    EventHandler externalEvent;
    void Example1()
    {
        CancellationTokenSource cts = new CancellationTokenSource();
        externalEvent +=
         (sender, obj) => { cts.Cancel(); }; //wire up an external requester 
        try
        {
            int val = LongRunningFunc(cts.Token);
        }
        catch (OperationCanceledException)
        {
            //cleanup after cancellation if required... 
        }
    }

    private static int LongRunningFunc(CancellationToken token)
    {
        int total = 0;
        for (int i = 0; i < 1000; i++)
        {
            for (int j = 0; j < 1000; j++)
            {
                total++;
            }
            if (token.IsCancellationRequested)
            { // observe cancellation 
                throw new OperationCanceledException(token); // acknowledge cancellation 
            }
        }
        return total;
    }

異步模式下的取消

另外一種常見的方式是在一些異步操作中，往往不能主動釋放，只能等待異步操作回調(diào)的時候才能操作結(jié)果。此時一般取消方法如下：

任務(wù)線程注冊異步操作完成的回調(diào)函數(shù)，開始異步操作。
UI線程接受取消指令，置取消標(biāo)記位，并主動執(zhí)行回調(diào)函數(shù)
回調(diào)函數(shù)中通過取消標(biāo)記位判斷該任務(wù)是已經(jīng)完成還是被取消的，并執(zhí)行相關(guān)析構(gòu)操作。

使用方式如下：

    void BlockingOperation(CancellationToken token)
    {
        ManualResetEvent mre = new ManualResetEvent(false);
        //register a callback that will set the MRE 
        CancellationTokenRegistration registration =
         token.Register(() => mre.Set());
        using (registration)
        {
            mre.WaitOne();
            if (token.IsCancellationRequested) //did cancellation wake us? 
                throw new OperationCanceledException(token);
        } //dispose the registration, which performs the deregisteration. 
    }

這里我們通過CancellationToken注冊了一個回調(diào)方法以通知任務(wù)等待線程，也可以以我們經(jīng)常使用的WaitHandle的那樣的方式使用。

    void Wait(WaitHandle wh, CancellationToken token)
    {
        WaitHandle.WaitAny(new[] { wh, token.WaitHandle });
        if (token.IsCancellationRequested) //did cancellation wake us? 
            throw new OperationCanceledException(token);
    }

高級應(yīng)用

由于例子比較簡單，這里就只列舉一下代碼，不多介紹了。

一個CancellationToken對應(yīng)多個任務(wù)

    void Example4()
    {
        CancellationTokenSource cts = new CancellationTokenSource();
        Func1(cts.Token);
        Func2(cts.Token);
        Func3(cts.Token);
        //... 
        cts.Cancel(); // all listeners see the same cancellation request. 
    }

一個任務(wù)對應(yīng)多個CancellationToken

    void LinkingExample(CancellationToken ct1, CancellationToken ct2)
    {
        CancellationTokenSource linkedCTS =
        CancellationTokenSource.CreateLinkedTokenSource(ct1, ct2);
        try
        {
            SlowFunc(linkedCTS.Token);
        }
        catch (OperationCanceledException oce)
        {
            if (ct1.IsCancellationRequested)
            {
                // ... 
            }
            else if (ct2.IsCancellationRequested)
            {
                // ... 
            }
        }
        linkedCTS.Dispose(); // clean up the linking. required. 
    }

最后我們再來一個并發(fā)查詢時取消的例子：

    private void RunQuery()
    {
        int[] data = { 1, 2, 3 };
        CancellationTokenSource cts = new CancellationTokenSource();
        var query = data.AsParallel()
                     .WithCancellation(cts.Token) // token given to library code 
                     .Select((x) => SlowFunc(x, cts.Token)); // token passed to user code 
    }

    private int SlowFunc(int x, CancellationToken token) 
    { 
        int result 
        while(...) 
        { 
            if (token.IsCancellationRequested) 
            throw new OperationCanceledException(token); 
            ... 
        } 
        return result; 
    }