HttpClient vs Webclient vs HttpWebRequest

System.Net.WebRequest is an abstract class, System.Net.HttpWebRequest and System.Net.FileWebRequest will inherit the WebRequest.

The System.Net.WebClient class provides common methods for sending data to or receiving data from any local, intranet, or Internet resource identified by a URI. The WebClient class uses the WebRequest class to provide access to resources. WebClient instances can access data with any WebRequest descendant registered with the WebRequest.RegisterPrefix method.

The System.Net.Http.HttpClient class is a base class for sending HTTP requests and receiving HTTP responses from a resource identified by a URI. 

The .NET 2.0 included WebClient class to communicate with web server using HTTP protocol. However, WebClient class had some limitations. The .NET 4.5 includes HttpClient class to overcome the limitation of WebClient. Microsoft doesn't recommend that you use the WebClient class for new development. Instead, use the System.Net.Http.HttpClient class.

HttpClient is intended to be instantiated once and re-used throughout the life of an application. Instantiating an HttpClient class for every request will exhaust the number of sockets available under heavy loads. This will result in SocketException errors.

Below is an example using HttpClient correctly.

public class GoodController : ApiController
{
    private static readonly HttpClient HttpClient;

    static GoodController()
    {
        HttpClient = new HttpClient();
    }
}