Fastest way to implement Duplicate Character Removal in String (C#)

Question

In C#, what is the fastest way to detect duplicate characters in a String and remove them (removal including 1st instance of the duplicated character)?

Answer 1

This preserves order and, based on my tests, is 4 times faster than using a HashSet. This assumes your character range is 0-255 but you can extend that easily. If you're planning on using this in a loop, move the int[] c = new int[255]; out and in the function do an Array.Clear(c,0,255).

        private static string RemoveDuplicates(string s)
        {
            int[] c = new int[255];
            for (int i = 0; i < s.Length; i++)
            {
                c[s[i]]++;
            }
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < s.Length; i++)
            {
                if (c[s[i]] == 1) sb.Append(s[i]);
            }
            return sb.ToString();
        }