Double To String (Improved)

gistfile1.cs

public static string dtoa(double value) {

            long trunc = (long)value;

            double mantissa = (trunc < 0) ? value - trunc : trunc - value;

            var left = (long)(1000000000000000 * (mantissa > 0 ? mantissa : -mantissa));

            char* buf = stackalloc char[64];
            char* bf = buf;
            lltoa(bf, trunc);

            if (value < 0) {
                bf[0] = '-';
            }

            while (*(++buf) != '\0') ;
            *(++buf) = '.';

            lltoa(buf, left);

            var str = new string(bf);

            var cstr = String.Concat(ltoa(trunc), ".", ltoa(left));

            //return "";
            return String.Concat(ltoa(trunc), ".", ltoa(left));
        }