Drawing horizontal and vertical gridlines

To draw horizontal and vertical gridlines we essentially use the same method used for the vertical gridline or column border used in the previous section. After drawing the vertical lines, we use GetItemRect() to get the item height and then we draw the horizontal grid lines based on this height. One implication of using GetItemRect() is that it fails when the list does not have any items in it and no horizontal line is drawn. Here is the complete code of the overridden OnPaint() function. Also note that the next version of the list view control will support the LVS_EX_GRIDLINES style and should make our code redundant.

void CMyListCtrl::OnPaint() 
	// First let the control do its default drawing.
	const MSG *msg = GetCurrentMessage();
	DefWindowProc( msg->message, msg->wParam, msg->lParam );

	// Draw the lines only for LVS_REPORT mode
	if( (GetStyle() & LVS_TYPEMASK) == LVS_REPORT )
		// Get the number of columns
		CClientDC dc(this );
		CHeaderCtrl* pHeader = (CHeaderCtrl*)GetDlgItem(0);
		int nColumnCount = pHeader->GetItemCount();

		// The bottom of the header corresponds to the top of the line 
		RECT rect;
		pHeader->GetClientRect( &rect );
		int top = rect.bottom;

		// Now get the client rect so we know the line length and
		// when to stop
		GetClientRect( &rect );

		// The border of the column is offset by the horz scroll
		int borderx = 0 - GetScrollPos( SB_HORZ );
		for( int i = 0; i < nColumnCount; i++ )
			// Get the next border
			borderx += GetColumnWidth( i );

			// if next border is outside client area, break out
			if( borderx >= rect.right ) break;

			// Draw the line.
			dc.MoveTo( borderx-1, top);
			dc.LineTo( borderx-1, rect.bottom );

		// Draw the horizontal grid lines

		// First get the height
		if( !GetItemRect( 0, &rect, LVIR_BOUNDS ))

		int height = rect.bottom - rect.top;

		GetClientRect( &rect );
		int width = rect.right;

		for( i = 1; i <= GetCountPerPage(); i++ )
			dc.MoveTo( 0, top + height*i);
			dc.LineTo( width, top + height*i );
	// Do not call CListCtrl::OnPaint() for painting messages

The vertical grid line is actually drawn one pixel left of the column border. This aligns it better with the column header. It also introduces a bug. When you increase a column width, the column area below the last visible item is not updated, thus leaving traces of the previous line. There are two approaches you can take to resolve this. First, draw the line exactly on the column border (e.i. do not subtract 1 from borderx). The second approach is to handle the HDN_TRACK notification from the header control and invalidate the exposed area so that it gets redrawn.

BTW, Paul Gerhart has also implemented this using an owner-drawn CListCtrl. He has made the source code available with a sample app. You can find it at http://www.voicenet.com/~pgerhart/_shware.html

This article was originally published on August 6th, 1998
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