Why Don't C# Arrays Directly Implement `IList`, and How Do They Still Seem To?

Arrays in C#: A Deeper Dive into Interface Implementation

Although arrays in C# appear to implement IList, they do not publicly expose the Count property. This may seem like a contradiction to C# interface implementation rules.

In reality, arrays implement System.Collections.IList, not System.Collections.Generic.IList. Generic interfaces like IList cannot be implemented by non-generic types like Array.

However, the CLR generates specific array types that could potentially implement these interfaces. But even for these concrete array types, casting to IEnumerable works, while retrieving the interface map throws an exception.

This is due to "quacks-like-a-duck" typing, where the compiler and CLR recognize arrays as implementing IList. The cast instruction is handled by a hidden wrapper class System.SZArrayHelper that provides an implementation of IList over the array.

Even though the Count property is not explicitly implemented, the CLR provides an internal getter using a cast to the T[] array.

This behavior, while somewhat unconventional, allows for a seamless and efficient implementation of IList for arrays. It's a clever technique that showcases the hidden mechanisms in C# and the CLR that contribute to its flexibility and power.

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