using System;

using System.Net;

using System.Text;

using System.IO;

public class Program

{

    public static void Main()

    {

        dynamic dynamicString = "{ \"test\" : \"value\" }";

        string json = "{ \"test\" : \"value\" }";

        string test = new HttpWebRequestBuilder()

            .SetRequestType()

            //.SetBody(json) //uncomment this and it works

            .SetBody(dynamicString) //uncomment this and it breaks

            .Build()

            .ExtensionMethod();

        Console.WriteLine(test);

    }

​

}

​

public class HttpWebRequestBuilder

{

    private readonly HttpWebRequest _request;

    public HttpWebRequestBuilder() 

    {

        Uri uri = new Uri("http://www.google.com");

        _request = WebRequest.CreateHttp(uri);

    }

    public HttpWebRequestBuilder SetRequestType() 

    {

        _request.Method = "POST";

        _request.ContentType = "application/json";

        return this;

    }

    public HttpWebRequestBuilder SetBody(string json) 

    {

        byte[] bytes = Encoding.UTF8.GetBytes(json);

        _request.ContentLength = bytes.Length;

        using (Stream writer = _request.GetRequestStream())

        {

            writer.Write(bytes, 0, bytes.Length);

            writer.Flush();

        }

        return this;

    }

    public HttpWebRequest Build() 

    {

        return _request;

    }

}

​

public static class WebRequestExtensions 

{

    public static string ExtensionMethod(this HttpWebRequest webRequest) 

    {

        return "extension method worked";

    }

}