Question

How can I prevent users from entering anything other than a numeric value or a decimal value with 1 decimal place?

The user should be allowed to enter any length of characters (if decimal value, before the decimal).

Answer 1

Try using Regex. This pattern should work: Regex match = new Regex(@"^[1-9]\d*(\.\d{1})?$"), put that in your validating event of the textbox. If its no match, Undo() or delete the Textbox.Text property.

    Regex match = new Regex(@"^[1-9]\d*(\.\d{1})?$");

    private void textBox1_Validating(object sender, CancelEventArgs e)
    {
        if (!match.IsMatch(textBox1.Text))
        {
            textBox1.Undo(); 
        }
    }

To actually undo the input immediatly, you have to use

    private void textBox1_KeyUp(object sender, KeyEventArgs e)
    {
        if (!match.IsMatch(textBox1.Text))
        {
            textBox1.Undo();
        }
    }

Because if you use KeyDown, the TextBox has no Undo State.

2nd Edit: If you want both cases to match, you have to do the check in the Validating Event or a similar one. Since the regex uses “$” to make sure, no characters are added in the end, you cannot enter “.” or else you’d end up having a number like 1. which would require additional checking.