Enumerating via interface - performance loss

Question

I had a little dispute (which was very close to holy war:) ) with my colleage, about the performance of access to list via indeces VS via enumerator. To operat

Answer 1

You can see the code here:

static void Main()
{

    List list = new List(Enumerable.Range(1,10000));

    int total = 0;
    foreach (var i in list)
    {
        total += i;
    }
    IEnumerable enumerable = list;
    foreach (var i in enumerable)
    {
        total += i;
    }
    Console.ReadLine();
}

Which generates this IL. Notice the difference between

System.Collections.Generic.List`1/Enumerator

and

System.Collections.Generic.IEnumerable`1 

and note that it is a ValueType (struct):

.method private hidebysig static void  Main() cil managed
{
  .entrypoint
  // Code size       146 (0x92)
  .maxstack  2
  .locals init ([0] class [mscorlib]System.Collections.Generic.List`1 list,
           [1] int32 total,
           [2] int32 i,
           [3] class [mscorlib]System.Collections.Generic.IEnumerable`1 enumerable,
           [4] valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator CS$5$0000,
           [5] bool CS$4$0001,
           [6] class [mscorlib]System.Collections.Generic.IEnumerator`1 CS$5$0002)
  IL_0000:  nop
  IL_0001:  ldc.i4.1
  IL_0002:  ldc.i4     0x2710
  IL_0007:  call       class [mscorlib]System.Collections.Generic.IEnumerable`1 [System.Core]System.Linq.Enumerable::Range(int32,
                                                                                                                                  int32)
  IL_000c:  newobj     instance void class [mscorlib]System.Collections.Generic.List`1::.ctor(class [mscorlib]System.Collections.Generic.IEnumerable`1)
  IL_0011:  stloc.0
  IL_0012:  ldc.i4.0
  IL_0013:  stloc.1
  IL_0014:  nop
  IL_0015:  ldloc.0
  IL_0016:  callvirt   instance valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator class [mscorlib]System.Collections.Generic.List`1::GetEnumerator()
  IL_001b:  stloc.s    CS$5$0000
  .try
  {
    IL_001d:  br.s       IL_002d
    IL_001f:  ldloca.s   CS$5$0000
    IL_0021:  call       instance !0 valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator::get_Current()
    IL_0026:  stloc.2
    IL_0027:  nop
    IL_0028:  ldloc.1
    IL_0029:  ldloc.2
    IL_002a:  add
    IL_002b:  stloc.1
    IL_002c:  nop
    IL_002d:  ldloca.s   CS$5$0000
    IL_002f:  call       instance bool valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator::MoveNext()
    IL_0034:  stloc.s    CS$4$0001
    IL_0036:  ldloc.s    CS$4$0001
    IL_0038:  brtrue.s   IL_001f
    IL_003a:  leave.s    IL_004b
  }  // end .try
  finally
  {
    IL_003c:  ldloca.s   CS$5$0000
    IL_003e:  constrained. valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator
    IL_0044:  callvirt   instance void [mscorlib]System.IDisposable::Dispose()
    IL_0049:  nop
    IL_004a:  endfinally
  }  // end handler
  IL_004b:  nop
  IL_004c:  ldloc.0
  IL_004d:  stloc.3
  IL_004e:  nop
  IL_004f:  ldloc.3
  IL_0050:  callvirt   instance class [mscorlib]System.Collections.Generic.IEnumerator`1 class [mscorlib]System.Collections.Generic.IEnumerable`1::GetEnumerator()
  IL_0055:  stloc.s    CS$5$0002
  .try
  {
    IL_0057:  br.s       IL_0067
    IL_0059:  ldloc.s    CS$5$0002
    IL_005b:  callvirt   instance !0 class [mscorlib]System.Collections.Generic.IEnumerator`1::get_Current()
    IL_0060:  stloc.2
    IL_0061:  nop
    IL_0062:  ldloc.1
    IL_0063:  ldloc.2
    IL_0064:  add
    IL_0065:  stloc.1
    IL_0066:  nop
    IL_0067:  ldloc.s    CS$5$0002
    IL_0069:  callvirt   instance bool [mscorlib]System.Collections.IEnumerator::MoveNext()
    IL_006e:  stloc.s    CS$4$0001
    IL_0070:  ldloc.s    CS$4$0001
    IL_0072:  brtrue.s   IL_0059
    IL_0074:  leave.s    IL_008a
  }  // end .try
  finally
  {
    IL_0076:  ldloc.s    CS$5$0002
    IL_0078:  ldnull
    IL_0079:  ceq
    IL_007b:  stloc.s    CS$4$0001
    IL_007d:  ldloc.s    CS$4$0001
    IL_007f:  brtrue.s   IL_0089
    IL_0081:  ldloc.s    CS$5$0002
    IL_0083:  callvirt   instance void [mscorlib]System.IDisposable::Dispose()
    IL_0088:  nop
    IL_0089:  endfinally
  }  // end handler
  IL_008a:  nop
  IL_008b:  call       string [mscorlib]System.Console::ReadLine()
  IL_0090:  pop
  IL_0091:  ret
} // end of method Program2::Main