using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;  

​

public class Program

{

    public static void Main()

    {

        List<string> hugeList = Enumerable.Range(1, 2000).Select(n => n.ToString()).ToList(); 

        //METHODE 1

        string concatResult = "";

        foreach (string val in hugeList)

        {

            concatResult += val;

        }

        //METHODE 2

        StringBuilder stringBuilder = new StringBuilder();

        String stringBuilderResult = "";

        foreach (String s in hugeList)

        {

            stringBuilder.Append(s);

        }

        stringBuilderResult = stringBuilder.ToString();

    }

}