Custom borders

Mathew Hall

Hi there. I'm working on a custom control where each side can have a different border thickness. I found this code while googling which allows me to modify the controls non-client area so I can draw my borders:

[StructLayout(LayoutKind.Sequential)]
private struct RECT
{
public int Left;
public int Top;
public int Right;
public int Bottom;
}

enum WM_MESSAGE
{
WM_NCCALCSIZE = 131,
WM_NCPAINT = 133,
WM_NCHITTEST = 132,
WM_NCLBUTTONDOWN = 161,
}

[StructLayout(LayoutKind.Sequential)]
struct NCCALCSIZE_PARAMS
{
public RECT rgrc0, rgrc1, rgrc2;
public IntPtr lppos;
}

[DllImport("User32.dll")]
private extern static IntPtr GetWindowDC( IntPtr hWnd );

[DllImport("User32.dll")]
private extern static int ReleaseDC( IntPtr hWnd, IntPtr hDC );

private Rectangle _rcButton = Rectangle.Empty;

protected override void WndProc(ref Message m)
{
switch (m.Msg)
{
case (int)WM_MESSAGE.WM_NCCALCSIZE :
if (m.WParam == IntPtr.Zero)
{
// shrink the client area
NCCALCSIZE_PARAMS csp;
csp = (NCCALCSIZE_PARAMS)Marshal.PtrToStructure(m.LParam, typeof(NCCALCSIZE_PARAMS));
csp.rgrc0.Left += 1;
csp.rgrc0.Top += 5;
csp.rgrc0.Right -= 1;
csp.rgrc0.Bottom -= 1;
Marshal.StructureToPtr( csp, m.LParam, false );
}
break;

  case (int)WM\_MESSAGE.WM\_NCPAINT :
  {
     // draw the borders in the non client area
     IntPtr hDC = GetWindowDC(m.HWnd);
     Graphics g = Graphics.FromHdc(hDC);
     g.FillRectangle(SystemBrushes.Highlight, 0, 0, this.Width, 5); // top
     g.FillRectangle(SystemBrushes.Highlight, 0, 0, 1, this.Height); // left
     g.FillRectangle(SystemBrushes.Highlight, this.Width-1, 0, 1, this.Height); // right
     g.FillRectangle(SystemBrushes.Highlight, 0, this.Height-1, this.Width, 1); // bottom
     ReleaseDC( m.HWnd, hDC );
     break;
  }

}

base.WndProc(ref m);
}

This works fine until the control is resized - it's client area gets reset. Any ideas on how to fix this? "I think I speak on behalf of everyone here when I say huh?" - Buffy

John Fisher

My guess is that the reason it works until the control is resized is that the client area was never successfully change the the size you are wanting. I took a look at the WM_NCCALCSIZE documentation here[^]. It says that if WParam is FALSE (zero), that a RECT is passed in LParam, not an NCCALCSIZE_PARAMS structure. On the other hand, if WParam is TRUE (non-zero), then you are supposed to get an NCCALCSIZE_PARAMS in LParam. So, it looks like the WM_NCCALCSIZE handling code is never doing anything. I suspect that if you put a breakpoint on the first line of the WM_NCCALCSIZE handling code, then stepped through it, that it would throw an exception at

csp.rgrc0.Left += 1;

John
"You said a whole sentence with no words in it, and I understood you!" -- my wife as she cries about slowly becoming a geek.

Mathew Hall

Thanks for the tip. I managed to get it to "remember" the resized non-client area by changing the offending case statememt to:

case (int)WM_MESSAGE.WM_NCCALCSIZE :
if ( m.WParam == IntPtr.Zero )
{
RECT rect = (RECT) Marshal.PtrToStructure(m.LParam, typeof(RECT));
rect.Left += 1;
rect.Top += 5;
rect.Right -= 1;
rect.Bottom -= 1;
Marshal.StructureToPtr(rect, m.LParam, false);
}
else
{
NCCALCSIZE_PARAMS csp;
csp = (NCCALCSIZE_PARAMS)Marshal.PtrToStructure(m.LParam, typeof(NCCALCSIZE_PARAMS));
csp.rgrc0.Left += 1;
csp.rgrc0.Top += 5;
csp.rgrc0.Right -= 1;
csp.rgrc0.Bottom -= 1;
Marshal.StructureToPtr(csp, m.LParam, false);
}

Only problem was that sometimes the right and bottom border widths were out by 1. Managed to fix this by overriding OnSizeChanged:

[DllImport("User32.dll")]
public static extern int RedrawWindow(IntPtr hwnd, IntPtr lprcUpdate, IntPtr hrgnUpdate, int fuRedraw);

protected override void OnSizeChanged(EventArgs e)
{
base.OnSizeChanged(e);
// force redraw of the entire control (including non-client area)
RedrawWindow(this.Handle, IntPtr.Zero, IntPtr.Zero, RDW_FRAME | RDW_UPDATENOW | RDW_INVALIDATE);
}

Works like a charm :) "I think I speak on behalf of everyone here when I say huh?" - Buffy

John Fisher

Great! John
"You said a whole sentence with no words in it, and I understood you!" -- my wife as she cries about slowly becoming a geek.