Wednesday, 29 June 2016

Wrapping Asynchronous Programming Model (APM) to Task-based Asynchronous Pattern (TAP)

Let's look at how we can wrap classic Begin and End methods used in APM to the newer Task-based Asynchronous Pattern (TAP). Many methods of older framework Versions of .NET support such APM methods and we want to wrap or adapt them to support TAP and async await. Example code:

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

namespace ApmToTap
{
    class Program
    {

        static void Main(string[] args)
        {
            DownloadDemo();

            Console.WriteLine();
            Console.ReadKey();
        }

        private static async void DownloadDemo()
        {
            WebRequest wr = WebRequest.Create("https://t.co/UrkiLgN1BC");
            try
            {
                var response = await wr.GetResponseFromAsync();
                using (Stream stream = response.GetResponseStream())
                {
                    StreamReader reader = new StreamReader(stream, Encoding.UTF8);
                    Console.WriteLine(reader.ReadToEnd());
                }
            }
            catch (Exception err)
            {
                Console.WriteLine(err.Message);
            }
        }
    }

    public static class WebRequestExtensions
    {

        public static Task<WebResponse> GetResponseFromAsync(this WebRequest request)
        {
            return Task<WebResponse>.Factory.FromAsync(request.BeginGetResponse,
                request.EndGetResponse, null);
        }

    }

}

We use the Task<T>Factory.FromAsync method and provide the delegates for the Begin and End methods used in APM. We then provide just null as the AsyncState parameter, as this is not needed. We then can await the Task we create here and get the functionality Task provides such as information of how the asynchronous operation went, exceptions and so on. And of course we can also get the result we usually retrieve in the End method using APM. So there you have it. To use TAP With APM methods, you can use the Task<T>FromAsync method.

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