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1.原始的异步方法的调用

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我们来看个简单的例子，在这里演示调用 WebClient.DownloadStringAsync 方法(这个方法不是 TAP)，然后由 WebClient.DownloadStringCompleted 事件通知 UI 更新，这是大多数人都会用的方法。

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1. private void DownloadString(string address)
2. {
3. ????WebClient wc = new WebClient();
4. ????wc.DownloadStringCompleted += (sender, e) =>
5. ????{
6. ????????if (e.Cancelled)
7. ????????????this.textBox1.Text = "Cancel";
8. ????????else if (e.Error != null)
9. ????????????this.textBox1.Text = "Error";
10. ????????else
11. ????????????this.textBox1.Text = e.Result;
12. ????};
13. ?
14. ????wc.DownloadStringAsync(new Uri(address));
15. }

客户端调用

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1. private void button_DownloadString_Click(object sender, EventArgs e)
2. {
3. ????DownloadString("https://www.google.com.tw/");
4. }

这是一个很简单的例子，一旦若项目里有成千上万的通知事件跟 UI 绑在一起，维护起来会相当的痛苦。

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2.将 EAP 转换成 TAP步骤

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• 命名规则以 Async 为后缀
• 返回 Task 或是 Task<TResult>
• 调用 TaskCompletionSource 方法

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改变 Task 状态可调用以下三个方法：SetCanceled、SetException、SetResult

1. private Task<string> DownloadStringAsync(string address)
2. {
3. ????var tcs = new TaskCompletionSource<string>();
4. ????WebClient wc = new WebClient();
5. ????wc.DownloadStringCompleted += (sender, e) =>
6. ????{
7. ????????if (e.Cancelled)
8. ????????????tcs.SetCanceled();
9. ????????else if (e.Error != null)
10. ????????????tcs.SetException(e.Error);
11. ????????else
12. ????????????tcs.SetResult(e.Result);
13. ????};
14. ?
15. ????wc.DownloadStringAsync(new Uri(address));
16. ????return tcs.Task;
17. }

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客户端调用

1. private async void button_DownloadStringAsync_Click(object sender, EventArgs e)
2. {
3. ????var task = DownloadStringAsync("https://www.google.com.tw/");
4. ????await task;
5. ????if (task.IsCanceled)
6. ????{
7. ????????this.textBox1.Text = "Cancel";
8. ????}
9. ????else if (task.IsFaulted)
10. ????{
11. ????????this.textBox1.Text = "Error";
12. ????}
13. ????else if (task.IsCompleted)
14. ????{
15. ????????this.textBox1.Text = task.Result;
16. ????}
17. }

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转自：http://www.it165.net/pro/html/201308/6710.html

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3.微软的封装

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1. public static Task<byte[]> DownloadDataTask(this WebClient webClient, Uri address)
2. {
3. ????// Create the task to be returned
4. ????var tcs = new TaskCompletionSource<byte[]>(address);
5. ?
6. ????// Setup the callback event handler
7. ????DownloadDataCompletedEventHandler handler = null;
8. ????handler = (sender, e) => EAPCommon.HandleCompletion(tcs, e, () => e.Result, () => webClient.DownloadDataCompleted -= handler);
9. ????webClient.DownloadDataCompleted += handler;
10. ?
11. ????// Start the async work
12. ????try
13. ????{
14. ????????webClient.DownloadDataAsync(address, tcs);
15. ????}
16. ????catch(Exception exc)
17. ????{
18. ????????// If something goes wrong kicking off the async work,
19. ????????// unregister the callback and cancel the created task
20. ????????webClient.DownloadDataCompleted -= handler;
21. ????????tcs.TrySetException(exc);
22. ????}
23. ?
24. ????// Return the task that represents the async operation
25. ????return tcs.Task;
26. }

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1. internal class EAPCommon
2. {
3. ????internal static void HandleCompletion<T>(
4. ????????TaskCompletionSource<T> tcs, AsyncCompletedEventArgs e, Func<T> getResult, Action unregisterHandler)
5. ????{
6. ????????// Transfers the results from the AsyncCompletedEventArgs and getResult() to the
7. ????????// TaskCompletionSource, but only AsyncCompletedEventArg‘s UserState matches the TCS
8. ????????// (this check is important if the same WebClient is used for multiple, asynchronous
9. ????????// operations concurrently). Also unregisters the handler to avoid a leak.
10. ????????if (e.UserState == tcs)
11. ????????{
12. ????????????if (e.Cancelled) tcs.TrySetCanceled();
13. ????????????else if (e.Error != null) tcs.TrySetException(e.Error);
14. ????????????else tcs.TrySetResult(getResult());
15. ????????????unregisterHandler();
16. ????????}
17. ????}
18. }

利用TaskCompletionSource将EAP转换成TAP

原文：http://www.cnblogs.com/pengzhen/p/4831339.html