测试结果：

 

double->none 366ms

double->long 161ms

double->long2 188ms

double->format 564ms

double->Round 393ms

 

代码：

 

using System;using System.Collections.Generic;using System.Diagnostics;using System.Linq;using System.Text;using System.Threading.Tasks;namespace testStringbuilder{    class Program    {        static void Main(string[] args)        {            const int count = 1000000;            RunTest("double->none",()=>            {                StringBuilder sb = new StringBuilder(1000*1024*32);                double a = 3.1415926;                for (int i = 0; i < count; i++)                {                    sb.Append(a);                    sb.Append(',');                }            });            RunTest("double->long", () =>            {                StringBuilder sb = new StringBuilder(1000 * 1024 * 32);                double a = 3.1415926;                for (int i = 0; i < count; i++)                {                    sb.Append((long)a);                    sb.Append(',');                }            });            RunTest("double->long2", () =>            {                StringBuilder sb = new StringBuilder(1000 * 1024 * 32);                double a = 3.1415926;                for (int i = 0; i < count; i++)                {                    sb.Append((long)(a * 100));                    sb.Append(',');                }            });            RunTest("double->format", () =>            {                StringBuilder sb = new StringBuilder(1000 * 1024 * 32);                double a = 3.1415926;                for (int i = 0; i < count; i++)                {                    sb.AppendFormat("{0:f3}",a);                    sb.Append(',');                }            });            RunTest("double->Round", () =>            {                StringBuilder sb = new StringBuilder(1000 * 1024 * 32);                double a = 3.1415926;                for (int i = 0; i < count; i++)                {                    sb.Append(Math.Round(a, 3));                    sb.Append(',');                }            });        }        private static void RunTest(string key, Action action)        {            double milli = 0;            for (int i = 0; i < 3; i++)            {                Stopwatch watch = Stopwatch.StartNew();                try                {                    action();                }                catch (Exception ex)                {                    Console.WriteLine(ex);                }                watch.Stop();                milli += watch.ElapsedMilliseconds;            }            Console.WriteLine("{0}\t{1}ms", key, (long) (milli/3));        }    }}