Testing null array index

Question

Here's the thing:

object[] arrayText = new object[1];

if (arrayText[1] == null)
{
    MessageBox.Show("Is null");
}

We know that is going to be null, but it throws an exception, but I don't want to handle it in a try/catch block because that is nested in a loop and try/catch will slow it down, also it doesn't look really good:

object[] arrayText = new object[1];
try
{
    if (arrayText[1] == null)
    {

    }
}
catch (Exception ex)
{
    MessageBox.Show("Is null");
}

Thanks for you suggestions!

Answer 1

null is not the problem here, but the index is invalid. Arrays in C# are 0-based, so if you create an array with 1 element, only index 0 is valid:

array[0] == null

You can avoid that by checking the bounds manually before accessing the index:

if (index < array.Length) {
    // access array[index] here
} else {
    // no exception, this is the "invalid" case
}

Answer 2

You're accessing an index that's outside the array's bounds. The array initializer takes a number for the number of elements, not the maximum index (like VB.NET). Since arrays are zero-based, your maximum index is 0 in this case.

Answer 3

object[] arrayText = new object[1];

if (arrayText[0] == null)
{
    MessageBox.Show("Is null");
}

Try that? Arrays are 0 based, so trying to access arrayText[1] will give you an OutOfBoundsException. And the try/catch won't really impact your performance that much there, there isn't much in the stack at that point.

Answer 4

Check the .Length of the array inside the loop, or better yet, set your loop parameters to be limited to the length of the array.

object[] arrayText = new object[1];
for (int i = 0; i < arrayText.Length; i++)
{
    doWork(arrayText[i]);
}

Answer 5

If you read the description on the exception that is being thrown you will see that it is "Index was outside the bounds of the array."

The new object[1] indicates that the array has one element. (1 is the count) However C# array's start indexing at 0 not 1 so an array of one element is index 0. So arrayText[0] will return null but arrayText[1] will throw an exception.

Answer 6

I think the problem is not that arrayText[1] is null, it's that arrayText[1] doesnt exist - so you should get an IndexOutOfRangeException and not a null

if you're up a creek and cant easily change the code to verify the lenght you might consider adding a function that inspects the Length property (see snippit below) or overloading operator[]... both of these are a little gross but if you're in a pickle... :)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace array
{
    class Program
    {
        static object Index(object [] array, int idx)
        {
            if (idx >= array.Length)
                return null;
            else
                return array[idx];
        }
        static void Main(string[] args)
        {
            object[] blah = new object[10];
            object o = Index(blah, 10);
        }
    }
}