How can I make a WPF TextBox care about a decimal point?

Question

So I have a TextBox that only allows numbers and decimal points. I want the user to only be allowed to enter 1 decimal point.

Here is the code that is triggered on the PreviewTextInput event: (Some of the code is a little redundant, but it does the job)

    private void PreviewTextInput(object sender, TextCompositionEventArgs e)
    {

        TextBox textBox = (TextBox)sender;

        if (e.Text == ".")
        {

            if (textBox.Text.Contains("."))
            {

                e.Handled = true;
                return;

            }

            else
            {
                //Here I am attempting to add the decimal point myself
                textBox.Text = (textBox.Text + ".");
                e.handled = true;
                return;

            }

        }
        else
        {
            e.Handled = !IsTextAllowed(e.Text);
            return;
        }
    }

    private static bool IsTextAllowed(string text)
    {
        Regex regex = new Regex("[^0-9.-]+"); //regex that matches disallowed text
        return !regex.IsMatch(text);
    }

The problem is that the first decimal point that is entered is not "significant" until it is followed by a number. So, if the user enters 123. and you were to set a breakpoint and check the value of textBox.text it would be 123. I know this is happening because the textBox is bound to a Double so it is trying to be "smart" and forget about those currently "insignificant" values (The ".").

There shouldn't be anything wrong with my code, I was just hoping to force the textBox to hopefully skip over some unnecessary(?) auto-formatting.

Is there a way to make the textBox "care" about that first decimal point?

Possible Duplicate that was never answered.

or

*Is there a different approach to limiting the number of decimals?" (I have done a lot of research in this area, and I don't think there are any other options.)

Answer 1

if just limiting the charcters you want maybe something like binding with the string format would meet your needs

here is a good example of the format for double

And this would be an example of doing the binding in code to your ViewModel

    <TextBox Text="{Binding LimitedDouble,StringFormat={}{0:00.00}}"></TextBox>

Answer 2

private void txtDecimal_KeyPress(object sender, KeyPressEventArgs e)
{
    if (!Char.IsDigit(e.KeyChar) && e.KeyChar != '\b' && e.KeyChar!='.')
    {
        e.Handled = true;
    }
    if (e.KeyChar == '.' && txtDecimal.Text.Contains("."))
    {
        e.Handled = true;
    }
}