c# - 等待 2 个线程中的 1 个完成

Question

我的代码中有一个位置，我需要等待传感器识别手指，或者用户按下一个键来中止此操作并返回主菜单。
我尝试将条件变量与 Monitor 和锁定概念一起使用，但是当我尝试提醒主线程时，没有任何反应。

代码:

private static object _syncFinger = new object(); // used for syncing

private static bool AttemptIdentify()
{
    // waiting for either the user cancels or a finger is inserted
    lock (_syncFinger)
    {
        Thread tEscape = new Thread(new ThreadStart(HandleIdentifyEscape));
        Thread tIdentify = new Thread(new ThreadStart(HandleIdentify));
        tEscape.IsBackground = false;
        tIdentify.IsBackground = false;
        tEscape.Start();
        tIdentify.Start();
        Monitor.Wait(_syncFinger); // -> Wait part
    }

    // Checking the change in the locked object

    if (_syncFinger is FingerData) // checking for identity found
    {
        Console.WriteLine("Identity: {0}", ((FingerData)_syncFinger).Guid.ToString());
    }
    else if(!(_syncFinger is Char)) // char - pressed a key to return
    {
        return false; // returns with no error
    }

    return true;
}

private static void HandleIdentifyEscape()
{
    do
    {
        Console.Write("Enter 'c' to cancel: ");
    } while (Console.ReadKey().Key != ConsoleKey.C);
    _syncFinger = new Char();
    LockNotify((object)_syncFinger);
}

private static void HandleIdentify()
{
    WinBioIdentity temp = null;
    do
    {
        Console.WriteLine("Enter your finger.");
        try // trying to indentify
        {
            temp = Fingerprint.Identify(); // returns FingerData type
        }
        catch (Exception ex)
        {
            Console.WriteLine("ERROR: " + ex.Message);
        }
        // if couldn't identify, temp would stay null
        if(temp == null)
        {
            Console.Write("Invalid, ");
        }
    } while (temp == null);

    _syncFinger = temp;
    LockNotify(_syncFinger);
}

private static void LockNotify(object syncObject)
{
    lock(syncObject)
    {
        Monitor.Pulse(syncObject); 
    }
}

Answer 1

when i try to alert the main thread, nothing happens.

那是因为主线程正在等待这里创建的对象的监视器:

private static object _syncFinger = new object(); // used for syncing

但是您的每个线程都会替换该对象值，然后向监视器发送信号以获取新 对象。主线程不知道新对象，因此向监视器发出新对象的信号当然不会对主线程产生影响。

首先，任何时候你为了使用 with lock 而创建一个对象时，将其设为readonly:

private static readonly object _syncFinger = new object(); // used for syncing

这样做总是正确的，这将防止您在线程等待时犯下更改受监视对象的错误。

接下来，创建一个单独的字段来保存 WinBioIdentity 值，例如:

private static WinBioIdentity _syncIdentity;

然后使用 that 将结果中继回主线程:

private static bool AttemptIdentify()
{
    // waiting for either the user cancels or a finger is inserted
    lock (_syncFinger)
    {
        _syncIdentity = null;
        Thread tEscape = new Thread(new ThreadStart(HandleIdentifyEscape));
        Thread tIdentify = new Thread(new ThreadStart(HandleIdentify));
        tEscape.IsBackground = false;
        tIdentify.IsBackground = false;
        tEscape.Start();
        tIdentify.Start();
        Monitor.Wait(_syncFinger); // -> Wait part
    }

    // Checking the change in the locked object

    if (_syncIdentity != null) // checking for identity found
    {
        Console.WriteLine("Identity: {0}", ((FingerData)_syncIdentity).Guid.ToString());
        return true;
    }

    return false; // returns with no error
}

private static void HandleIdentifyEscape()
{
    do
    {
        Console.Write("Enter 'c' to cancel: ");
    } while (Console.ReadKey().Key != ConsoleKey.C);
    LockNotify((object)_syncFinger);
}

private static void HandleIdentify()
{
    WinBioIdentity temp = null;
    do
    {
        Console.WriteLine("Enter your finger.");
        try // trying to indentify
        {
            temp = Fingerprint.Identify(); // returns FingerData type
        }
        catch (Exception ex)
        {
            Console.WriteLine("ERROR: " + ex.Message);
        }
        // if couldn't identify, temp would stay null
        if(temp == null)
        {
            Console.Write("Invalid, ");
        }
    } while (temp == null);

    __syncIdentity = temp;
    LockNotify(_syncFinger);
}

总而言之，您应该更喜欢使用现代的 async/await 习惯用法:

private static bool AttemptIdentify()
{
    Task<WinBioIdentity> fingerTask = Task.Run(HandleIdentify);
    Task cancelTask = Task.Run(HandleIdentifyEscape);

    if (Task.WaitAny(fingerTask, cancelTask) == 0)
    {
        Console.WriteLine("Identity: {0}", fingerTask.Result.Guid);
        return true;
    }

    return false;
}

private static void HandleIdentifyEscape()
{
    do
    {
        Console.Write("Enter 'c' to cancel: ");
    } while (Console.ReadKey().Key != ConsoleKey.C);
}

private static WinBioIdentity HandleIdentify()
{
    WinBioIdentity temp = null;
    do
    {
        Console.WriteLine("Enter your finger.");
        try // trying to indentify
        {
            temp = Fingerprint.Identify(); // returns FingerData type
        }
        catch (Exception ex)
        {
            Console.WriteLine("ERROR: " + ex.Message);
        }
        // if couldn't identify, temp would stay null
        if(temp == null)
        {
            Console.Write("Invalid, ");
        }
    } while (temp == null);

    return temp;
}

以上是一个最低限度的例子。最好使 AttemptIdentify() 方法 async 本身，然后使用 await Task.WhenAny() 而不是 Task .WaitAny()。最好包含一些中断任务的机制，即一旦一个任务完成，您应该想要中断另一个任务，这样它就不会继续尝试它的工作。

但这些类型的问题并不是 async/await 版本独有的，不需要解决以改进您现在的代码。