What is an IndexOutOfRangeException or ArgumentOutOfRangeException and how do I fix it?

What is an IndexOutOfRangeException  or ArgumentOutOfRangeException and how do I fix it?

 What Is It?

This exception means that you're trying to access a collection item by index, using an invalid index. An index is invalid when it's lower than the collection's lower bound or greater than or equal to the number of elements it contains.

When It Is Thrown

Given an array declared as:

byte[] array = new byte[4];

You can access this array from 0 to 3, values outside this range will cause IndexOutOfRangeException to be thrown. Remember this when you create and access an array.

Array Length

In C#, usually, arrays are 0-based. It means that first element has index 0 and last element has index Length - 1 (where Length is total number of items in the array) so this code doesn't work:

array[array.Length] = 0;

Arrays Do Not Grow

An array is fast. Very fast in linear search compared to every other collection. It is because items are contiguous in memory so memory address can be calculated (and increment is just an addition). No need to follow a node list, 

simple math! You pay this with a limitation: they can't grow, if you need more elements you need to reallocate that array (this may take a relatively long time if old items must be copied to a new block). You resize them with Array.Resize<T>(), this example adds a new entry to an existing array:

Array.Resize(ref array, array.Length + 1);