c# - 为什么是 str = str.Replace().Replace();比 str = str.Replace(); 快str = str.替换()？

Question

我正在做一个本地测试来比较 C# 中 String 和 StringBuilder 的 Replace 操作性能，但是对于 String 我使用了以下代码:

String str = "String to be tested. String to be tested. String to be tested."
str = str.Replace("i", "in");
str = str.Replace("to", "ott");
str = str.Replace("St", "Tsr");
str = str.Replace(".", "\n");
str = str.Replace("be", "or be");
str = str.Replace("al", "xd");

但是，在注意到 String.Replace() 比 StringBuilder.Replace() 更快之后，我开始针对上面的代码测试以下代码:

String str = "String to be tested. String to be tested. String to be tested."
str = str.Replace("i", "in").Replace("to", "ott").Replace("St", "Tsr").Replace(".", "\n").Replace("be", "or be").Replace("al", "xd");

最后一个结果快了大约 10% 到 15% 倍，关于为什么它更快有什么想法吗？为同一个变量赋值是否很昂贵？

Answer 1

我做了这个基准:

namespace StringReplace
{
    using BenchmarkDotNet.Attributes;
    using BenchmarkDotNet.Running;

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

        private String str = "String to be tested. String to be tested. String to be tested.";

        [Benchmark]
        public string Test1()
        {
            var a = str;
            a = a.Replace("i", "in");
            a = a.Replace("to", "ott");
            a = a.Replace("St", "Tsr");
            a = a.Replace(".", "\n");
            a = a.Replace("be", "or be");
            a = a.Replace("al", "xd");

            return a;
        }

        [Benchmark]
        public string Test2()
        {
            var a = str;
            a = a.Replace("i", "in").Replace("to", "ott").Replace("St", "Tsr").Replace(".", "\n").Replace("be", "or be").Replace("al", "xd");

            return a;
        }
    }
}

结果:

BenchmarkDotNet=v0.10.0
OS=Microsoft Windows NT 6.2.9200.0
Processor=Intel(R) Core(TM) i7-7700 CPU 3.60GHz, ProcessorCount=8
Frequency=3515629 Hz, Resolution=284.4441 ns, Timer=TSC
Host Runtime=Clr 4.0.30319.42000, Arch=32-bit RELEASE
GC=Concurrent Workstation
JitModules=clrjit-v4.7.2600.0
Job Runtime(s):
    Clr 4.0.30319.42000, Arch=32-bit RELEASE


 Method |      Mean |    StdDev |    Median |
------- |---------- |---------- |---------- |
  Test1 | 1.3768 us | 0.0354 us | 1.3704 us |
  Test2 | 1.3941 us | 0.0325 us | 1.3778 us |

如您所见， Release模式下的结果是相同的。所以，我认为由于变量的过度分配， Debug模式可能会有很小的差异。但在 Release模式下，编译器可以对其进行优化。