C＃Windows窗体FormatException未处理

Question

I am trying to put a check in textbox for value not greater then 100 but i am getting formatExeception can anybody help in this regard.. Thanks in Advance.

private void textBox1_TextChanged(object sender, EventArgs e)
{
    if (Int32.Parse(textBox1.Text) > 100)
    {
        MessageBox.Show("No. Of Elements Must be Less Then 100");
    }
}

Answer 1

You need to surround your statement with a try-catch in case the content of the textbox cannot be parsed into Int32. Assuming you run into the exception, you can then get a message describing the reason for the error.

Answer 2

Probably better to use a Numeric Updown if you only want the user to enter numbers but the error is happening because the text in the textbox cannot be parsed to a number. Use int.TryParse. It wont throw an exception if it cannot parse the string to a number

        int numElements = 0;
        int.TryParse(textBox1.Text, out numElements);
        if(numElements >100){
            MessageBox.Show("No. Of Elements Must be Less Then 100");
        }

Answer 3

private void textBox1_TextChanged(object sender, EventArgs e)
{
    int parsed = 0;
    if (!int.TryParse(textBox1.Text), out parsed)
    {
        MessageBox.Show("No. You must enter a number!");
        return;
    }
    if (parsed > 100)
    {
        MessageBox.Show("No. Of Elements Must be Less Then 100");
    }
}

Answer 4

That property of Parse to throw exceptions on parsing error is quite Vexing . It is so Vexing, the Framework Developers added TryParse with the 2.0 version. If you want to parse strings, you should always be using TryParse once you get past the initial development phase.

Or ideally a appraoch to validation/input that does not allow faulty inputs (like the Numerical Up/Down Ken Tucker noted.

If you somehow lack access to TryParse, I wrote a copy of it way back:

//Parse throws ArgumentNull, Format and Overflow Exceptions.
//And they only have Exception as base class in common, but identical handling code (output = 0 and return false).

bool TryParse(string input, out int output){
  try{
    output = int.Parse(input);
  }
  catch (Exception ex){
    if(ex is ArgumentNullException ||
      ex is FormatException ||
      ex is OverflowException){
      //these are the exceptions I am looking for. I will do my thing.
      output = 0;
      return false;
    }
    else{
      //Not the exceptions I expect. Best to just let them go on their way.
      throw;
    }
  }

  //I am pretty sure the Exception replaces the return value in exception case. 
  //So this one will only be returned without any Exceptions, expected or unexpected
  return true;

}