これが最強の桁数カウントだ

Benchmark-report-github.md

BenchmarkDotNet=v0.10.14, OS=Windows 10.0.17692
Intel Core i7-3770 CPU 3.40GHz (Ivy Bridge), 1 CPU, 8 logical and 4 physical cores
  [Host]     : .NET Framework 4.7.1 (CLR 4.0.30319.42000), 32bit LegacyJIT-v4.7.3062.0
  DefaultJob : .NET Framework 4.7.1 (CLR 4.0.30319.42000), 32bit LegacyJIT-v4.7.3062.0

Method	Source	Mean	Error	StdDev	Median
UseToString	123.456	441.00 ns	5.8036 ns	5.1447 ns	440.59 ns
UseDecimalUnsafe	123.456	47.29 ns	0.5086 ns	0.4508 ns	47.27 ns
UseDecimalSafe	123.456	95.44 ns	2.1743 ns	5.0394 ns	93.50 ns

Benchmark.cs

using System;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

namespace ConsoleApp9
{
    class Program
    {
        static void Main(string[] args)
        {
            BenchmarkRunner.Run<Benchmark>();
        }
    }

    public class Benchmark
    {
        [Params(123.456)]
        public double Source;

        [Benchmark]
        public (int seisu, int syosu) UseToString()
        {
            var xStr = this.Source.ToString("F99", System.Globalization.CultureInfo.InvariantCulture);
            var dotIndex = xStr.IndexOf('.');

            var syosuCount = 0;
            for (var i = dotIndex + 1; i < xStr.Length; i++)
            {
                if (xStr[i] != '0') syosuCount = i - dotIndex;
            }

            return (dotIndex, syosuCount);
        }

        [Benchmark]
        public (int seisu, int syosu) UseDecimalUnsafe()
        {
            var tmpDouble = this.Source;
            var seisuCount = 0;

            while (tmpDouble >= 1.0)
            {
                tmpDouble /= 10.0;
                seisuCount++;
            }

            int syosuCount;
            unsafe
            {
                var tmpDecimal = (decimal)this.Source;
                var flags = *(uint*)&tmpDecimal;
                syosuCount = (int)((flags >> 16) & 0xFF);
            }

            return (seisuCount, syosuCount);
        }

        [Benchmark]
        public (int seisu, int syosu) UseDecimalSafe()
        {
            var tmpDouble = this.Source;
            var seisuCount = 0;

            while (tmpDouble >= 1.0)
            {
                tmpDouble /= 10.0;
                seisuCount++;
            }

            var tmpDecimal = (decimal)this.Source;
            var flags = (uint)Decimal.GetBits((decimal)this.Source)[3];
            var syosuCount = (int)((flags >> 16) & 0xFF);

            return (seisuCount, syosuCount);
        }
    }
}