List.Enumerator IEnumerator.Reset() method impl

Question

Despite the fact, that IEnumerator.Reset method should never be used I found strange behavior of the method implementation within List<T>.

No matter how you examine the .NET Framework Source Code (tried with reference source and ILSpy) the method is implemented as following:

void System.Collections.IEnumerator.Reset() {
    if (version != list._version) {
        ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumFailedVersion);
    }

    index = 0;
    current = default(T);
}

However, it looks like the method is never called at all! Consider the code:

var list = new List<int>(1) { 3 };
using (var e = list.GetEnumerator())
{
    Console.WriteLine(e.MoveNext());
    Console.WriteLine(e.Current);

    ((IEnumerator)e).Reset();

    Console.WriteLine(e.MoveNext());
    Console.WriteLine(e.Current);
}

It's pretty clear, that it should print True and 3 twice. Instead of that the result is

True
3
False
0

Any simple explanation I'm missing?

Answer 1

Any simple explanation I'm missing?

Yes: you're boxing the List.Enumerator here:

((IEnumerator)e).Reset();

That takes a copy of the existing one and resets it - leaving the original in one piece.

To reset the actual enumerator, you'd need something like this:

var list = new List<int>(1) { 3 };
var e = list.GetEnumerator();
// Can't use "ref" with a using statement
try
{
    Console.WriteLine(e.MoveNext());
    Console.WriteLine(e.Current);

    Reset(ref e);

    Console.WriteLine(e.MoveNext());
    Console.WriteLine(e.Current);
}
finally
{
    e.Dispose();
}

static void Reset<T>(ref T enumerator) where T : IEnumerator
{
    enumerator.Reset();
}

It's tricky because it uses explicit interface implementation.

I haven't tested it, but I think that should work for you. Obviously it's a bad idea to do this...

EDIT: Alternatively, just change your variable type to IEnumerator or IEnumerator<int> to start with. Then it will be boxed once, and the Reset method will mutate the boxed value:

var list = new List<int>(1) { 3 };
using (IEnumerator e = list.GetEnumerator())
{
    Console.WriteLine(e.MoveNext());
    Console.WriteLine(e.Current);

    e.Reset();

    Console.WriteLine(e.MoveNext());
    Console.WriteLine(e.Current);
}