4321

using System;

namespace ConsoleApp5
{
    class Program
    {
        static void Main()
        {
            var str = "asxd xsa";
            var result = "" + str[0];
            for (int i = 1; i < str.Length; i++)
            {
                bool end = false;
                for (int j = i - 1; j >= 0; j--)
                {
                    if (str[i] == str[j])
                    {
                        end = true;
                        break;
                    }
                }
                if (!end) result += str[i];
            }
            Console.WriteLine(result);
            Console.ReadKey();
        }
    }
}