Fixed aspect ratio in C# window

I needed to create a window with a fixed aspect ratio (for video), and I wanted that the resize frame will change with the user cursor drag.

I came around with this post. Which is almost perfect, but when you drag the left or upper bounds you get the window moving... Here is the fixed code:

#region Keep video window aspect ratio
[StructLayout(LayoutKind.Sequential)]
public struct RECT
{
    public int Left;
    public int Top;
    public int Right;
    public int Bottom;
}

public class MyRect
{
    public int Left;
    public int Top;
    public int Right;
    public int Bottom;
}

public const int WM_SIZING = 0x0214;
int _lastWidth = 0;
MyRect _lastRect = null;
double _aspectRatio = 0.5;

protected override void WndProc(ref Message m)
{
    if (m.Msg == WM_SIZING)
    {
        RECT rc;
        rc = (RECT)Marshal.PtrToStructure(m.LParam, typeof(RECT));

        if (_lastRect == null)
        {
            _lastRect = new MyRect();
            _lastRect.Top = rc.Top;
            _lastRect.Bottom = rc.Bottom;
            _lastRect.Left = rc.Left ;
            _lastRect.Right = rc.Right;
        }

        if (rc.Right - rc.Left != _lastWidth)
        {
            double newHeight = ((double)(rc.Right - rc.Left)) 
                                   / _aspectRatio;
            rc.Bottom = (int)newHeight + _lastRect.Top;
            Marshal.StructureToPtr(rc, m.LParam, true);
        }
        else
        {
            double newWidth = ((double)(rc.Bottom - rc.Top)) 
                                  * _aspectRatio;

            rc.Right = (int)newWidth + _lastRect.Left;
            Marshal.StructureToPtr(rc, m.LParam, true);
        }

        _lastWidth = rc.Right - rc.Left;
        _lastRect.Top = rc.Top;
        _lastRect.Bottom = rc.Bottom;
        _lastRect.Left = rc.Left;
        _lastRect.Right = rc.Right;
    }

    base.WndProc(ref m);
}
#endregion