c# - C# 是否在循环/lambda 语句中自动优化代码？

Question

例如，在 Javascript 中，强烈建议将函数调用放在循环之外以获得更好的性能:

var id = someIdType.ToString();
someList.Where(a => a.id == id) ...

C# 怎么样？相同情况还是编译器/运行时使用内部优化/缓存？

someList.Where(a => a.id == someIdType.ToString()) ...

可能是菜鸟问题，之前有人问过，但找不到引用。

Answer 1

C#代码:

List<string> list = new List<string>();
list.Where(a => a == typeof(String).ToString());

MSIL 中的 Lambda 表达式，调试配置:

.method private hidebysig static bool  '<Main>b__0'(string a) cil managed
{
  .custom instance void     [mscorlib]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = ( 01 00 00 00 ) 
  // Code size       26 (0x1a)
  .maxstack  2
  .locals init ([0] bool CS$1$0000)
  IL_0000:  ldarg.0
  IL_0001:  ldtoken    [mscorlib]System.String
  IL_0006:  call       class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle)
  IL_000b:  callvirt   instance string [mscorlib]System.Object::ToString()
  IL_0010:  call       bool [mscorlib]System.String::op_Equality(string,
                                                             string)
  IL_0015:  stloc.0
  IL_0016:  br.s       IL_0018
  IL_0018:  ldloc.0
  IL_0019:  ret
} // end of method Program::'<Main>b__0'

MSIL 中的 Lambda 表达式，发布配置:

.method private hidebysig static bool  '<Main>b__0'(string a) cil managed
{
  .custom instance void     [mscorlib]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = ( 01 00 00 00 ) 
  // Code size       22 (0x16)
  .maxstack  8
  IL_0000:  ldarg.0
  IL_0001:  ldtoken    [mscorlib]System.String
  IL_0006:  call       class [mscorlib]System.Type     [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle)
  IL_000b:  callvirt   instance string [mscorlib]System.Object::ToString()
  IL_0010:  call       bool [mscorlib]System.String::op_Equality(string,
                                                             string)
  IL_0015:  ret
} // end of method Program::'<Main>b__0'

两个版本都调用 typeof(String).ToString())，此 lambda 会在每次迭代时调用。没有对 IL 级别的优化，JIT 编译不会在此处添加任何内容。原因是:函数可能有副作用。