WinForms textbox strange behaivor after 10 numbers

Question

I'm trying to put some controls on my TextBox using Visual Studio 2019 WinForms.

So this is my big control

int myNumber;
private void txtNumberControl_TextChanged(object sender, EventArgs e)
{
    try
    {
        if (sender != null)
        {
            myNumber = Convert.ToInt32(txtNumberControl.Text);
        }
    }
    catch (Exception)
    {
        MessageBox.Show("You can write only numbers!", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
    }
        
}

As you can see from the code above, I'm trying to make sure that the user inputs only numeric values. The strange behavior starts after 10 numbers writed. So if I write 12345678912 it goes in catch and shows me the MessageBox error message.

Here is a picture of it (it's in Italian but the error is the same I just translated it).
myErrorMessage

But the most strange part is that I can put down how many 00000000000000000000000000 I want and It just works as expected.

I'm seeking for help, can someone gently explain me why is this happening?

Answer 1

Your program goes into the catch because the number you wanted to enter is now to high to be converted into an integer.

Max Value for an integer is: 2147483647

You've gone over that limit with: 12345678912

Because of that your program fails to convert and prints the message even tough your input is a legit number.

To actually check if the whole inputed string is a number even if it is over the integer limit. You can use .IsDigit() combined with .All() to check if any of the letters is not a number.

Example:

public bool CheckForNumber(string yourString) {
    // Check if all chars in this string are digits.
    if (yourString.All(char.IsDigit)) {
        return true;
    }

    return false;
}

We can now use that function in combination with the User input to check if it is a valid number.

private void txtNumberControl_TextChanged(object sender, EventArgs e) {
    // Check the text of the object that triggered this Event.
    if (!CheckForNumber((sender as Control).Text)) {
        MessageBox.Show("You can write only numbers!", "Error", 
            MessageBoxButtons.OK, MessageBoxIcon.Error);
        return;
    }

    // Try to parse the text into a long (is 0 if the input is invalid).
    if (!long.TryParse(txtNumberControl.Text, out long number)) {
        // Not validated
    }   
}