Detect decimal separator

Solution:

private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
    char a = Convert.ToChar(Thread.CurrentThread.CurrentCulture.NumberFormat.NumberDecimalSeparator);
    if (e.KeyChar == a)
    {
        e.Handled = true;
        textBox1.Text = "0";
    }
}

That way, when you hit . or , you will have a 0 in your TextBox.

EDIT:

If you want to insert a 0 everytime you hit the decimal separator, this is the code:

char a = Convert.ToChar(CultureInfo.CurrentCulture.NumberFormat.NumberDecimalSeparator);
if (e.KeyChar == a)
{
    e.KeyChar="0";
}