Precise ToString() for double

gistfile1.txt

using static System.Double;
using static System.Math;
using static System.Numerics.BigInteger;
using System.Numerics;

public static class DoubleConvert {
    public static string Precisely(this double a) {
        if (IsInfinity(a) || IsNaN(a)) return a.ToString();
        if (a == 0) return 1 / a < 0 ? "-0.0" : "0.0";
        BigInteger n = 0, d = 1, p = 1;
        string r = a < 0 ? "-" : "";
        for (a = Abs(a); a < 0.5; a *= 2) d *=2;
        for (; a >= 1; a /= 2) p *= 2;
        for (; a > 0; a %= 1, d *= 2) n = n * 2 + p * (int)(a *= 2);
        r += DivRem(n, d, out n) + ".";
        do r += DivRem(n * 10, d, out n); while (n > 0);
        return r;
    }
}