Question

I’m currently trying to make a C# API visible in COM and I’m currently stuck on making the non generic GetEnumerator method visible in COM for classes that also inherit IEnumerable<T>. If I implement another method on the ITestIEnumerable, it is visible. I’ve also found that non generic GetEnumerator methods are visible on classes that derive from Lists.

I don’t really understand why this isn’t working, it seems to be a special case for classes that inherit IEnumerable<T>. Is there anyway to expose this to COM so I can use the For Each construct in VB6?

For example

[ClassInterface(ClassInterfaceType.AutoDual)]
[ProgId("Test")]
[Guid("1aa2bdec-0530-44e8-b62d-b75914a6de22")]
public class TestIEnumerable : ITestIEnumerable
{
    #region IEnumerable<Item> Members

    [ComVisible(false)]
    public IEnumerator<Item> GetEnumerator()
    {
        return _collection.GetEnumerator();
    }

    #endregion

    #region ITestIEnumerable Members

    [DispId(-4)]
    [ComVisible(true)]
    IEnumerator ITestIEnumerable.GetEnumerator()
    {
        return _collection.GetEnumerator();
    }

    #endregion

    #region IEnumerable Members

    [DispId(-4)]
    [ComVisible(true)]
    IEnumerator IEnumerable.GetEnumerator()
    {
        return _collection.GetEnumerator();
    }

    #endregion
}

[InterfaceType(ComInterfaceType.InterfaceIsDual)]
[Guid("187f3fed-9d4a-4a52-9a7c-a2fdeb56b9fa")]
[ComImport]
public interface ITestIEnumerable : IEnumerable<Item>
{
    [DispId(-4)]
    [ComVisible(true)]
    new IEnumerator GetEnumerator();
}

Answer 1

You can’t have more than one public method of the same name with the same signature. As such, two of your GetEnumerator methods are being implemented using explicit interface implementation:

IEnumerator ITestIEnumerable.GetEnumerator()
{
}

As such, they’re treated as private members – it’s not even visible to other .NET code unless that code casts your TestIEnumerable object to ITestIEnumerable.

So, decide which one of your GetEnumerator() methods you wish to expose, and make it public:

public IEnumerator GetEnumerator()
{
}

And switch the generic one to an explicit implementation:

IEnumerator<Item> IEnumerable<Item>.GetEnumerator()
{
}