GetCurrentCursorPosition.cs

[DllImport("user32.dll")]
        [return: MarshalAs(UnmanagedType.Bool)]
        static extern bool GetCursorPos(ref Win32Point pt);

[StructLayout(LayoutKind.Sequential)]
struct Win32Point
{
    public Int32 X;
    public Int32 Y;
};

public static Point GetCurrentCursorPosition(Visual relativeTo)
{
    Win32Point w32Mouse = new Win32Point();
    GetCursorPos(ref w32Mouse);
    if (relativeTo == null)
        return new Point(w32Mouse.X, w32Mouse.Y);
    else
        return relativeTo.PointFromScreen(new Point(w32Mouse.X, w32Mouse.Y));
}