First Non repeated Character in a String using an Array

Here is how to find the first non repeated character in an a string using an array
the assumption is the String is ASCII other wise check for the hash table function

For example If the input is "TEETOTALAR" The First non repeated Character is "O"

public static char FirstNonRepeatedArray(string stringToCheck)
        {
            int length = stringToCheck.Length;
            int i = 0;
            int[] intCollection = new int[256];
            char returnChar = '\0';

            for (i = 0; i < length; i++)
            {
                intCollection[stringToCheck[i]] = intCollection[stringToCheck[i]] + 1;
            }
            for (i = 0; i < length; i++)
            {
                if (intCollection[stringToCheck[i]] == 1)
                {
                    returnChar = stringToCheck[i];
                    break;
                }
            }
            return returnChar;
        }