Question

I’m having problems implementing IEnumerable<T> in my custom collection class in C++/CLI. Here is the relevant part of the code:

using namespace System::Collections::Generic;

ref class MyCollection : IEnumerable<MyClass^>
{
public:
    MyCollection()
    {
    }  

    virtual IEnumerator<MyClass^>^ GetEnumerator()
    {
        return nullptr;
    }
};

When compiled, this results in the following errors:

error C2392:
‘System::Collections::Generic::IEnumerator
^MyCollection::GetEnumerator(void)’:
covariant returns types are not
supported in managed types, otherwise
‘System::Collections::IEnumerator
^System::Collections::IEnumerable::GetEnumerator(void)’
would be overridden error C3766:
‘MyCollection’ must provide an
implementation for the interface
method
‘System::Collections::IEnumerator
^System::Collections::IEnumerable::GetEnumerator(void)’

This makes sense, since IEnumerable<T> derives from IEnumerable. However, I’m not sure how to fix this compile error. If this was C#, I would implicitly implement IEnumerable, however I’m not sure how to do that in C++/CLI (if that’s even possible) like this:

class MyCollection : IEnumerable<MyClass>
{
    public MyCollection()
    {
    }

    public IEnumerator<MyClass> GetEnumerator()
    {
        return null;
    }

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

If I do add an implementation of IEnumerable::GetEnumerator(), the compiler complains about two methods that differ only by return type (which also makes sense).

So, how do I implement IEnumerable<T> in a C++/CLI class?

Answer 1

You must provide an explicit implementation of the non-generic GetEnumerator() method and include the non-generic namespace:

using namespace System::Collections;

....

virtual IEnumerator^ EnumerableGetEnumerator() = IEnumerable::GetEnumerator
{
    return GetEnumerator<MyClass^>();
}

Update: As mentioned in the comments, the explicit version of GetEnumerator must be named different to avoid name clash, thus I’ve named it EnumerableGetEnumerator.

Similarly, in C# you would have to do it like this:

using System.Collections.Generic;

public class MyCollection : IEnumerable<MyClass>
{
    public MyCollection()
    {
    }  

    public IEnumerator<MyClass> GetEnumerator()
    {
        return null;
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator<MyClass>();
    }
}