为什么在MouseUp事件发生后会发生MouseMove事件？

Question

In WindowsForms I just added event handlers as follows:

    private void Form1_MouseDown(object sender, MouseEventArgs e)
    {
        Debug.WriteLine($"=> Form1_MouseDown, Clicks: {e.Clicks}, Location: {e.Location}");
    }

    private void Form1_MouseUp(object sender, MouseEventArgs e)
    {
        Debug.WriteLine($"=> Form1_MouseUp, Clicks: {e.Clicks}, Location: {e.Location}");
    }

    private void Form1_MouseMove(object sender, MouseEventArgs e)
    {
        Debug.WriteLine($"=> Form1_MouseMove, Clicks: {e.Clicks}, Location: {e.Location}");
    }

And the output is:

=> Form1_MouseMove, Clicks: 0, Location: {X=17,Y=21}
=> Form1_MouseDown, Clicks: 1, Location: {X=17,Y=21}
=> Form1_MouseUp,   Clicks: 1, Location: {X=17,Y=21}
=> Form1_MouseMove, Clicks: 0, Location: {X=17,Y=21}

You can see that all events occurs in the same location, So my question is why is there a MouseMove event after MouseUp event?

Also I tried similar code in WPF and MouseMove event NOT occurred.

And I tried similar code in C++ and MouseMove event NOT occurred:

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    switch (message)
    {
     ...

    case WM_MOUSEMOVE:
        OutputDebugString(L"WM_MOUSEMOVE\n");
        break;

    case WM_LBUTTONDOWN:
        OutputDebugString(L"WM_LBUTTONDOWN\n");
        break;

    case WM_LBUTTONUP:
        OutputDebugString(L"WM_LBUTTONUP\n");
        break;

    default:
        return DefWindowProc(hWnd, message, wParam, lParam);
    }
    return 0;
}

Answer 1

If your mouse had previously been focused on a separate window, then clicking on a new window and shifting the focus of the mouse will generate a mouse move event (even if the mouse didn't move immediately before or after you clicked your mouse).

Here is a link to a similar StackOverflow response "Ghost" MouseMove Event

Answer 2

This is because the mouse capture by the MouseDown is released on MouseUp . And this extra MouseMove may be to ensure the cursor position. As a workaround you can do this

        Point LastLocation = Point.Empty;

        private void Form1_MouseDown(object sender, MouseEventArgs e)
        {
            Debug.WriteLine("=> Form1_MouseDown, Clicks: " + e.Location + ", Location: " + e.Location + "");
        }

        private void Form1_MouseUp(object sender, MouseEventArgs e)
        {
            Debug.WriteLine("=> Form1_MouseUp, Clicks: " + e.Location + ", Location: " + e.Location + "");

        }

        private void Form1_MouseMove(object sender, MouseEventArgs e)
        {
            if (LastLocation != e.Location)
            {
                LastLocation = e.Location;
                Debug.WriteLine("=> Form1_MouseMove, Clicks: " + e.Location + ", Location: " + e.Location + "");
            }
        }

Answer 3

This is the intended behavior and will also be trigger whenever app is being switched (Eg: Alt+Tab).

You should go with workaround as suggested by @VishnuBabu's workaround. And to ignore initial mousemove trigger, you can get the current position of cursor once window is loaded instead of setting the LastLocation to Empty.