Question

I want to draw a margin line at 80 characters in a WinForms TextBox. Here is what I’ve tried, in my TextBox subclass:

protected override void WndProc(ref Message m)
{
    base.WndProc(ref m);

    const int WM_PAINT = 0x00F;
    if (m.Msg == WM_PAINT) {
        DrawMargin();
    }
}

void DrawMargin()
{
    using (Pen pen = new Pen(Color.Gray, 1)) {
        using (Graphics graphics = CreateGraphics()) {
            float charWidth = graphics.MeasureString("M", Font).Width;
            graphics.DrawLine(pen, charWidth * 80, 0, charWidth * 80, Height);
        }
    }
}

There are at least three problems with this:

1. When the user enters some text, part of the line gets blanked out (goes white).
2. When the user selects some text with the mouse, the above happens again.
3. The line flickers when the TextBox is scrolled.

I notice that TED Notepad (which uses a Win32 EDIT control) is able to draw a margin without any problems, so it seems that it’s possible to do what I want. Could anyone advise me how?

Answer 1

As far as I can tell, the best way of doing this is simply to place a WinForms.Panel over the TextBox:

class FooTextBox : TextBox
{
    public FooTextBox()
    {
        margin = new Panel();

        margin.Enabled   = false;
        margin.BackColor = Color.LightGray;
        margin.Top       = 0;
        margin.Height    = ClientSize.Height;
        margin.Left      = <whatever>;
        margin.Width     = 1;

        Controls.Add(margin);
    }

    Panel margin;
}

Since the panel is not Enabled, it doesn’t take mouse input.