foreach or IEnumerator

1-source.cs

class Program
{
    static void Main(string[] args)
    {
        var array = new int[1000000];
        foreach (var i in array){}
    
        var enumerator = array.GetEnumerator();
        while (enumerator.MoveNext()){}
    
        var lines = System.IO.File.ReadLines("c:\temp");
        foreach (var line in lines){}
    
        var fileEnum = lines.GetEnumerator();
        while (fileEnum.MoveNext()) { }
    }
}

2-decompiled.cs

// Using Telerik JustDecompile
private static void Main(string[] args)
{
   int[] array = new int[1000000];
   int[] numArray = array;
   for (int i = 0; i < (int)numArray.Length; i++)
   {
      int num = numArray[i];
   }
   
   IEnumerator enumerator = array.GetEnumerator();
   while (enumerator.MoveNext()){}
   
   IEnumerable<string> lines = File.ReadLines("c:\temp");
   foreach (string str in lines){}
   
   IEnumerator<string> fileEnum = lines.GetEnumerator();
   while (fileEnum.MoveNext()){}
}

3-source.il

.method private hidebysig static void Main (
            string[] args
        ) cil managed 
    {
        .entrypoint
        .locals init (
            [0] int32[] 'array',
            [1] class [mscorlib]System.Collections.IEnumerator enumerator,
            [2] class [mscorlib]System.Collections.Generic.IEnumerable`1<string> lines,
            [3] class [mscorlib]System.Collections.Generic.IEnumerator`1<string> fileEnum,
            [4] int32[] CS$6$0000,
            [5] int32 CS$7$0001,
            [6] class [mscorlib]System.Collections.Generic.IEnumerator`1<string> CS$5$0002
        )

        IL_0000: ldc.i4 1000000
        IL_0005: newarr [mscorlib]System.Int32
        IL_000a: stloc.0
        IL_000b: ldloc.0
        IL_000c: stloc.s CS$6$0000
        IL_000e: ldc.i4.0
        IL_000f: stloc.s CS$7$0001
        IL_0011: br.s IL_001f
        .loop
        {
            IL_0013: ldloc.s CS$6$0000
            IL_0015: ldloc.s CS$7$0001
            IL_0017: ldelem.i4
            IL_0018: pop
            IL_0019: ldloc.s CS$7$0001
            IL_001b: ldc.i4.1
            IL_001c: add
            IL_001d: stloc.s CS$7$0001

            IL_001f: ldloc.s CS$7$0001
            IL_0021: ldloc.s CS$6$0000
            IL_0023: ldlen
            IL_0024: conv.i4
            IL_0025: blt.s IL_0013
        }

        IL_0027: ldloc.0
        IL_0028: callvirt instance class [mscorlib]System.Collections.IEnumerator [mscorlib]System.Array::GetEnumerator()
        IL_002d: stloc.1
        .loop
        {
            IL_002e: ldloc.1
            IL_002f: callvirt instance bool [mscorlib]System.Collections.IEnumerator::MoveNext()
            IL_0034: brtrue.s IL_002e
        }
        IL_0036: ldstr "c:\temp"
        IL_003b: call class [mscorlib]System.Collections.Generic.IEnumerable`1<string> [mscorlib]System.IO.File::ReadLines(string)
        IL_0040: stloc.2
        IL_0041: ldloc.2
        IL_0042: callvirt instance class [mscorlib]System.Collections.Generic.IEnumerator`1<string> class [mscorlib]System.Collections.Generic.IEnumerable`1<string>::GetEnumerator()
        IL_0047: stloc.s CS$5$0002
        .try
        {
            IL_0049: br.s IL_0053
            .loop
            {
                IL_004b: ldloc.s CS$5$0002
                IL_004d: callvirt instance string class [mscorlib]System.Collections.Generic.IEnumerator`1<string>::get_Current()
                IL_0052: pop

                IL_0053: ldloc.s CS$5$0002
                IL_0055: callvirt instance bool [mscorlib]System.Collections.IEnumerator::MoveNext()
                IL_005a: brtrue.s IL_004b
            }

            IL_005c: leave.s IL_006a
        }
        finally
        {
            IL_005e: ldloc.s CS$5$0002
            IL_0060: brfalse.s IL_0069

            IL_0062: ldloc.s CS$5$0002
            IL_0064: callvirt instance void [mscorlib]System.IDisposable::Dispose()

            IL_0069: endfinally
        }

        IL_006a: ldloc.2
        IL_006b: callvirt instance class [mscorlib]System.Collections.Generic.IEnumerator`1<string> class [mscorlib]System.Collections.Generic.IEnumerable`1<string>::GetEnumerator()
        IL_0070: stloc.3
        .loop
        {
            IL_0071: ldloc.3
            IL_0072: callvirt instance bool [mscorlib]System.Collections.IEnumerator::MoveNext()
            IL_0077: brtrue.s IL_0071
        }
        IL_0079: ret
    }