C# Winforms: Is there any way to improve this simple custom Textbox code?

Question

I have been working on Custom TextBox control, which only allows alphanumeric characters. Note that I have not restricted characters from KeyPress, KeyUp event as I don't want to restrict copy/paste or any other operation that should be allowed generally. I have only trimmed non-alphanumeric character on paste operation. However, I am not sure whether the code I wrote is good or bad as I have very little experience of desktop application.

using System;
using System.Windows.Forms;
using System.Text.RegularExpressions;

namespace SaintThomas.UserControls
{
    class TextBoxForUserName : TextBox
    {
    protected override void OnTextChanged(EventArgs e)
    {
        base.OnTextChanged(e);

        this.SuspendLayout();
        int startPos = this.SelectionStart;
        if (Regex.IsMatch(this.Text, "[^0-9_A-Z]", RegexOptions.IgnoreCase))
        {
        int reduceStartPos = this.Text.Length;
        this.Text = Regex.Replace(this.Text, "[^0-9_A-Z]", "", RegexOptions.IgnoreCase);
        startPos = (startPos <= 0) ? 0 : startPos - (reduceStartPos - this.Text.Length);
        if (this.Text.Length < startPos)
        {
            startPos = this.Text.Length;
        }
        this.SelectionStart = startPos;
        }
        this.ResumeLayout();
    }
    }
}

Answer 1

Are you reinventing the MaskedTextBox?

Answer 2

Aside from the indentation issues...

• You could simply use @"\W" for your regular expression (with RegexOptions.None).
• A simpler way to deal with the cursor is to measure the distance from the end of the string, since you can know that will remain constant.
• Using Regex.Match followed by Regex.Replace seems pointless to me. If the string doesn't need to be changed, Regex.Replace should be able to determine that quickly.

• Calls to SuspendLayout also seem unnecessary, since the layout of your control (i.e., its bounds, position) is not changing.

  protected override void OnTextChanged(EventArgs e) {
      base.OnTextChanged(e);
      int charsFromEnd = this.Text.Length - this.SelectionStart;
      this.Text = Regex.Replace(this.Text, @"\W", "", RegexOptions.None);
      this.SelectionStart = this.Text.Length - charsFromEnd;
  }