Question

The problem is simple: I have a form with textboxes, and in one of those textboxes, I want users to enter either a positive or negative 2-digit number. I’m looking for an easy way to enforce this restriction, i.e. without having to parse the number and check whether its absolute value is below 100.

If I set the textbox maxlength to 2, they cannot enter numbers below -9. If the maxlength is 3, they can enter numbers above 99.

A masked textbox has no solution, I cannot make the “-” literal optional, or at least not that I know of.

What would be the simplest solution to this restriction: “An empty textbox will accept 2 input characters if no “-” is typed, otherwise it accepts 3″? Handling the change event to see if a “-” was typed and resetting maxlength seems a bit overkill…

Answer 1

You actually want a NumericUpDownControl. Use that, and set its Minimum and Maximum properties. No validation code required at all, and easily solves your problem.

If you’re stuck on using a TextBox then in short you’re not going to get the desired functionality you want without actually checking the number. You could write a UserControl to do this and you could reuse it, but you still have to write some validation code.

Just validate the integer using ASP.NET/Javascript style validation. This example uses a label with a ForeColor of Red to display validation messages.

private void numberTextBox_TextChanged(object sender, EventArgs e) {
    int number;

    bool isValid = int.TryParse(numberTextBox.Text, out number);

    if (!isValid) {
        validationLabel.Text = "Must be a two-digit number.";
        validationLabel.Visible = true;
        return;
    }

    if (number < -99 || number > 99) {
        validationLabel.Text = "Must be between -99 and 99";
        validationLabel.Visible = true;
        return;
    }

    if (isValid) {
        validationLabel.Visible = false;
        // Do something else with your number
        // if you need to.
    }
}

There’s nothing wrong with having to write code to do validation. You probably won’t achieve good results using existing designer properties otherwise.