将async / await转换为Task.ContinueWith

这个问题是由对这个问题的评论引发的：

如何在没有Microsoft.Bcl.Async情况下将非线性async/await代码反向移植到.NET 4.0？

在链接的问题中，我们有一个WebRequest操作，如果它一直失败，我们想要重试有限次数。 Async/await代码可能如下所示：

 async Task GetResponseWithRetryAsync(string url, int retries) { if (retries < 0) throw new ArgumentOutOfRangeException(); var request = WebRequest.Create(url); while (true) { WebResponse task = null; try { task = request.GetResponseAsync(); return (HttpWebResponse)await task; } catch (Exception ex) { if (task.IsCanceled) throw; if (--retries == 0) throw; // rethrow last error // otherwise, log the error and retry Debug.Print("Retrying after error: " + ex.Message); } } } 

从第一个想法，我使用TaskCompletionSource ，就像这样（未经测试）：

 Task GetResponseWithRetryAsync(string url, int retries) { if (retries < 0) throw new ArgumentOutOfRangeException(); var request = WebRequest.Create(url); var tcs = new TaskCompletionSource(); Action<Task> proceesToNextStep = null; Action doStep = () => request.GetResponseAsync().ContinueWith(proceedToNextStep); proceedToNextStep = (prevTask) => { if (prevTask.IsCanceled) tcs.SetCanceled(); else if (!prevTask.IsFaulted) tcs.SetResult((HttpWebResponse)prevTask.Result); else if (--retries == 0) tcs.SetException(prevTask.Exception); else doStep(); }; doStep(); return tcs.Task; } 

问题是， 如何在没有TaskCompletionSource情况下执行此TaskCompletionSource ？

 static public Task GetResponseTapAsync(this WebRequest request) { return Task.Factory.FromAsync( (asyncCallback, state) => request.BeginGetResponse(asyncCallback, state), (asyncResult) => request.EndGetResponse(asyncResult), null); } 

 static Task GetResponseWithRetryAsync(string url, int retries) { if (retries < 0) throw new ArgumentOutOfRangeException(); var request = WebRequest.Create(url); Func, Task> proceedToNextStep = null; Func> doStep = () => { return request.GetResponseTapAsync().ContinueWith(proceedToNextStep).Unwrap(); }; proceedToNextStep = (prevTask) => { if (prevTask.IsCanceled) throw new TaskCanceledException(); if (prevTask.IsFaulted && --retries > 0) return doStep(); // throw if failed or return the result return Task.FromResult((HttpWebResponse)prevTask.Result); }; return doStep(); }