Simplest method to do a REST API calls in c# with basic .NET classes (no external library needed). I use it when communicationg with Recurly.com REST services.

RestApi.cs

    /*
      Simplest method to do a REST API calls in c# with basic .NET classes.
      
      I use it when communicationg with Recurly.com REST services.
    */

using System;
using System.Collections.Generic;
using System.Data;
using System.IO;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
using System.Transactions;
using System.Web.Mvc;
using System.Xml;


    /// <summary>
    /// Helping class, that solves problem that Microsoft has done. See remarks.
    /// </summary>
    /// <remarks>
    /// 1. When you use <see cref="System.Net.WebClient"/>, you cannot get response status (200 - OK, 404 - Not Found, ...)
    /// 
    /// 2. When you use <see cref="System.Net.WebRequest"/>, you cannot change HTTP headers before sending request
    /// 
    /// 3. Use <see cref="System.Net.HttpWebRequest"/> directly is not possible.
    /// 
    /// Solution:
    /// 
    /// Microsoft hase done a "great job" again. Most useful method <see cref="System.Net.WebClient.GetWebRequest"/> 
    /// is <c>protected</c>. So we must use this class as a hack.
    /// </remarks>
    public class BetterWebClient : System.Net.WebClient
    {
        /// <summary>
        /// See <see cref="System.Net.WebRequest.GetWebRequest"/>
        /// </summary>
        /// <param name="address">See <see cref="System.Net.WebRequest.GetWebRequest"/></param>
        /// <returns>See <see cref="System.Net.WebRequest.GetWebRequest"/></returns>
        public virtual System.Net.WebRequest GetWebRequestX(Uri address)
        {
            return this.GetWebRequest(address);
        }
    }
    
    public class RestApi
    {
      /// <summary>
        /// Makes a REST API call (I'm using it with Recurly.com service)
        /// </summary>
        /// <remarks>
        /// It will return either a response from a REST API call.
        /// If any error was during comunication, this method returns empty string. 
        /// </remarks>
        /// <param name="url">URL to fetch</param>
        /// <returns>Returns a string, representing XML of the object.</returns>
        /// <param name="method"><see cref="System.Net.WebRequestMethods.Http"/></param>
        public string Fetch(string url, string method = System.Net.WebRequestMethods.Http.Get, string xmlToSend = "")
        {
            DateTime startedOnUtc = DateTime.UtcNow;
            string responseData = null;
            string responseHeaders = null;
            int responseStatusCode = 0;

            try {
                BetterWebClient webClient = new BetterWebClient();
                webClient.Proxy = null;
                webClient.Headers.Add(System.Net.HttpRequestHeader.Accept, "application/xml");
                
                // You can use BASIC authorization like so (you must implement your own Recurly.ApiKeyBase64String): 
                webClient.Headers.Add(System.Net.HttpRequestHeader.Authorization, "Basic " + Recurly.ApiKeyBase64String);

                if (false == String.IsNullOrWhiteSpace(xmlToSend))
                {
                    webClient.Headers.Add(System.Net.HttpRequestHeader.ContentType, "application/xml; charset=utf-8");
                }
                var webRequest = webClient.GetWebRequestX(new Uri(url));
                webRequest.Method = method;

                if (false == String.IsNullOrWhiteSpace(xmlToSend))
                {
                    using (StreamWriter writer = new StreamWriter(webRequest.GetRequestStream(), Encoding.UTF8))
                    {
                        writer.WriteLine(xmlToSend);
                    }
                }
                else
                {
                    if (method == System.Net.WebRequestMethods.Http.Put)
                    {
                        webRequest.ContentLength = 0; // ContentLength == 0 is required for PUT requests
                    }
                }

                using (System.Net.WebResponse response = webRequest.GetResponse())
                {
                    ProcessResponse(response, out responseData, out responseHeaders, out responseStatusCode);
                }
            }
            catch (System.Net.WebException ex)
            {
                using (ex.Response)
                {
                    ProcessResponse(ex.Response, out responseData, out responseHeaders, out responseStatusCode);
                }

                responseData = "<!-- " + ex.Message + " -->\r\n\r\n" + responseData;
            }
            finally
            {
              /* YOU CAN (you should) LOG ALL YOUR COMMUNICATION WITH SERVER HERE like this:
              
                var log = Log.CreateCommunicationLogObject();
                
                log.StartedOnUtc = startedOnUtc;
                log.FinishedOnUtc = DateTime.UtcNow;
                log.RequestMethod = method;
                log.RequestData = String.IsNullOrWhiteSpace(xmlToSend) ? null : xmlToSend;
                log.RequestUrl = url;
                log.ResponseData = responseData;
                log.ResponseStatusCode = responseStatusCode;
                log.ResponseHeaders = responseHeaders;
                
                Log.Save();
              */
            }
            
            if (responseStatusCode >= 200 && responseStatusCode < 300)
            {
                // All 2xx HTTP statuses are OK
                return responseData;
            }

            return String.Empty;
        }
        
        private static void ProcessResponse(System.Net.WebResponse response, out string recurlyResponse, out string responseHeaders, out int responseStatusCode)
        {
            recurlyResponse = responseHeaders = String.Empty;
            responseStatusCode = 0;

            using (var responseStream = response.GetResponseStream())
            {
                using (System.IO.StreamReader sr = new System.IO.StreamReader(responseStream, Encoding.UTF8))
                {
                    recurlyResponse = sr.ReadToEnd();
                }
            }

            responseHeaders = response.Headers.ToString();
            var httpWebResponse = response as System.Net.HttpWebResponse;
            if (httpWebResponse != null)
            {
                responseStatusCode = (int)httpWebResponse.StatusCode;
            }
        }
    }