Really easy C# question

theheyes · 16 Jun 2009 at 00:01

Okay, I'm pretty embarrassed by the fact I've been outsmarted by a handful of lines of code. I'm trying to follow this guide to get myself started with C#. The guide is quite good, but I'm stumped on part of it. Whether its because it's late or the beer I'm doing my nut in trying to work this out.

I'm up to the "Adding things to do" section of the guide linked to above, which is basically passing an argument from the command line and doing something with it. I keep getting an error when trying to run what I've written. I've boiled it down somewhat from the guide for simplicity - the error is the same:

Code:

using System;
//using System.IO;

namespace toDoList
{
	class MainClass
	{
		public static void Main(string[] args)
		{
			
			if (args[0] == "add") 
			{
				Console.WriteLine("Hello");
			} 
			else 
			{
				Console.WriteLine("Goodbye");
			}
			
		}
	}
}

And the error is:

Code:

Unhandled Exception: System.IndexOutOfRangeException: Array index is out of range.
  at toDoList.MainClass.Main (System.String[] args) [0x00000] in /home/mh/toDoList/toDoList/Main.cs:16

I should be able to do "$ mono ./toDoList.exe add" to make it spit out "Hello" and anything else to spit out "Goodbye".

I'm going to go to bed now and hope someone has pointed out the obvious to me when I check back tomorrow.

Cheers guys & girls.

Dj_Jestar · 16 Jun 2009 at 00:16

It's basically saying there is no indice at position 0 on the array. I've not used mono, but check if arguments are passed differently than if on windows.

BullBoyShoes · 16 Jun 2009 at 10:12

args[0] is pointing to the first element in an array, which cant hold an entire string, only a single character, so therefore cant == "add"

maybe

Inquisitor · 16 Jun 2009 at 10:16

BullBoyShoes said:
args[0] is pointing to the first element in an array, which cant hold an entire string, only a single character, so therefore cant == "add"

maybe

No; args is an array of strings, so args[0] is a string.

mk17 · 16 Jun 2009 at 11:19

You need to add a check that the args array actually contains anything in the first place:

Code:

if (args.Length != 0)
{
    if( args[0] == "add" )
    {
        ...
        ...

You're getting the Index exception because there is nothing at position zero, and I don't mean null or string.Empty - the array's length is zero because no argument has been passed.

Dj_Jestar · 16 Jun 2009 at 11:38

or, to be more useful, take a look at what is in the arg array rather than negatively testing, investigate..

Code:

foreach (string arg in args)
{
  Console.WriteLine(arg);
}

theheyes · 16 Jun 2009 at 16:30

Cheers I'm going to try those later on, I was thinking along those lines a little. I might be back though!

Ladforce · 16 Jun 2009 at 17:08

its safer to loop through all your arguments with foreach(string arg in args) then a comparison block within this loop.

.Andy · 16 Jun 2009 at 17:21

args[0] is the program name you need args[1]

theheyes · 16 Jun 2009 at 17:47

Argh still no worky, I've tried all the suggestions. Interestingly if I use:

Code:

if (args.Length != 0)

or

Code:

foreach (string arg in args)

... the program will compile (I thought we'd cracked it), but when I come to run it with the argument "add" it throws out the exception error. i.e.

Code:

mh@mh-lin:~/toDoList/toDoList/bin/Debug$ mono toDoList.exe add

Unhandled Exception: System.IndexOutOfRangeException: Array index is out of range.
  at toDoList.MainClass.Main (System.String[] args) [0x00000]

Same error, different place.

A real head scratcher this one. I've made a post at the end of that guide, whether someone responds to it is another matter. I'm still open to more suggestions, even if you just post how you would do it might give me ideas. Cheers all.

lokkers · 16 Jun 2009 at 17:57

It seems to work for me, but the I'm running Windows. Can you step through it? Check for args == null?

Dj_Jestar · 16 Jun 2009 at 18:04

hmm.. try exactly this..

Code:

using System;

namespace toDoList
{
  class MainClass
  {
    static void Main(string[] args)
    {
      Console.WriteLine("args size: " + args.Length.ToString());
      Console.WriteLine("args:");
      Console.WriteLine("=====");
      foreach (string arg in args)
      {
        Console.WriteLine(arg);
      }

      Console.WriteLine("\nPress any key to continue...");
      Console.ReadKey();
    }
  }
}

and see the output..

theheyes · 17 Jun 2009 at 22:40

@lokkers - it's encouraging that it works in Windows. Perhaps its my version of Mono? The Help > About says it is Mono version 1.0 which sounds iffy but surely there can't be a bug for something so simple. Did you copy/paste the code from my first post exactly?

@Dj_Jestar - that works fine. See output:

Code:

mh@mh-lin:~/toDoList/toDoList/bin/Debug$ mono toDoList.exe apple bear car
args size: 3
args:
=====
apple
bear
car

Press any key to continue...

mh@mh-lin:~/toDoList/toDoList/bin/Debug$

I've actually already completed a similar task for printing out elements in an array, it just seems that whenever you try to reference it directly it throws an exception.

Cheers guys I really appreciate the help.

Lightnix · 18 Jun 2009 at 00:50

I know it's not exactly the most desirable solution, but could you copy args[] into a List of strings using the foreach loop?

Edit: Or perhaps try args.ToList()[0]?

Dj_Jestar · 18 Jun 2009 at 02:46

utterly bizzarre.

I can't remember how to do it, but dump the array's keys to see if it's not 0 indexed.

Inquisitor · 18 Jun 2009 at 08:27

That is weird. The only explanation I can think of is that it's not 0-indexed, like Jestar said. Try printing args.GetLowerBound(0) and see what that says, or try accessing args[1] to see what that contains.