A simple line in windows form

Handle Control.Paint event (for a control or a form) or override Control.OnPaint. In both cases, use System.Drawing.Graphics instance obtained from even argument. Using this instance, use System.Drawing.Graphics.Draw... methods to render whatever you want.

[EDIT]

After clarification by OP:

My method will work for this. You can use custom Control and override OnPaint or, more simply, use just Panel and Paint event. When you change the key combination, don't forget: you need to call Control.Invalidate. That's it. It will do the trick.

Alternative suggestion: may be you graphical element is too simple. Try to find some good character in Unicode (use standard "Character Map" application to see what's available) and use just a read-only TextBox and just text in it. For a vertical line, you can use code point '|': 0x007C :-) or '∣' 0x2223 (Mathematical Operations — "Divides" or even '│' 0x2502 or '┃' 0x2503 (Box Drawing, Light Vertical / Heavy Vertical).
Why making things more complex than they can be?

—SA