Basic C# Code to Find Out the Occurrence of Each Character in a String

Recently I went to an interview where the interviewer asked me to write some basic code to find out the occurrence of each character in a string.
I managed to do that using some loop, but it was not an optimal solution. The complexity, O(n), for that code was fairly high. I’ve continued to play with the code since, and have found a lot of options to solve this problem.