Have a look at this: Counting Lines in a String
] it's based on lines (so there are some bits you can ignore) but newline is just a character in a text file, so most oif the methods it shows will work for any character.
To find out how many of each character there are in your string in one line, this will work:
string input = "ABCDABDFGHI";
Dictionary<char, int> counts = input.GroupBy(x => x).ToDictionary(x => x.Key, x => x.Count());
Each element of the dictionary will be the character and how many times it appears.