Custom form designer, move/resize controls using WinAPI

Question

I have to fix some problems and enchance form designer written long ago for a database project. In Design Panel class code I encountered these lines

private void DesignPanel_MouseDown(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Left)
    {
     (sender as Control).Capture = false;
     switch (FMousePosition)
     {
     case MousePosition.mpNone: 
      SendMessage((sender as Control).Handle, WM_SYSCOMMAND, 0xF009, 0);
      break;// Move
     case MousePosition.mpRightBottom: 
      SendMessage((sender as Control).Handle, WM_SYSCOMMAND, 0xF008, 0);
      break;//RB
     case MousePosition.mpLeftBottom: 
      SendMessage((sender as Control).Handle, WM_SYSCOMMAND, 0xF007, 0); 
            // ... here are similar cases ...
     case MousePosition.mpLeft:
      SendMessage((sender as Control).Handle, WM_SYSCOMMAND, 0xF001, 0);
      break;//L  
     }
    }
}

FMousePosition indicates whether mouse was over any edge of selected control.

What confusing me is these windows messages: it seems there is no documentation on WM_SYSCOMMAND with parameters 0xF001-0xF009 (maybe it starts some kind of 'drag/resize sequence'). Any ideas?

If my suggestion is right, then how can I cancel these sequences?

Answer 1

See APIViewer and check out constants starting with SC_

Answer 2

They are undocumented parameters. After searching I managed to find this list.

• 0xF000 (Centre cursor on the form)
• 0xF001 (Resize from left)
• 0xF002 (Resize from right)
• 0xF003 (Resize from up)
• 0xF004 (Lock the bottom right corner of the form, the up left corner move for resize)
• 0xF005 (Same from bottom left corner)
• 0xF006 (Lock up right and left border, resize other)
• 0xF007 (Lock up and right border, resize other border)
• 0xF008 (Lock left and up border and resize other)
• 0xF009 (Drag from anywhere)
• 0xF010 (Put cursor centred at the upper order)
• 0xF020 (Auto-Minimize Form)
• 0xF030 (Auto-Maximize Form)

Reference: http://www.delphi3000.com/articles/article_1054.asp#Comments

Answer 3

Based on my Win32 Programming (Rector and Newcomer) p902-903 explains WM_SYSCOMMAND is sent when the user selects an item from the system menu (rather than sending the normal WM_COMMAND).

The MSDN help says SC_SIZE = 0xF000 and it and Win32 Programming also say Windows uses the four low-order bits of the predefined system menu IDs internally but doesn't go on to clarify their use. Thanks stukelly for clarifying them.