Question

So as you may know, arrays in C# implement IList<T>, among other interfaces. Somehow though, they do this without publicly implementing the Count property of IList<T>! Arrays have only a Length property.

Is this a blatant example of C#/.NET breaking its own rules about the interface implementation or am I missing something?

Answer 1

New answer in the light of Hans’s answer

Thanks to the answer given by Hans, we can see the implementation is somewhat more complicated than we might think. Both the compiler and the CLR try very hard to give the impression that an array type implements IList<T> – but array variance makes this trickier. Contrary to the answer from Hans, the array types (single-dimensional, zero-based anyway) do implement the generic collections directly, because the type of any specific array isn’t System.Array – that’s just the base type of the array. If you ask an array type what interfaces it supports, it includes the generic types:

foreach (var type in typeof(int[]).GetInterfaces())
{
    Console.WriteLine(type);
}

Output:

System.ICloneable
System.Collections.IList
System.Collections.ICollection
System.Collections.IEnumerable
System.Collections.IStructuralComparable
System.Collections.IStructuralEquatable
System.Collections.Generic.IList`1[System.Int32]
System.Collections.Generic.ICollection`1[System.Int32]
System.Collections.Generic.IEnumerable`1[System.Int32]

For single-dimensional, zero-based arrays, as far as the language is concerned, the array really does implement IList<T> too. Section 12.1.2 of the C# specification says so. So whatever the underlying implementation does, the language has to behave as if the type of T[] implements IList<T> as with any other interface. From this perspective, the interface is implemented with some of the members being explicitly implemented (such as Count). That’s the best explanation at the language level for what’s going on.

Note that this only holds for single-dimensional arrays (and zero-based arrays, not that C# as a language says anything about non-zero-based arrays). T[,] doesn’t implement IList<T>.

From a CLR perspective, something funkier is going on. You can’t get the interface mapping for the generic interface types. For example:

typeof(int[]).GetInterfaceMap(typeof(ICollection<int>))

Gives an exception of:

Unhandled Exception: System.ArgumentException: Interface maps for generic
interfaces on arrays cannot be retrived.

So why the weirdness? Well, I believe it’s really due to array covariance, which is a wart in the type system, IMO. Even though IList<T> is not covariant (and can’t be safely), array covariance allows this to work:

string[] strings = { "a", "b", "c" };
IList<object> objects = strings;

… which makes it look like typeof(string[]) implements IList<object>, when it doesn’t really.

The CLI spec (ECMA-335) partition 1, section 8.7.1, has this:

A signature type T is compatible-with a signature type U if and only if at least one of the following holds

…

T is a zero-based rank-1 array V[], and U is IList<W>, and V is array-element-compatible-with W.

(It doesn’t actually mention ICollection<W> or IEnumerable<W> which I believe is a bug in the spec.)

For non-variance, the CLI spec goes along with the language spec directly. From section 8.9.1 of partition 1:

Additionally, a created vector with element type T, implements the interface System.Collections.Generic.IList<U>, where U := T. (§8.7)

(A vector is a single-dimensional array with a zero base.)

Now in terms of the implementation details, clearly the CLR is doing some funky mapping to keep the assignment compatibility here: when a string[] is asked for the implementation of ICollection<object>.Count, it can’t handle that in quite the normal way. Does this count as explicit interface implementation? I think it’s reasonable to treat it that way, as unless you ask for the interface mapping directly, it always behaves that way from a language perspective.

What about ICollection.Count?

So far I’ve talked about the generic interfaces, but then there’s the non-generic ICollection with its Count property. This time we can get the interface mapping, and in fact the interface is implemented directly by System.Array. The documentation for the ICollection.Count property implementation in Array states that it’s implemented with explicit interface implementation.

If anyone can think of a way in which this kind of explicit interface implementation is different from “normal” explicit interface implementation, I’d be happy to look into it further.

Old answer around explicit interface implementation

Despite the above, which is more complicated because of the knowledge of arrays, you can still do something with the same visible effects through explicit interface implementation.

Here’s a simple standalone example:

public interface IFoo
{
    void M1();
    void M2();
}

public class Foo : IFoo
{
    // Explicit interface implementation
    void IFoo.M1() {}

    // Implicit interface implementation
    public void M2() {}
}

class Test    
{
    static void Main()
    {
        Foo foo = new Foo();

        foo.M1(); // Compile-time failure
        foo.M2(); // Fine

        IFoo ifoo = foo;
        ifoo.M1(); // Fine
        ifoo.M2(); // Fine
    }
}