c# – 为什么StringBuilder比字符串连接慢？

为什么StringBuilder与连接相比较慢？
StringBuilder旨在避免额外的对象创建,但是为什么会影响性能？

static void Main(string[] args)
    {
        int max = 1000000;
        for (int times = 0; times < 5; times++)
        {
            Console.WriteLine("\ntime: {0}",(times+1).ToString());
            Stopwatch sw = Stopwatch.StartNew();
            for (int i = 0; i < max; i++)
            {
                string msg = "Your total is ";
                msg += "$500 ";
                msg += DateTime.Now;
            }
            sw.Stop();
            Console.WriteLine("String +\t: {0}ms",((int)sw.ElapsedMilliseconds).ToString().PadLeft(6));

            sw = Stopwatch.StartNew();
            for (int j = 0; j < max; j++)
            {
                StringBuilder msg = new StringBuilder();
                msg.Append("Your total is ");
                msg.Append("$500 ");
                msg.Append(DateTime.Now);
            }
            sw.Stop();
            Console.WriteLine("StringBuilder\t: {0}ms",((int)sw.ElapsedMilliseconds).ToString().PadLeft(6));
        }
        Console.Read();
    }

编辑：根据建议移出范围变量：

解决方法

更改,以使StringBuilder不被实例化,而不是.Clear()它：

time: 1
String +    :   3348ms
StringBuilder   :   3151ms

time: 2
String +    :   3346ms
StringBuilder   :   3050ms

等等

请注意,这仍然测试完全相同的功能,但尝试重新使用资源一点更聪明.

代码：(也住在http://ideone.com/YuaqY)

using System;
using System.Text;
using System.Diagnostics;

public class Program
{
    static void Main(string[] args)
    {
        int max = 1000000;
        for (int times = 0; times < 5; times++)
        {
            {
                Console.WriteLine("\ntime: {0}",(times+1).ToString());
                Stopwatch sw = Stopwatch.StartNew();
                for (int i = 0; i < max; i++)
                {
                    string msg = "Your total is ";
                    msg += "$500 ";
                    msg += DateTime.Now;
                }
                sw.Stop();
                Console.WriteLine("String +\t: {0}ms",((int)sw.ElapsedMilliseconds).ToString().PadLeft(6));
            }

            {
                Stopwatch sw = Stopwatch.StartNew();
                StringBuilder msg = new StringBuilder();
                for (int j = 0; j < max; j++)
                {
                    msg.Clear();
                    msg.Append("Your total is ");
                    msg.Append("$500 ");
                    msg.Append(DateTime.Now);
                }
                sw.Stop();
                Console.WriteLine("StringBuilder\t: {0}ms",((int)sw.ElapsedMilliseconds).ToString().PadLeft(6));
            }
        }
        Console.Read();
    }
}