c# - Why can't I use the enumerator of an array, instead of implementing it myself?

Question

I have some code like this:

public class EffectValues : IEnumerable<object>
{
    public object [ ] Values { get; set; }

    public IEnumerator<object> GetEnumerator ( )
    {
        return this.Values.GetEnumerator ( );
    }

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator ( )
    {
        return this.GetEnumerator ( );
    }
}

But the compiler complains saying:

"Cannot implicitly convert type 'System.Collections.IEnumerator' to 'System.Collections.Generic.IEnumerator'. An explicit conversion exists (are you missing a cast?)"

I thought the Array type implemented both IEnumerable interfaces, does it not? Because I can use Linq features on the Values instance directly.

Answer 1

This is a subtle and a bit unfortunate. The easy workaround is:

public IEnumerator<object> GetEnumerator ( )
{
     return ((IEnumerable<object>)this.Values).GetEnumerator ( );     
} 

I thought the Array type implemented both IEnumerable interfaces, does it not?

The rules are:

• System.Array implements IEnumerable "implicitly", with public methods.
• every array type T[] inherits from System.Array.
• every array type T[] implements IList<T>, IEnumerable<T> and so on.
• therefore every array type T[] is convertible to IEnumerable<T>

Notice that the third point was NOT

• every array type T[] implements IList<T>, IEnumerable<T> and so on with public methods and properties defined on T[] that implicitly implement the members

And there you go. When you look up GetEnumerator, we look it up on object[] and don't find it, because object[] implements IEnumerable<object> explicitly. It is convertible to IEnumerable<object>, and convertibility doesn't count for lookups. (You wouldn't expect a method of "double" to appear on int just because int is convertible to double.) We then look at the base type, and find that System.Array implements IEnumerable with a public method, so we've found our GetEnumerator.

That is, think about it like this:

namespace System
{
    abstract class Array : IEnumerable
    {
        public IEnumerator GetEnumerator() { ... }
        ...
    }
}

class object[] : System.Array, IList<object>, IEnumerable<object>
{
    IEnumerator<object> IEnumerable<object>.GetEnumerator() { ... }
    int IList<object>.Count { get { ... } }
    ...
}

When you call GetEnumerator on object[], we don't see the implementation that is an explicit interface implementation, so we go to the base class, which does have one visible.

How do all the object[], int[], string[], SomeType[] classes get generated "on the fly"?

Magic!

This is not generics, right?

Right. Arrays are very special types and they are baked in at a deep level into the CLR type system. Though they are very similar to generics in a lot of ways.

It seems like this class object [] : System.Array is something that can't be implemented by a user, right?

Right, that was just to illustrate how to think about it.

Which one do you think is better: Casting the GetEnumerator() to IEnumerable<object>, or just use foreach and yield?

The question is ill-formed. You don't cast the GetEnumerator to IEnumerable<object>. You either cast the array to IEnumerable<object> or you cast the GetEnumerator to IEnumerator<object>.

I would probably cast Values to IEnumerable<object> and call GetEnumerator on it.

I will probably use casting but I am wondering if this is a place where you or some programmer who could read the code, would think it's less clear.

I think it's pretty clear with the cast.

when you said implicit implementation, you mean in the form of Interface.Method, right?

No, the opposite:

interface IFoo { void One(); void Two(); }
class C : IFoo
{
    public void One() {} // implicitly implements IFoo.One
    void IFoo.Two() {} // explicitly implements IFoo.Two
}

The first declaration silently implements the method. The second is explicit about what interface method it implements.

What's the reason for implementing IEnumerable<T> like that, instead of implicit implementation with public methods? I got curious because you said "This is a subtle and a bit unfortunate", so it seems like it's because of an older decision that forced you to do this I imagine?

I don't know who made this decision. It is kind of unfortunate though. It's confused at least one user -- you -- and it confused me for a few minutes there too!

I would have thought the Array type would be something like this: public class Array<T> : IEnumerable<T> etc. But instead there is some magical code about it then, right?

Right. As you noted in your question yesterday, things would have been a lot different if we'd had generics in CLR v1.

Arrays are essentially a generic collection type. Because they were created in a type system that did not have generics, there has to be lots of special code in the type system to handle them.

Next time you design a type system put generics in v1 and make sure you get strong collection types, nullable types and non-nullable types baked in to the framework from the beginning. Adding generics and nullable value types post hoc was difficult.