为什么列表 <T> 实现IList <T> ，ICollection <T> 和IEnumerable <T> ？

Question

If you go to definition of List<T> you would see the following:

public class List<T> : IList<T>, ICollection<T>, IEnumerable<T>

IList<T> already inherits from both ICollection<T> and IEnumerable<T> .

Wouldn't it have been sufficient if List<T> only implemented IList<T> ?

Answer 1

Yes, it makes no difference in this case. In some cases it can make a difference, if you're using a base class which already implements an interface but you wish to reimplement it yourself explicitly - but in this case there's no base class (other than the implicit object ) and it would have behaved exactly the same way.

Contrary to my recollections, I don't believe there's a difference in the way the class is represented in metadata whether the code explicitly declares all the interfaces or not. Here's an example:

interface IFoo {}
interface IBar : IFoo {}

class FooBar1 : IBar {}
class FooBar2 : IBar, IFoo {}

Both ildasm and Reflector show the same information for FooBar1 and FooBar2 ... it shows both of them implementing IBar and IFoo .

In other words, we can't tell whether the original source code for List<T> actually specifies all the interfaces or not. Maybe it does, maybe it doesn't - but it doesn't matter either way.

EDIT: For completeness, I also checked the cases where you're extending two interfaces with another interface. I can't find a difference in the metadata in that case, either. I'm sure I remember some situation in which it was evident, but I can't find it now.

Answer 2

Yes it would. IList<T> itself implements the other two.

The object browser shows you all the interfaces the class implements, whether directly ( IList<T> ) or indirectly ( ICollection<T> , IEnumerable<T> , through IList<T> ).

Answer 3

That's not the way it was actually coded behind the scenes. That's just what tools like Reflector show you when it turns the IL back into C#.

Answer 4

If you looked at the source code:

https://referencesource.microsoft.com/#mscorlib/system/collections/generic/list.cs

You can observe that the signature is:

public class List<T> : IList<T>, System.Collections.IList, IReadOnlyList<T>

This can only mean that the metadata browser de-normalizes the inheritance hierarchy for us.