using System;

using System.Text;

using System.Collections.Generic;

using System.Linq;

​

// http://www.csharpstar.com/remove-duplicate-characters-from-string-in-csharp/

​

public class Program

{

    public static void Main()

    {

        var instr = "Hello World";

        var outstr = RemoveDuplicateCharacters(instr);

        Console.WriteLine(outstr);

    }

    private static string RemoveDuplicateCharacters(string s)

    {

        var sb = new StringBuilder();

        var dict = new Dictionary<char, int>();

        for (var i = 0; i < s.Length; i++)

        {

            if (!dict.ContainsKey(s[i]))

            {

                dict[s[i]] = 1;

                sb.Append(s[i]);

            }

        }

        return sb.ToString();

    }

    private static string RemoveDupes(string s)

    {

        return new string(s.Distinct(new CharacterComparer()).ToArray());

    }

}

​

public class CharacterComparer : IEqualityComparer<char>

{

    public bool Equals(char x, char y)

    {

        return char.ToLower(x).Equals(char.ToLower(y));

    }

    public int GetHashCode(char obj)

    {

        return char.ToLower(obj).GetHashCode();

    }

}