Does C# optimize code automatically in looped/lambda statements?

Question

In Javascript for example, one is strongly encouraged to place function calls outside of loops for better performance:

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

How about C#? Same case or does the compiler/runtime employ internal optimization/caching?

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

Probably a noob question and has been asked before, but can't find a reference.

Answer 1

C# code:

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

Lambda expression in MSIL, Debug configuration:

.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'

Lambda expression in MSIL, Release configuration:

.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'

Both versions call typeof(String).ToString()), this lambda is called on every iteration. No optimization on IL level, JIT compilation will not add anything here. The reason is: function may have side effects.