In local scopes#

Ensure that the line pointed to by the stack trace is accessing an index that is within the limits of the collection. The index needs to be 0 or above, and less than the length of the collection.

Use if statements to validate your indexer and avoid invalid access, or restructure your logic entirely.

Common mistakes include:

• Accessing an empty collection (.Length or .Count is 0).
• An improperly written for or while loop.
• Using an index from a different loop, i instead of j for example.

You can use the debugger to step over your code, inspecting variables and execution to assess what is wrong.

Inside lambdas#

If you have code like this:

for (int i = 0; i < values.Length; i++)
{
    values[i].onClick.AddListener(() => values[i].enabled = false);
}

The i in the delegate is not copied inside the for loop, it is created before it, reused, and increased as the counter of the loop. The value in the listener will increase to values.Length at the end of the loop. To fix this, declare a local version of the counter that is used in the delegate:

for (int i = 0; i < values.Length; i++)
{
    int iLocal = i;
    values[i].onClick.AddListener(() => values[iLocal].enabled = false);
}

See anonymous methods and closures for more information.