How do arrays in C# partially implement IList?

So as you may know, arrays in C# implement IList<T>, among other interfaces

Well, yes, erm no, not really. This is the declaration for the Array class in the .NET 4 framework:

[Serializable, ComVisible(true)]
public abstract class Array : ICloneable, IList, ICollection, IEnumerable, 
                              IStructuralComparable, IStructuralEquatable
{
    // etc..
}

It implements System.Collections.IList, not System.Collections.Generic.IList<>. It can’t, Array is not generic. Same goes for the generic IEnumerable<> and ICollection<> interfaces.

But the CLR creates concrete array types on the fly, so it could technically create one that implements these interfaces. This is however not the case. Try this code for example:

using System;
using System.Collections.Generic;

class Program {
    static void Main(string[] args) {
        var goodmap = typeof(Derived).GetInterfaceMap(typeof(IEnumerable<int>));
        var badmap = typeof(int[]).GetInterfaceMap(typeof(IEnumerable<int>));  // Kaboom
    }
}
abstract class Base { }
class Derived : Base, IEnumerable<int> {
    public IEnumerator<int> GetEnumerator() { return null; }
    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { return GetEnumerator(); }
}

The GetInterfaceMap() call fails for a concrete array type with “Interface not found”. Yet a cast to IEnumerable<> works without a problem.

This is quacks-like-a-duck typing. It is the same kind of typing that creates the illusion that every value type derives from ValueType which derives from Object. Both the compiler and the CLR have special knowledge of array types, just as they do of value types. The compiler sees your attempt at casting to IList<> and says “okay, I know how to do that!”. And emits the castclass IL instruction. The CLR has no trouble with it, it knows how to provide an implementation of IList<> that works on the underlying array object. It has built-in knowledge of the otherwise hidden System.SZArrayHelper class, a wrapper that actually implements these interfaces.

Which it doesn’t do explicitly like everybody claims, the Count property you asked about looks like this:

    internal int get_Count<T>() {
        //! Warning: "this" is an array, not an SZArrayHelper. See comments above
        //! or you may introduce a security hole!
        T[] _this = JitHelpers.UnsafeCast<T[]>(this);
        return _this.Length;
    }

Yes, you can certainly call that comment “breaking the rules” 🙂 It is otherwise darned handy. And extremely well hidden, you can check this out in SSCLI20, the shared source distribution for the CLR. Search for “IList” to see where the type substitution takes place. The best place to see it in action is clr/src/vm/array.cpp, GetActualImplementationForArrayGenericIListMethod() method.

This kind of substitution in the CLR is pretty mild compared to what happens in the language projection in the CLR that allows writing managed code for WinRT (aka Metro). Just about any core .NET type gets substituted there. IList<> maps to IVector<> for example, an entirely unmanaged type. Itself a substitution, COM doesn’t support generic types.

Well, that was a look at what happens behind the curtain. It can be very uncomfortable, strange and unfamiliar seas with dragons living at the end of the map. It can be very useful to make the Earth flat and model a different image of what’s really going on in managed code. Mapping it to everybody favorite answer is comfortable that way. Which doesn’t work so well for value types (don’t mutate a struct!) but this one is very well hidden. The GetInterfaceMap() method failure is the only leak in the abstraction that I can think of.