HOW TO：註冊取消要求的回呼

下列範例示範如何註冊委派，該委派會於 IsCancellationRequested 屬性因為在已建立語彙基元的物件上呼叫 Cancel 而變為 true 時叫用。 請使用這種技巧取消原本並不支援統一取消架構的非同步作業，並且對可能會等候非同步作業完成的方法解除封鎖。

注意事項
啟用 "Just My Code" 時，Visual Studio 有時候會在擲回例外狀況的程式碼行處中斷，並顯示「使用者程式碼未處理的例外狀況」之類的錯誤訊息。這是良性的錯誤。您可以按 F5 從中斷的地方繼續，並看到下面範例所示的例外處理行為。若要防止 Visual Studio 在遇到第一個錯誤時就中斷，只要取消選取 [工具]、[選項]、[偵錯]、[一般] 下的 [Just My Code] 核取方塊即可。

範例

在下列範例中，CancelAsync 方法會註冊為透過取消語彙基元提出取消要求時要叫用的方法。

Class CancelWithCallback


    Shared Sub Main()

        Dim cts As New CancellationTokenSource()

        ' Start cancelable task.
        Dim t As Task = Task.Factory.StartNew(Sub() DoWork(cts.Token))

        Console.WriteLine("Press 'c' to cancel.")
        Dim ch As Char = Console.ReadKey().KeyChar
        If ch = "c"c Then

            cts.Cancel()
        End If
        Console.WriteLine("Press any key to exit.")
        Console.ReadKey()
    End Sub

    Shared Sub DoWork(ByVal token As CancellationToken)

        Dim wc As New WebClient()

        ' Create an event handler to receive the result.
        AddHandler wc.DownloadStringCompleted, Sub(obj, e)

                                                   ' Checks status of WebClient, not external token
                                                   If e.Cancelled = False Then

                                                       Console.WriteLine(e.Result + "\r\nPress any key.")

                                                   Else
                                                       Console.WriteLine("Download was canceled.")
                                                   End If
                                               End Sub

        token.Register(Sub() wc.CancelAsync())

        Console.WriteLine("Starting request")
        wc.DownloadStringAsync(New Uri("https://www.contoso.com"))
    End Sub
End Class

namespace Cancel3
{
    using System;
    using System.Net;
    using System.Threading;
    using System.Threading.Tasks;

    class CancelWithCallback
    {

        static void Main(string[] args)
        {
            var cts = new CancellationTokenSource();

            // Start cancelable task.
            Task t = Task.Factory.StartNew(() =>
            {
                DoWork(cts.Token);
            });

            Console.WriteLine("Press 'c' to cancel.");
            char ch = Console.ReadKey().KeyChar;
            if (ch == 'c')
            {
                cts.Cancel();
            }
            Console.WriteLine("Press any key to exit.");
            Console.ReadKey();
        }

        static void DoWork(CancellationToken token)
        {
            WebClient wc = new WebClient();

            // Create an event handler to receive the result.
            wc.DownloadStringCompleted += (obj, e) =>
            {
                // Checks status of WebClient, not external token
                if (!e.Cancelled)
                {
                    Console.WriteLine(e.Result + "\r\nPress any key.");
                }
                else
                    Console.WriteLine("Download was canceled.");
            };

            // Do not initiate download if the external token
            // has already been canceled.
            if (!token.IsCancellationRequested)
            {
                // Register the callback to a method that can unblock.
                // Dispose of the CancellationTokenRegistration object
                // after the callback has completed.
                using (CancellationTokenRegistration ctr = token.Register(() => wc.CancelAsync()))
                {
                    Console.WriteLine("Starting request");
                    wc.DownloadStringAsync(new Uri("https://www.contoso.com"));
                }
            }
        }
    }
}

如果取消已經於回呼註冊時要求提出取消要求，仍然保證會呼叫回呼。 在此特別的案例中，如果沒有非同步作業正在執行中，則 CancelAsync 方法不會執行任何動作，因此呼叫方法並不會有問題。

請參閱

概念

取消