How to make a textbox only accep a-z and nothing else?

Question

I have 4 text boxes at a Windows form. I would like to change it to only accept letters the letters a to z and nothing else, even when content is pasted in. If the user pastes in a mix of letters and unwanted characters, only the letters should show up in the textbox.

The last thing that I would like to have is the numlock pad. Those numbers are the same as the number row at a top of a keyboard, but I want to them to block them too!

Answer 1

I'd suppose to introduce a custom control derived from TextBox, by analogy with this post:

public class LettersTextBox : TextBox
{
    protected override void OnKeyPress(KeyPressEventArgs e)
    {
        base.OnKeyPress(e);

        string c = e.KeyChar.ToString();

        if (e.KeyChar >= 'a' && e.KeyChar <= 'z' || char.IsControl(e.KeyChar))
            return;

        e.Handled = true;
    }

    protected override void WndProc(ref System.Windows.Forms.Message m)
    {
        const int WM_PASTE = 0x0302;
        if (m.Msg == WM_PASTE)
        {
            string text = Clipboard.GetText();
            if (string.IsNullOrEmpty(text))
                return;

            if (text.Any(c => c < 'a' || c > 'z'))
            {
                if (text.Any(c => c >= 'a' || c <= 'z'))
                    SelectedText = new string(text.Where(c => c >= 'a' && c <= 'z').ToArray());
                return;
            }
        }
        base.WndProc(ref m);
    }
}