How to create an async method in C# 4 according to

问题:

Consider the following code snippet:

public static Task<string> FetchAsync()
{
    string url = "http://www.example.com", message = "Hello World!";

    var request = (HttpWebRequest)WebRequest.Create(url);
    request.Method = WebRequestMethods.Http.Post;

    return Task.Factory.FromAsync<Stream>(request.BeginGetRequestStream, request.EndGetRequestStream, null)
        .ContinueWith(t =>
        {
            var stream = t.Result;
            var data = Encoding.ASCII.GetBytes(message);
            Task.Factory.FromAsync(stream.BeginWrite, stream.EndWrite, data, 0, data.Length, null, TaskCreationOptions.AttachedToParent)
                .ContinueWith(t2 => { stream.Close(); });
        })
        .ContinueWith<string>(t =>
        {
            var t1 =
                Task.Factory.FromAsync<WebResponse>(request.BeginGetResponse, request.EndGetResponse, null)
                .ContinueWith<string>(t2 =>
                {
                    var response = (HttpWebResponse)t2.Result;
                    var stream = response.GetResponseStream();
                    var buffer = new byte[response.ContentLength > 0 ? response.ContentLength : 0x100000];
                    var t3 = Task<int>.Factory.FromAsync(stream.BeginRead, stream.EndRead, buffer, 0, buffer.Length, null, TaskCreationOptions.AttachedToParent)
                        .ContinueWith<string>(t4 =>
                        {
                            stream.Close();
                            response.Close();
                            if (t4.Result < buffer.Length)
                            {
                                Array.Resize(ref buffer, t4.Result);
                            }
                            return Encoding.ASCII.GetString(buffer);
                        });
                    t3.Wait();
                    return t3.Result;
                });
            t1.Wait();
            return t1.Result;
        });
}

It should return Task<string>, send HTTP POST request with some data, return a result from webserver in a form of string and be as much efficient as possible.

• Did you spot any problems regarding async flow in the example above?
• Is it OK to have .Wait() inside .ContinueWith() in this example
• Do you see any other problems with this peace of code (keeping aside exception handling for now)?

回答1:

If async related C# 4.0 code is huge and ugly - there is a chance that it's implemented properly. If it's nice and short, then most likely it's not ;)

..though, you may get it look more attractive by creating extension methods on WebRequest, Stream classes and cleanup the main method.

P.S.: I hope C# 5.0 with it's new async keyword and library will be released soon.

Reference: http://msdn.microsoft.com/en-us/vstudio/async.aspx

回答2:

You're correct in thinking that the Waits are unnecessary - Result will block until a result is ready.

---

However, an even easier way would be to base it off use the examples provided in the ParallelExtensionsExtras library.

They have made extensions for WebClient which do exactly what you're looking for:

static Task<string> FetchAsync()
{
    string url = "http://www.example.com", message = "Hello World!";

    return new WebClient().UploadStringTask(url, "POST", message);
}

You can read more about it in this post on the Parallel Programming with .NET blog.