Remove Duplicate characters from String, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            string value1 = RemoveDuplicateChars("Csharpstar");
            string value2 = RemoveDuplicateChars("Google");
            string value3 = RemoveDuplicateChars("Yahoo");
            string value4 = RemoveDuplicateChars("CNN");
            string value5 = RemoveDuplicateChars("Line1\nLine2\nLine3");
 
            Console.WriteLine(value1);
            Console.WriteLine(value2);
            Console.WriteLine(value3);
            Console.WriteLine(value4);
            Console.WriteLine(value5);
        }
 
        static string RemoveDuplicateChars(string key)
        {
            // --- Removes duplicate chars using string concats. ---
            // Store encountered letters in this string.
            string table = "";
 
            // Store the result in this string.
            string result = "";
 
            // Loop over each character.
            foreach (char value in key)
            {
                // See if character is in the table.
                if (table.IndexOf(value) == -1)
                {
                    // Append to the table and the result.
                    table += value;
                    result += value;
                }
            }
            return result;
        }
    }
}

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Compilation time: 0,17 sec, absolute running time: 0,13 sec, cpu time: 0,14 sec, average memory usage: 17 Mb, average nr of threads: 3