How to Remove Duplicate characters from String in C#

Hello All,
In this Program, I will show you, how to make a program to remove duplicate characters from strings using c#

using System;
using System.Collections.Generic;

namespace testconsole

{
    class Program
    {
        static void Main(string[] args)
        {
            string Stringvalue = "this is Apple";
            string result = "";
            Console.WriteLine("Before remove duplicate character :{0} ", Stringvalue);

            foreach (char ch in Stringvalue)
            {
                if(result.Contains(ch.ToString()))
                {
                   
                }
                else
                {
                    result = result + ch;
                }
            }
            Console.WriteLine("after remove duplicate character :{0} ", result);
            Console.ReadKey();
        }

     
       
    }

}