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.

Answer 2

The extra interfaces are shown because they are implied by IList. If you implement IList, you must also implement ICollection and IEnumerable.