Effectively, we can add an infinite number of items to a list:

List<int> numbers = new List<int>();

for (int i = 0 ; i < 1000; i++)
{
  numbers.Add(i);
}

We can access and edit them using bracket notation:

int numberFive = numbers[4];
numbers[4] = 523;

However this doesn’t mean that we can access any random index. If we try to access the 1001st element in this list, we’ll get an error:

Console.WriteLine(numbers[1001]);

The above code causes the error:

Unhandled Exception:
System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.

We can only access indices which have been added to the list. The last available index in the numbers list would be 999.

Here are two tips to avoid this issue:

1. Imagine the list growing every time we add a number and shrinking every time we remove a number. Unlike arrays, there is no set length.
2. Check the Count of your lists before accessing an index, as shown below.

int index = 1001;
if (index < numbers.Count)
{
  Console.WriteLine(numbers[index])
}