StringBuilder

using System;
using System.Text;

namespace MyCshap1
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = string.Empty;
            for(int i = 1; i <= 100000; i++)
            {
                s += i.ToString();
            }

            Console.WriteLine(s);

            StringBuilder sb = new StringBuilder();
            for (int i = 1; i <= 100000; i++)
            {
                sb.Append(i);   
            }

            Console.WriteLine(sb);
        }          
    }    
}