非同步和等待(async和await)

   在.Net 4.5中，通過async和await兩個關鍵字，引入了一種新的基於任務的非同步編程模型（TAP）。在這種方式下，可以通過類似同步方式編寫非同步代碼，極大簡化了非同步編程模型。如下式一個簡單的實例：

static async void DownloadStringAsync2(Uri uri)
    {
        var webClient = new WebClient();
        var result = await webClient.DownloadStringTaskAsync(uri);
        Console.WriteLine(result);
    }

　　而之前的方式是這樣的：

static void DownloadStringAsync(Uri uri)
    {
        var webClient = new WebClient();
        webClient.DownloadStringCompleted += (s, e) =>
            {
                Console.WriteLine(e.Result);
            };
        webClient.DownloadStringAsync(uri);
    }

也許前面這個例子不足以體現async和await帶來的優越性，下麵這個例子就明顯多了：

public void CopyToAsyncTheHardWay(Stream source, Stream destination)
    {
        byte[] buffer = new byte[0x1000];
        Action<IAsyncResult> readWriteLoop = null;
        readWriteLoop = iar =>
        {
            for (bool isRead = (iar == null); ; isRead = !isRead)
            {
                switch (isRead)
                {
                    case true:
                        iar = source.BeginRead(buffer, 0, buffer.Length,
                            readResult =>
                            {
                                if (readResult.CompletedSynchronously) return;
                                readWriteLoop(readResult);
                            }, null);
                        if (!iar.CompletedSynchronously) return;
                        break;
                    case false:
                        int numRead = source.EndRead(iar);
                        if (numRead == 0)
                        {
                            return;
                        }
                        iar = destination.BeginWrite(buffer, 0, numRead,
                            writeResult =>
                            {
                                if (writeResult.CompletedSynchronously) return;
                                destination.EndWrite(writeResult);
                                readWriteLoop(null);
                            }, null);
                        if (!iar.CompletedSynchronously) return;
                        destination.EndWrite(iar);
                        break;
                }
            }
        };
        readWriteLoop(null);
    }

    public async Task CopyToAsync(Stream source, Stream destination)
    {
        byte[] buffer = new byte[0x1000];
        int numRead;
        while ((numRead = await source.ReadAsync(buffer, 0, buffer.Length)) != 0)
        {
            await destination.WriteAsync(buffer, 0, numRead);
        }
    }

可參看微軟的官方網站：Visual Studio Asynchronous Programming，其官方文檔Task-Based Asynchronous Pattern Overview介紹的非常詳細， VisualStudio中自帶的CSharp Language Specification中也有一些說明。

源自:http://blog.csdn.net/wulex/article/details/73499500