How C# enumerator methods look like at IL code level

EnumeratorMethodShowcase.txt

// The following C# code from LINQPad (free windows app) uses a method (Test) that
// returns an enumeration. Have a look how the IL code that the compiler generates
// looks like:

static IEnumerable<int> Test()
{
	yield return 100;
	var n = Rand.Range(200, 500);
	yield return n;
	yield return 600;
}

// The real method is rather short and contains almost no code:

Test:
IL_0000:  ldc.i4.s    FE 
IL_0002:  newobj      UserQuery+<Test>d__1..ctor      <- create instance of the hidden class
IL_0007:  ret         

// The compiler has generated a hidden class called <Test>d__1
// and it has Dispose(), MoveNext(), Reset(), GetEnumerator() as 
// methods and Current as a getter property.

<Test>d__1.System.IDisposable.Dispose:
IL_0000:  ret         

All code from the original Test() method is now contained in
MoveNext() as one big switch statement. The current element
is calculated and stored in UserQuery+<Test>d__1.<>2__current
and returned by the Current property. The state machine has
a numerical state that starts at case 0 and it is stored in
UserQuery+<Test>d__1.<>1__state:

<Test>d__1.MoveNext:
IL_0000:  ldarg.0     
IL_0001:  ldfld       UserQuery+<Test>d__1.<>1__state
IL_0006:  stloc.0     
IL_0007:  ldloc.0     
IL_0008:  switch      (IL_001F, IL_0037, IL_005E, IL_0079)    <---- switch statement
IL_001D:  ldc.i4.0    
IL_001E:  ret         
IL_001F:  ldarg.0                                             <---- first case/yield
IL_0020:  ldc.i4.m1   
IL_0021:  stfld       UserQuery+<Test>d__1.<>1__state
IL_0026:  ldarg.0     
IL_0027:  ldc.i4.s    100 
IL_0029:  stfld       UserQuery+<Test>d__1.<>2__current
IL_002E:  ldarg.0     
IL_002F:  ldc.i4.1    
IL_0030:  stfld       UserQuery+<Test>d__1.<>1__state
IL_0035:  ldc.i4.1    
IL_0036:  ret         
IL_0037:  ldarg.0                                             <---- second case/yield
IL_0038:  ldc.i4.m1   
IL_0039:  stfld       UserQuery+<Test>d__1.<>1__state
IL_003E:  ldc.i4      200 
IL_0043:  ldc.i4      500 
IL_0048:  call        Verse.Rand.Range
IL_004D:  stloc.1     // n
IL_004E:  ldarg.0     
IL_004F:  ldloc.1     // n
IL_0050:  stfld       UserQuery+<Test>d__1.<>2__current
IL_0055:  ldarg.0     
IL_0056:  ldc.i4.2    
IL_0057:  stfld       UserQuery+<Test>d__1.<>1__state
IL_005C:  ldc.i4.1    
IL_005D:  ret         
IL_005E:  ldarg.0                                             <---- third case/yield
IL_005F:  ldc.i4.m1   
IL_0060:  stfld       UserQuery+<Test>d__1.<>1__state
IL_0065:  ldarg.0     
IL_0066:  ldc.i4      600 
IL_006B:  stfld       UserQuery+<Test>d__1.<>2__current
IL_0070:  ldarg.0     
IL_0071:  ldc.i4.3    
IL_0072:  stfld       UserQuery+<Test>d__1.<>1__state
IL_0077:  ldc.i4.1    
IL_0078:  ret         
IL_0079:  ldarg.0                                             <---- end
IL_007A:  ldc.i4.m1   
IL_007B:  stfld       UserQuery+<Test>d__1.<>1__state
IL_0080:  ldc.i4.0    
IL_0081:  ret         

<Test>d__1.System.Collections.Generic.IEnumerator<System.Int32>.get_Current:
IL_0000:  ldarg.0     
IL_0001:  ldfld       UserQuery+<Test>d__1.<>2__current
IL_0006:  ret         

<Test>d__1.System.Collections.IEnumerator.Reset:
IL_0000:  newobj      System.NotSupportedException..ctor
IL_0005:  throw       

<Test>d__1.System.Collections.IEnumerator.get_Current:
IL_0000:  ldarg.0     
IL_0001:  ldfld       UserQuery+<Test>d__1.<>2__current
IL_0006:  box         System.Int32
IL_000B:  ret         

<Test>d__1.System.Collections.Generic.IEnumerable<System.Int32>.GetEnumerator:
IL_0000:  ldarg.0     
IL_0001:  ldfld       UserQuery+<Test>d__1.<>1__state
IL_0006:  ldc.i4.s    FE 
IL_0008:  bne.un.s    IL_0022
IL_000A:  ldarg.0     
IL_000B:  ldfld       UserQuery+<Test>d__1.<>l__initialThreadId
IL_0010:  call        System.Environment.get_CurrentManagedThreadId
IL_0015:  bne.un.s    IL_0022
IL_0017:  ldarg.0     
IL_0018:  ldc.i4.0    
IL_0019:  stfld       UserQuery+<Test>d__1.<>1__state
IL_001E:  ldarg.0     
IL_001F:  stloc.0     
IL_0020:  br.s        IL_0029
IL_0022:  ldc.i4.0    
IL_0023:  newobj      UserQuery+<Test>d__1..ctor
IL_0028:  stloc.0     
IL_0029:  ldloc.0     
IL_002A:  ret         

<Test>d__1.System.Collections.IEnumerable.GetEnumerator:
IL_0000:  ldarg.0     
IL_0001:  call        UserQuery+<Test>d__1.System.Collections.Generic.IEnumerable<System.Int32>.GetEnumerator
IL_0006:  ret         

<Test>d__1..ctor:
IL_0000:  ldarg.0     
IL_0001:  call        System.Object..ctor
IL_0006:  ldarg.0     
IL_0007:  ldarg.1     
IL_0008:  stfld       UserQuery+<Test>d__1.<>1__state
IL_000D:  ldarg.0     
IL_000E:  call        System.Environment.get_CurrentManagedThreadId
IL_0013:  stfld       UserQuery+<Test>d__1.<>l__initialThreadId
IL_0018:  ret