Question

How can I efficiently tell if the mouse is over a top-level window?

By “over”, I mean that the mouse pointer is within the client rectangle of the top-level window and there is no other top-level window over my window at the location of the mouse pointer. In other words, if the user clicked the event would be sent to my top-level window (or one of its child windows).

I am writing in C# using Windows Forms, but I don’t mind using p/invoke to make Win32 calls.

Answer 1

You could use the WinAPI function WindowFromPoint. Its C# signature is:

[DllImport("user32.dll")]
static extern IntPtr WindowFromPoint(POINT Point);

Note that POINT here is not the same as System.Drawing.Point, but PInvoke provides a declaration for POINT that includes an implicit conversion between the two.

If you don’t already know the mouse cursor position, GetCursorPos finds it:

[DllImport("user32.dll")]
static extern bool GetCursorPos(out POINT lpPoint);

However, the WinAPI calls lots of things “windows”: controls inside a window are also “windows”. Therefore, you might not get a top-level window in the intuitive sense (you might get a radio button, panel, or something else). You could iteratively apply the GetParent function to walk up the GUI hierarchy:

[DllImport("user32.dll", ExactSpelling=true, CharSet=CharSet.Auto)]
public static extern IntPtr GetParent(IntPtr hWnd);

Once you find a window with no parent, that window will be a top-level window. Since the point you originally passed in belongs to a control that is not covered by another window, the top-level window is necessarily the one the point belongs to.