Finding

Index method

starts searching at the beginning of the array

System.Array.Index 

If the value does not exist it returns -1.

LastIndexOf method

starts searching at the end of the array

System.Array.LastIndexOf 

If the value does not exist then it returns -1

BinarySearch method

This provides a faster search
Your array must be sorted first and therefore implement IComparable or a type that implements IComparer

int myArray[] = new int[] 

System.Array.BinarySearch(aiArray, 9)