as a deserialized model, list of models, byte array, string, or stream (or add your own formats):

     string idea = await client
        .GetAsync("ideas", new Idea())
        .AsString();

If you don't need the response, you can just wait for the request to complete. (This will still raise errors for you.)

     await client.PostAsync("ideas", new Idea());

You can fluently configure the request, from HTTP headers to query string arguments. The client will take care of the details (like sanitizing input and encoding the URL):

     Idea idea = await client
        .GetAsync("ideas")
        .WithHeader("Content-Type", "application/json")
        .WithArguments(new { id = 14, tenant = "tenant-name" }) // equivalent to .WithArgument("id", 14).WithArgument("tenant", "tenant-name")
        .As<Idea>();

You can even configure a range of features like credentials and cookies using the default HTTP message handler:

     client.MessageHandler.Credentials = new NetworkCredential("username", "password");
     client.MessageHandler.CookieContainer.Add(new Cookie(...));
     client.MessageHandler.Proxy = new WebProxy(...);

Not every feature is shown in these examples, but every method is fully code-documented for IntelliSense so it's easy to just use the client.

HTTP errors (such as HTTP Not Found) will be raised as ApiException, and you can add your own validation by overriding Request.ValidateResponse. For example, you could raise application errors from the API as client exceptions. (You can disable these exceptions by setting IRequest.RaiseErrors = false.)

When an HTTP request fails, you can find out why by checking the exception object. This contains the HttpResponseMessage (which includes the HTTP details like the request message, HTTP status code, headers, and response body) and IResponse (which provides a convenient way to read the response body).

For example:

     /// <summary>Get a value from the API.</summary>
	 /// <param name="key">The value key.</param>
	 /// <exception cref="KeyNotFoundException">The key could not be found.</exception>
	 /// <exception cref="CustomApiExeption">The remote application returned an error message.</exception>
     public async Task<string> GetValue(string key)
     {
        try
        {
           return await client
              .Get("api/dictionary")
              .WithArgument("key", key)
              .AsString();
        }
        catch(ApiException exception)
        {
           // key not found
           if(exception.ResponseMessage.StatusCode == HttpStatusCode.NotFound)
              throw new KeyNotFoundException("The key could not be found.")
           
           // remote application error
           if(exception.ResponseMessage.Content != null)
           {
              exception.Response.RaiseErrors = false; // disable validation so we can read response content
              throw new CustomApiException(await exception.Response.AsString(), exception);
           }
           
           // unhandled exception
           throw;
        }
     }

The client is designed to take advantage of the async and await keywords in .NET 4.5, but you can use the client synchronously:

     Idea idea = client
        .GetAsync("ideas/14")
        .AsString()
        .Result;

Or if you don't need the response:

     client.PostAsync("ideas", new Idea()).Wait();

Note: Result and Await() will wrap any exceptions thrown by the client into an AggregateException.

Beware: mixing blocking and asynchronous code within UI applications (like a web project) can lead to deadlocks. (If the only asynchronous code is the client itself, you should be fine doing this.) For further information, see Parallel Programming with .NET: Await, and UI, and deadlocks! Oh my! (Stephen Toub, MSDN) and Don't Block on Async Code (Stephen Cleary).

The fluent client is available as the Pathoschild.Http.FluentClient NuGet package.

Optional addons:

• Pathoschild.Http.Formatters.Core includes Pathoschild.Http.Formatters;
• Pathoschild.Http.Formatters.JsonNet includes Pathoschild.Http.Formatters.JsonNet and Pathoschild.Http.Formatters.

The client uses .NET's MediaTypeFormatters for serializing and deserializing HTTP messages, and uses content negotiation to select the relevant format. These are the same ones used by the HttpClient itself and the ASP.NET Web API. When creating a client for an ASP.NET Web API, this lets you seamlessly use the same formatters on both sides.

You can use any of the many implementations already available (including the Json.NET formatters below), or create your own (MediaTypeFormatterBase might help). For example, to replace the default JSON formatter with the formatter below:

     IClient client = new FluentClient("http://example.org/api/");
     client.Formatters.Remove(client.Formatters.JsonFormatter);
     client.Formatters.Add(new JsonNetFormatter());

The Pathoschild.Http.Formatters.JsonNet package provides three formats using Json.NET: BSON (application/bson), JSON (application/json, text/json), and JSONP (application/javascript, application/ecmascript, text/javascript, text/ecmascript). JSONP requests can include an optional callback query parameter that specifies the JavaScript method name to invoke.

     client.Formatters.Add(new JsonNetBsonFormatter());
     client.Formatters.Add(new JsonNetFormatter());

You can inject your own HTTP message handler to do pretty much anything you want. For example, you could easily create a custom handler for unit tests which talks directly to a mock without actual HTTP calls:

     UnitTestHandler handler = new UnitTestHandler() { WasCalled = false };
     IClient<UnitTestHandler> client = new FluentClient<UnitTestHandler>(new HttpClient(handler), handler);
     bool wasCalled = client.MessageHandler.WasCalled; // strongly-typed access to the handler

You can create your own implementations of the client interfaces (IClient and IRequest) — the default classes have virtual methods, so you can subclass them to override individual methods and properties.

You can inject your own implementations of the interfaces using the IFactory, which is called whenever the client needs an implementation. You can subclass the default Factory and only override the methods you're interested in.

     IClient client = new FluentClient("http://example.org/api/", new CustomFactory());

You can combine the factory with the decorator pattern using the delegating implementations (DelegatingFluentClient and DelegatingRequest) to inject specialized behaviour. These implementations let you override individual methods and properties while delegating everything else to another implementation.

For example, this delegating response tracks time spent waiting for HTTP requests using MiniProfiler:

     public class ProfiledRequest : DelegatingRequest
     {
        public ProfiledRequest(IRequest request)
           : base(request) { }

        public override Task<HttpResponseMessage> AsMessage()
        {
           using (MiniProfiler.Current.Step("Waiting for API"))
              return base.AsMessage();
        }
     }

You can then combine decorators to inject the behaviour you want:

     // override factory
     public class CustomFactory : Factory
     {
        public override IRequest GetRequest(HttpRequestMessage message, MediaTypeFormatterCollection formatters, Func<IRequest, Task<HttpResponseMessage>> dispatcher)
        {
           IRequest request = base.GetRequest(message, formatters, dispatcher);
           request = new ProfiledRequest(request);
           request = new AuditedRequest(request);
           return request;
        }
     }
	 
     // use the client without worrying about what behaviour is injected
     IClient client = new FluentClient("http://example.org/api/", new CustomFactory());
     Idea idea = await client
        .Get("ideas/14")
        .As<Idea>();