using System;
using System.Collections.Generic;
using System.IO;
using System.Net;
using System.Text;
using System.Threading.Tasks;

public class AsyncHttpWebRequest
{
    private HttpWebRequest webRequest;

    public async Task<string> DoRequest(Uri uri, string method, string contentType, Dictionary<string, string> postParameters)
    {
        this.webRequest = (HttpWebRequest)HttpWebRequest.CreateHttp(uri);
        this.webRequest.Method = method;
        this.webRequest.ContentType = contentType;

        if (method == "POST")
        {
            await this.SendPostData(postParameters);
        }

        return await this.GetWebContent();
    }

    private async Task SendPostData(Dictionary<string, string> parameters)
    {
        await Task<Stream>.Factory.FromAsync(this.webRequest.BeginGetRequestStream, this.webRequest.EndGetRequestStream, this.webRequest)
            .ContinueWith(streamResult =>
            {
                using (Stream postStream = streamResult.Result)
                {
                    string postData = string.Empty;
                    int count = 0;

                    foreach (KeyValuePair<string, string> kvp in parameters)
                    {
                        postData += kvp.Key + "=" + kvp.Value;

                        if (++count < parameters.Count)
                        {
                            postData += "&";
                        }
                    }

                    byte[] byteArray = Encoding.UTF8.GetBytes(postData);

                    postStream.Write(byteArray, 0, byteArray.Length);
                }
            });
    }

    private async Task<string> GetWebContent()
    {
        return await Task<WebResponse>.Factory.FromAsync(this.webRequest.BeginGetResponse, this.webRequest.EndGetResponse, this.webRequest)
            .ContinueWith<string>(webResponseResult =>
            {
                string responseString = string.Empty;

                try
                {
                    using (HttpWebResponse response = (HttpWebResponse)webResponseResult.Result)
                    {
                        using (StreamReader streamRead = new StreamReader(response.GetResponseStream()))
                        {
                            responseString = streamRead.ReadToEnd();
                        }
                    }
                }
                catch (Exception e)
                {
                    responseString = null;
                }

                return responseString;
            });
    }
}