我如何在 c# 中捕获异常？

Question

I'm having trouble catching an exception in c#. I want to convert numbers from decimal systems to (other) systems, so for example, if the user enters dec num, which contains other characters ('a', 'b', 'c' etc.) program will show an error message.

try
{
    string numbers = "0123456789abcdef";
    for (int i=0; i<txt.Length; i++)
    {
        for (int j=0; j<16; i++)
        {
            if (txt[i] == numbers[j] && j >= 10)
                throw new Exception();
        }
    }
}
catch (Exception)
{
    MessageBox.Show("Error!");
}

Thank you!

Answer 1

Exceptions are designed for exceptional situations ; here you have a user input validation (no need in such means like exceptions); a simple loop ( foreach ) and if will be enough:

 foreach (char c in txt)
   if (c < '0' || c > '9') {
     MessageBox.Show("Error!");

     break; // at least one error, let's skip further validation 
   }

Answer 2

There are two articles on exception handling, that I link often:

• https://blogs.msdn.microsoft.com/ericlippert/2008/09/10/vexing-exceptions/
• https://www.codeproject.com/Articles/9538/Exception-Handling-Best-Practices-in.NET

You are in the non-eviable position of writing your own parsing and Exception throwing code. But it might actually be unessesary - .NET has support for innumerable Number formats and Numbering Systems. Hex to Decimal can be done in 2 commands: https://learn.microsoft.com/en-us/do.net/csharp/programming-guide/types/how-to-convert-between-hexadecimal-strings-and-numeric-types#example-2

It seems like you are trying to reinvent Int.TryPrase() and are doing just about everything wrong. You should try to use existing parse functions, before you try to reinvent the wheel.

What you wrote does not look like a proper TryParse function, where you can catch the Vexing exceptions but still communicate that they happened. And what you do catch is also way to wide. There seem to be fundamental design flaws, too many to simply fix.

If you are dead set on reinventing that wheel, I would advise first and foremost to read the two articles I linked, so you do not make the common Exception Handling mistakes. I did once write a TryParse approximation for someone stuck on Framework 1.1. This might give you some ideas:

//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;
}