How to make a textBox accept only Numbers and just one decimal point in Windows 8

Question

I am new to windows 8 phone. I am writing a calculator app that can only accept numbers in the textbox and just a single decimal point. how do I prevent users from inputting two or more decimal Points in the text box as the calculator cant handle that.

I have been using Keydown Event, is that the best or should I use Key up?

private void textbox_KeyDown(object sender, System.Windows.Input.KeyEventArgs e) {

}

Answer 1

Thanks all for the help. I finally did this and it worked very well in the Text changed event. That was after setting the inputscope to Numbers

string dika = Textbox1.Text;
    int countDot = 0;
    for (int i = 0; i < dika.Length; i++)
    {
        if (dika[i] == '.')
        {
            countDot++;
            if (countDot > 1)
            {
                dika = dika.Remove(i, 1); 
                i--;
                countDot--;
            }
        }
    }
Textbox1.Text = dika;
Textbox1.Select(dika.Text.Length, 0);