Run on .Net 2.0 or 3.5. If you think the result depends on the input you give it, then you haven't tried running it

gistfile1.cs

using System;

namespace ConsoleApplication4
{
    class Program
    {
        static void Main(string[] args)
        {
            CheckNull(null);
        }

        static bool CheckNull(string s)
        {
        start:
            bool tmp = Console.ReadLine() == "";
            if (tmp) goto end;
        bool tmp2 = true;
            tmp = String.IsNullOrEmpty(s);
            if (tmp2 == tmp)
            {
                goto start;
            }
            else
            {
                goto start;
            }
            end:
            goto start;
        }
    }
}