Question

How does one make a textbox highlighted with red color when an event occurred like invalid number format in c#?

Answer 1

It’s not terribly clear what you mean, but it sounds like you want to do something like:

• Hook up an event handler for whenever the text changes in a textbox
• In that event handler, check whether the text can be parsed as a number, ideally using int.TryParse (rather than just parsing it and catching an exception)
• Setting the foreground colour of the textbox to red or black, depending on whether the parsing attempt was successful

So something like:

textBox.TextChanged += (sender, args) => {
    int ignored;
    bool valid = int.tryParse(textBox.Text, out ignored);
    textBox.ForeColor = valid ? Color.Black : Color.Red;
};

(There may well be other rather more sophisticated ways of doinig this, including preventing the invalid input to start with… but for just changing the colour, this will work 🙂