What kind of class does yield return return

Question

So I've notice that this code works:

class Program
{
    public static void Main()
    {
        Int32[ ]numbers = {1,2,3,4,5};

        using (var enumerator = Data().GetEnumerator())
        {

        }
    }

    public static IEnumerable<String> Data()
    {
        yield return "Something";
    }
}

In particular, I'm curious about the using block, since:

Int32[] numbers = { 1, 2, 3, 4, 5, 6 };

using (var enumerator = numbers.GetEnumerator())
{

}

fails with a compiler error. Obviously, the class that yield return returns is IDisposable while a regular array enumerator is not. So now I'm curious: what exactly does yield return create?

Answer 1

IEnumerator<T> implements IDisposable, as you can see in the Object Browser or in MSDN.

The non-generic IEnumerator does not.

The base Array class implements IEnumerable but not IEnumerable<T>. (since Array is not generic)
Concrete array types do implement IEnumerable<T>, but they implement GetEnumerator() explicitly (I'm not sure why).
Therefore, the GetEnumerator() visible on any array type returns IEnumerator.

The generic IEnumerable<T> implementation returns a System.SZArrayHelper.SZGenericArrayEnumerator<T>.

The source code for this class (in Array.cs) has the following comment which partially explains this (remember, all support for generic arrays dates back to a time when IEnumerable<T> was not contraviant)

//--------------------------------------------------------------------------------------- 
// ! READ THIS BEFORE YOU WORK ON THIS CLASS. 
//
// The methods on this class must be written VERY carefully to avoid introducing security holes. 
// That's because they are invoked with special "this"! The "this" object
// for all of these methods are not SZArrayHelper objects. Rather, they are of type U[]
// where U[] is castable to T[]. No actual SZArrayHelper object is ever instantiated. Thus, you will
// see a lot of expressions that cast "this" "T[]". 
//
// This class is needed to allow an SZ array of type T[] to expose IList<T>, 
// IList<T.BaseType>, etc., etc. all the way up to IList<Object>. When the following call is 
// made:
// 
//   ((IList<T>) (new U[n])).SomeIListMethod()
//
// the interface stub dispatcher treats this as a special case, loads up SZArrayHelper,
// finds the corresponding generic method (matched simply by method name), instantiates 
// it for type <T> and executes it.
// 
// The "T" will reflect the interface used to invoke the method. The actual runtime "this" will be 
// array that is castable to "T[]" (i.e. for primitivs and valuetypes, it will be exactly
// "T[]" - for orefs, it may be a "U[]" where U derives from T.) 
//---------------------------------------------------------------------------------------