using System.Net;
using System.IO;

...

string requestUrl = "https://www.example.com/submit.html";
HttpWebRequest request = HttpWebRequest.CreateHttp(requestUrl);
request.Method = "POST";

// Optionally, set properties of the HttpWebRequest, such as:
request.AutomaticDecompression = DecompressionMethods.Deflate | DecompressionMethods.GZip;
request.ContentType = "application/x-www-form-urlencoded";
// Could also set other HTTP headers such as Request.UserAgent, Request.Referer,
// Request.Accept, or other headers via the Request.Headers collection.

// Set the POST request body data. In this example, the POST data is in 
// application/x-www-form-urlencoded format.
string postData = "myparam1=myvalue1&myparam2=myvalue2";
using (var writer = new StreamWriter(request.GetRequestStream()))
{
    writer.Write(postData);
}

// Submit the request, and get the response body from the remote server.
string responseFromRemoteServer;
using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
{
    using (StreamReader reader = new StreamReader(response.GetResponseStream()))
    {
        responseFromRemoteServer = reader.ReadToEnd();
    }
}

using System.Net;
using System.IO;

...

string requestUrl = "https://www.example.com/page.html";
HttpWebRequest request = HttpWebRequest.CreateHttp(requestUrl);

// Optionally, set properties of the HttpWebRequest, such as:
request.AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate;
request.Timeout = 2 * 60 * 1000; // 2 minutes, in milliseconds

// Submit the request, and get the response body.
string responseBodyFromRemoteServer;
using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
{
    using (StreamReader reader = new StreamReader(response.GetResponseStream()))
    {
        responseBodyFromRemoteServer = reader.ReadToEnd();
    }
}

using System.Net;

...

string serverResponse;
try 
{
    // Call a method that performs an HTTP request (per the above examples).
    serverResponse = PerformHttpRequest();
}
catch (WebException ex)
{
    if (ex.Status == WebExceptionStatus.ProtocolError)
    {
        HttpWebResponse response = ex.Response as HttpWebResponse;
        if (response != null)
        {
            if ((int)response.StatusCode == 404) // Not Found
            {
                // Handle the 404 Not Found error 
                // ...
            }
            else
            { 
                // Could handle other response.StatusCode values here.
                // ...
            }
        }
    }
    else
    {
        // Could handle other error conditions here, such as WebExceptionStatus.ConnectFailure.
        // ...
    }
}

public static async Task PostAsync(this Uri uri, object value)
{
    var content = new ObjectContext(value.GetType(), value, new JsonMediaTypeFormatter());

    using (var client = new HttpClient())
    {
        return await client.PostAsync(uri, content);
    }
}

. . .

var uri = new Uri("http://stackoverflow.com/documentation/c%23/1971/performing-http-requests");
await uri.PostAsync(new { foo = 123.45, bar = "Richard Feynman" });

string contents = "";
string url = "http://msdn.microsoft.com";

using (System.Net.WebClient client = new System.Net.WebClient())
{
    contents = client.DownloadString(url);
}

Console.WriteLine(contents);

public static async Task<TResult> GetAnsync<TResult>(this Uri uri)
{
    using (var client = new HttpClient())
    {
        var message = await client.GetAsync(uri);

        if (!message.IsSuccessStatusCode)
            throw new Exception();

        return message.ReadAsAsync<TResult>();
    }
}

. . .

public class Result
{
    public double foo { get; set; }

    public string bar { get; set; }
}

var uri = new Uri("http://stackoverflow.com/documentation/c%23/1971/performing-http-requests");
var result = await uri.GetAsync<Result>();