numericOnlyTextBox.cs

private void numericOnlyTextBox_KeyUp(object sender, KeyEventArgs e)
{
    TextBox textBoxControl = sender as TextBox;

    string[] invalidCharacters = { "*", "#", ",", "(", ")", "x", "-", " ", "@", "." };

    for (int index = 0; index < invalidCharacters.Length; ++index)
    {
        textBoxControl.Text = textBoxControl.Text.Replace(invalidCharacters[index], "");
    }

    textBoxControl.SelectionStart = textBoxControl.Text.Length;
}

private void numericOnlyTextBox_KeyDown(object sender, KeyEventArgs e)
{
    TextBox textBoxControl = sender as TextBox;
    int n = 2; //TODO: adjust this as the max number of digits

    if (textBoxControl.Text.Length > n)
    {
        e.Handled = true;
    }
}