Two relevant properties ReadOnly and Enabled. ReadOnly = true prevents editing grays out the background, but it still allows focus. Enabled = false grays out the background, text and prevents editing or focus.
Windows UI conventions dicate giving the user a visual cue that a control is readonly (that way they won't attempt to edit it and be subsequently frustrated). The grayed out disabled state is the defined system convention, but it's arguable too much of a cue (and not a legibile enough one).
The simplest route is probababy to set your control to ReadOnly, set the background to System.Drawing.SystemColors.Window and then block focus messages. You could do this by catching OnEnter events and immediately moving Focus to another control that's not readonly (say, a Close or Edit button). Or you could derive your own control and eat any WM_SETFOCUS messages. Example below.
I believe various third-party control sets give you additional options and granularity.
public class ReadOnlyTextBox : TextBox
{
const uint WM_SETFOCUS = 0x0007;
public ReadOnlyTextBox()
{
this.ReadOnly = true;
this.BackColor = System.Drawing.SystemColors.Window;
this.ForeColor = System.Drawing.SystemColors.WindowText;
}
protected override void WndProc(ref Message m)
{
// eat all setfocus messages, pass rest to base
if (m.Msg != WM_SETFOCUS)
base.WndProc(ref m);
}
}