abstract explicit interface implementation in C#

Question

I have this C# code:

abstract class MyList : IEnumerable<T>
{
    public abstract IEnumerator<T> GetEnumerator();

    //abstract IEnumerator IEnumerable.GetEnumerator();
}

As is, I get:

'Type' does not implement interface member 'System.Collections.IEnumerable.GetEnumerator()'.

remove the comment and I get:

The modifier 'abstract' is not valid for this item

How do I make an explicit implementation abstract

Answer 1

Interesting - I'm not sure you can. However, if this is your real code, do you ever want to implement the non-generic GetEnumerator() in any way other than by calling the generic one?

I'd do this:

abstract class MyList<T> : IEnumerable<T>
{
    public abstract IEnumerator<T> GetEnumerator();

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

That saves you from the tedium of having to implement it in every derived class - which would no doubt all use the same implementation.

Answer 2

While an explicit interface member may not be abstract (or virtual), it may be implemented in terms of an abstract (or virtual) member ¹ :

public abstract class Foo: IEnumerable {
    IEnumerator IEnumerable.GetEnumerator() { 
        return getEnumerator();    
    }

    protected abstract IEnumerator getEnumerator(); 
}

public class Foo<T>: Foo, IEnumerable<T> {
    private IEnumerable<T> ie;
    public Foo(IEnumerable<T> ie) {
        this.ie = ie;
    }

    public IEnumerator<T> GetEnumerator() {
        return ie.GetEnumerator();
    }

    protected override IEnumerator getEnumerator() {
        return GetEnumerator();
    }

    //explicit IEnumerable.GetEnumerator() is "inherited"
}

I've found the need for this in strongly typed ASP.NET MVC 3 partial views, which do not support generic type definition models (as far as I know).

Answer 3

You actually can do it, by forcing a class, which derives from an abstract class, to implement an interface, and still allow it to choose how to implement that interface - implicitly or explicitly:

namespace Test
{
    public interface IBase<T>
    {
        void Foo();
    }

    public abstract class BaseClass<T> 
        where T : IBase<T>  // Forcing T to derive from IBase<T>
    { }

    public class Sample : BaseClass<Sample>, IBase<Sample>
    {
        void IBase<Sample>.Foo() { }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Sample sample = new Sample();

            // Error CS1061  'Sample' does not contain a definition for 'Foo' 
            // and no extension method 'Foo' accepting a first argument of type 'Sample' 
            // could be found(are you missing a using directive or an assembly reference ?)
            sample.Foo();

            (sample as IBase<Sample>).Foo(); // No Error
        }
    }
}

Answer 4

I had a slightly more complicated case where I wanted a base class to implement the non generic interface explicitly and a derived class implement the generic interface.

Interfaces:

public interface IIdentifiable<TKey> : IIdentifiable
{
    TKey Id { get; }
}

public interface IIdentifiable
{
    object Id { get; }
}

I solved it by declaring an abstract getter method in the base class and letting the explicit implementation call it:

public abstract class ModelBase : IIdentifiable
{
    object IIdentifiable.Id
    {
        get { return GetId();  }
    }

    protected abstract object GetId();
}

public class Product : ModelBase, IIdentifiable<int>
{
    public int ProductID { get; set; }

    public int Id
    {
        get { return ProductID; }
    }

    protected override object GetId()
    {
        return Id;
    }
}

Note that the base class does not have the typed version of Id it could call.

Answer 5

It seems not possible to do an abstract explicit interface implementation, but you can do a workaround to get rid of the error, but still force to use the explicit interface implementation by the implicit one:

abstract class MyList : IEnumerable<T>
{
    public virtual IEnumerator<T> GetEnumerator() {
        (this as IEnumerable).GetEnumerator();
    }
}

However, as pointed out by a comment on the question above:

If you chose to have the member non abstract, the compiler will allow subclasses without (an own) implementation...