Question

I want to prevent one particular character * (asterisk) from being entered or pasted into a text box.

I tried:

1. key_press event – but it does not handle the case when user pastes an asterisk to the text box.
2. text_changed event – but when I remove the character, the cursor position goes back to the beginning of the text.

So I am wondering how to handle it, preferably in one event.

Answer 1

use the text changed event, but save the location of the cursor (the SelectionStart and SelectionEnd properties) before you remove the asterisk, then re set the cursor position (less the number of asterisks removed before the cursor).

    private void textBox1_TextChanged(object sender, TextChangedEventArgs e)
    {
        var currentText = textBox1.Text;
        var selectionStart = textBox1.SelectionStart;
        var selectionLength = textBox1.SelectionLength;

        int nextAsterisk;
        while ((nextAsterisk = currentText.IndexOf('*')) != -1)
        {
            if (nextAsterisk < selectionStart)
            {
                selectionStart--;
            }
            else if (nextAsterisk < selectionStart + selectionLength)
            {
                selectionLength--;
            }

            currentText = currentText.Remove(nextAsterisk, 1);
        }

        if (textBox1.Text != currentText)
        {
            textBox1.Text = currentText;
            textBox1.SelectionStart = selectionStart;
            textBox1.SelectionLength = selectionLength;
        }
    }