using System;

using System.Net;   

using System.Threading.Tasks;

using System.Text;

using System.Collections.Specialized;

​

public class Program

{

    public static void Main()

    {

        Task<byte[]> respuesta = SendMessage("http://microsoft.com",2,3);

        Console.WriteLine("Done!!");

    }

    public static Task<byte[]> SendMessage(string url, int intA, int intB)

    {

        var client = new WebClient();

        var data = string.Format("a={0}&b={1}",intA,intB);

        //return client.UploadStringTaskAsync(new Uri(url),data); //Compilation error (line 15, col 10): Cannot implicitly convert type 'System.Threading.Tasks.Task<string>' to 'System.Threading.Tasks.Task<byte[]>'

        //return client.UploadFileTaskAsync(new Uri(url),data); // OK

        //return client.UploadDataTaskAsync(new Uri(url),Encoding.UTF8.GetBytes(data)); // OK

        var nvc = new NameValueCollection() { {"a", intA.ToString() }, {"b",intB.ToString()}};

        return client.UploadValuesTaskAsync(new Uri(url),nvc);

    }

}

​