Question

I am subclassing TextBox:

class Editor : TextBox

I have overridden OnKeyDown, because I want tabs to be replaced by four spaces:

protected override void OnKeyDown(KeyEventArgs e)
{
    if (e.KeyCode == Keys.Tab) {
        SelectedText = "    ";
        e.SuppressKeyPress = true;
    }
}

This works, but unfortunately it also clears the undo buffer. The end result is that when the user presses tab, Ctrl+Z doesn’t work and ‘Undo’ on the right-click menu becomes disabled. The problem appears to be the “e.SuppressKeyPress = true;” part.

Does anyone have any idea of how to get around this?

For more info, I am creating a fairly simple text editor, and I’m handling not only the Tab key (as above), but also the Enter key. So I have this problem with Tab and Enter. I am aware that this problem doesn’t exist with RichTextBox, but for various reasons I want to use TextBox instead.

Any help would be much appreciated, as this is a show-stopping problem in my project.

Thanks,
Tom

Answer 1

I’ve finally found the solution, which is to use the Windows API as follows:

protected override void OnKeyDown(KeyEventArgs e)
{
    if (e.KeyCode == Keys.Tab) {
        WinApi.SendMessage(Handle, WinApi.WmChar, WinApi.VkSpace, (IntPtr)4);
        e.SuppressKeyPress = true;
    }
    base.OnKeyDown(e);
}

Here is my WinApi class:

using System;
using System.Runtime.InteropServices;

class WinApi
{ 
    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    public static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, IntPtr wParam, IntPtr lParam);

    public const           UInt32 WmChar  =         0x102;
    public static readonly IntPtr VkSpace = (IntPtr)0x20;
}