foreach Dictionary<>.Values or foreach Dictionary<>

Question

I want to know the details of this two styles of iterating Dictionary collections in C#:

Dictionary xydic = new Dictionary

        

Answer 1

In both cases you are getting iterator and not temporary collection. Iterator is using internal state machine to remember what is the current item, and it uses MoveNext method to get nex item.

If you look at IL code you will see more details what is going on inside of foreach. Here is the IL for

void Main()
{
    var dictionary = new Dictionary { { 1, "one" }, { 2, "two" }};

    foreach (var item in dictionary.Values)
    {
        Console.WriteLine(item);
    }
}

IL Code:

IL_0000:  nop         
IL_0001:  newobj      System.Collections.Generic.Dictionary..ctor
IL_0006:  stloc.1     
IL_0007:  ldloc.1     
IL_0008:  ldc.i4.1    
IL_0009:  ldstr       "one"
IL_000E:  callvirt    System.Collections.Generic.Dictionary.Add
IL_0013:  nop         
IL_0014:  ldloc.1     
IL_0015:  ldc.i4.2    
IL_0016:  ldstr       "two"
IL_001B:  callvirt    System.Collections.Generic.Dictionary.Add
IL_0020:  nop         
IL_0021:  ldloc.1     
IL_0022:  stloc.0     // dictionary
IL_0023:  nop         
IL_0024:  ldloc.0     // dictionary
IL_0025:  callvirt    System.Collections.Generic.Dictionary.get_Values
IL_002A:  callvirt    System.Collections.Generic.Dictionary+ValueCollection.GetEnumerator
IL_002F:  stloc.2     
IL_0030:  br.s        IL_0043
IL_0032:  ldloca.s    02 
IL_0034:  call        System.Collections.Generic.Dictionary+ValueCollection+Enumerator.get_Current
IL_0039:  stloc.3     // item
IL_003A:  nop         
IL_003B:  ldloc.3     // item
IL_003C:  call        System.Console.WriteLine
IL_0041:  nop         
IL_0042:  nop         
IL_0043:  ldloca.s    02 
IL_0045:  call        System.Collections.Generic.Dictionary+ValueCollection+Enumerator.MoveNext
IL_004A:  brtrue.s    IL_0032
IL_004C:  leave.s     IL_005D
IL_004E:  ldloca.s    02 
IL_0050:  constrained. System.Collections.Generic.Dictionary<,>+ValueCollection.Enumerator
IL_0056:  callvirt    System.IDisposable.Dispose
IL_005B:  nop         
IL_005C:  endfinally  
IL_005D:  ret