c# - 如何正确取消订阅事件

Question

我如何才能正确地取消订阅事件并确保现在不调用被调用的方法？

我的问题是这种代码:

public class MyClassWithEvent
{
    public event EventHandler MyEvent;
    public int Field;
}

public class MyMainClass
{
    private MyClassWithEvent myClass;

    public void Start()
    {
        myClass.MyEvent += new EventHandler(doSomething);
    }

    public void Stop()
    {
        myClass.MyEvent -= new EventHandler(doSomething);
        myClass = null;
    }

    private void doSomething()
    {
        myClass.Field = 42;
    }
}

如果在执行 doSomething 时调用 myClass = null，指令 myClass.Field = 42 会引发错误，因为 myClass 为 null。

如何确定 doSomething 在设置 myClass = null 之前没有执行？

编辑:

其他例子:

public void Stop()
{
    myClass.MyEvent -= new EventHandler(doSomething);

    // Can I add a function here to be sure that doSomething is not running ?

    myClass.Field = 101;
}

在那种情况下，我不确定 myClass.Field 是 42 还是 101。

编辑2:

显然我的问题并不像我想的那么简单。我将解释我的确切案例。

我的代码是:

public class MyMainClass
{
    object camera;//can be type uEye.Camera or DirectShowCamera
    bool isRunning = false;

    public void Start()
    {
        if (camera is uEye.Camera)
        {
            camera.EventFrame += new EventHandler(NewFrameArrived);
        }
        else if (camera is DirectShowCamera)
        {
            //other init
        }
        isRunning = true;
    }

    public void Stop()
    {
        if (camera is uEye.Camera)
        {
            camera.EventFrame -= new EventHandler(NewFrameArrived);
            camera.exit;
        }
        else if (camera is DirectShowCamera)
        {
            //other stop
        }
        isRunning = false;
    }

    public void ChangeCamera(object new camera)
    {
        if (isRunning)
            Stop()
        camera = new camera();
    }

    void NewFrameArrived(object sender, EventArgs e)
    {
        uEye.Camera Camera = sender as uEye.Camera;
        Int32 s32MemID;
        Camera.Memory.GetActive(out s32MemID);

        lock (_frameCameralocker)
        {
            if (_frameCamera != null)
                _frameCamera.Dispose();
            _frameCamera = null;
            Camera.Memory.ToBitmap(s32MemID, out _frameCamera);
        }

        Dispatcher.Invoke(new Action(() =>
        {
            lock (_frameCameralocker)
            {
                var bitmapData = _frameCamera.LockBits(
                    new System.Drawing.Rectangle(0, 0, _frameCamera.Width, _frameCamera.Height),
                    System.Drawing.Imaging.ImageLockMode.ReadOnly, _frameCamera.PixelFormat);

                if (_frameCamera.PixelFormat == System.Drawing.Imaging.PixelFormat.Format8bppIndexed)
                {
                    DeviceSource = System.Windows.Media.Imaging.BitmapSource.Create(
                                                        bitmapData.Width, bitmapData.Height, 96, 96, System.Windows.Media.PixelFormats.Gray8, null,
                                                        bitmapData.Scan0, bitmapData.Stride * bitmapData.Height, bitmapData.Stride);
                }

                _frameCamera.UnlockBits(bitmapData);

                if (OnNewBitmapReady != null)
                    OnNewBitmapReady(this, null);
            }
        }));
    }
}

当我将相机从 uEye 更改为 directshow 时，我在 DeviceSource = System.Windows.Media.Imaging.BitmapSource.Create(方法 NewFrameArrived)中遇到 AccessViolationException因为我尝试从退出的相机创建 BitmapSource

Answer 1

根据您更新的问题，您唯一需要做的就是将 Stop() 操作锁定为与 Dispatcher.Invoke

相同的锁

public void Stop()
{
    lock(_frameCameralocker)
    {
        if (camera is uEye.Camera)
        {
            camera.EventFrame -= new EventHandler(NewFrameArrived);
            camera.exit;
        }
        else if (camera is DirectShowCamera)
        {
            //other stop
        }
        isRunning = false;
    }
}

这将确保所有 NewFrameArrived 调用在您创建新相机之前完成或尚未开始。然后在调度程序内部检查您是否正在运行，以防帧在 Stop() 调用开始和完成之前排队。

    Dispatcher.Invoke(new Action(() =>
    {
        lock (_frameCameralocker)
        {
            if(!isRunning)
                return;

            var bitmapData = _frameCamera.LockBits(
                new System.Drawing.Rectangle(0, 0, _frameCamera.Width, _frameCamera.Height),
                System.Drawing.Imaging.ImageLockMode.ReadOnly, _frameCamera.PixelFormat);

            if (_frameCamera.PixelFormat == System.Drawing.Imaging.PixelFormat.Format8bppIndexed)
            {
                DeviceSource = System.Windows.Media.Imaging.BitmapSource.Create(
                                                    bitmapData.Width, bitmapData.Height, 96, 96, System.Windows.Media.PixelFormats.Gray8, null,
                                                    bitmapData.Scan0, bitmapData.Stride * bitmapData.Height, bitmapData.Stride);
            }

            _frameCamera.UnlockBits(bitmapData);

            if (OnNewBitmapReady != null)
                OnNewBitmapReady(this, null);
        }
    }));