How to allow only numbers and '+' to be entered in a textBox?

Question

WPF

A number is entered in the textBox in the format [phone], but if you enter the same number of letters, then there will be no error
Tried like this

if (!Char.IsDigit(e.Text, 0))
            {
                e.Handled = true;
            }

But then '+' can't be entered either

Answer 1

Something like this, only you can add one more condition that if the plus has already been entered, then do not let it be entered a second time. Or then filter and cut out unnecessary pluses.

if (!Char.IsDigit(e.Text, 0) || e.Text != '+')
            {
                e.Handled = true;
            }