int[] items = { 1, 2, 3, 4, 5 };
int size = 5;
int index = 4;

//thread 1
index = size;
size += 1;
int[] tmpArray = new int[size];
Array.Copy(items, tmpArray, items.Length);

//thread 2
size += 1;
index = size;

//thread 1

//throws an expection
tmpArray[index] = 4;
items = tmpArray;