GetSquareDigitChains

GetSquareDigitChains

        int resualt=GetSquareDigitChains(2,10000000);
         
         static int GetSquareDigitChains(int strt, int end) 
        {
            int i = 0;
            for (; strt < end; strt++) 
            {

                int nextstrt = 0;
                int strtTemp = strt;
                while (nextstrt != 89)
                {
                    strtTemp.ToString().ToCharArray().ToList().ForEach(x =>
                    {
                        nextstrt = nextstrt + int.Parse(x.ToString()) * int.Parse(x.ToString());
                    });
                    if (nextstrt == 1) { break; }
                    if (nextstrt == 89){ i++; break;}
                    else { strtTemp = nextstrt; nextstrt = 0; }
                     
                }

            
            }
            return i;
        }

A little Dynamic programming will get you better perf, also getting the digits using division instead of string comparison will help.

Ditto to "bashmohandes" comments.