inheritance - Why collections classes in C# (like ArrayList) inherit from multiple interfaces if one of these interfaces inherits from the remaining?

Question

When I press f12 on the ArrayList keyword to go to metadata generated from vs2008, I found that the generated class declaration as follows

public class ArrayList : IList, ICollection, IEnumerable, ICloneable

I know that the IList already inherits from ICollection and IEnumerable, so why does ArrayList redundantly inherit from these interfaces?

Answer 1

OK, I've done some research. If you create the following hierarchy:

  public interface One
    {
        void DoIt();
    }

    public interface Two : One
    {
        void DoItMore();
    }

    public class Magic : Two
    { 
        public void DoItMore()
        {
            throw new NotImplementedException();
        }

        public void DoIt()
        {
            throw new NotImplementedException();
        }
    }

And compile it, then reference the DLL in a different solution, type Magic and Press F12, you will get the following:

 public class Magic : Two, One
    {
        public Magic();

        public void DoIt();
        public void DoItMore();
    }

You will see that the interface hierarchy is flattened, or the compiler is adding the interfaces in? If you use reflector you get the same results too.

Update: If you open the DLL in ILDASM, you will see it saying:

implements ...Two

implements ...One.