Question

Example

Here is a code example I found at the Pete on Software blog:

var listThree = new string[] { "Pete", "On", "Software" };  
var listFour = new string[] { "Joel", "On", "Software" };  
stringExcept = listThree.Except(listFour);

The code compiles and runs. So far so good.

Question

However, I don’t understand why it works.

So, can anyone explain why I can use Enumerable.Except on a string array?

Perhaps, it will be clear to me if someone could explain how to read the signature of Enumerable.Except and give me a code example:

public static IEnumerable<TSource> Except<TSource>(
    this IEnumerable<TSource> first,
    IEnumerable<TSource> second
)

What I know

I know the concepts of generics and extension methods. But obviously not good enough to understand the code example above. I have also already used some basic Linq queries.

Answer 1

Except is an extension method which extends any type that implements IEnumerable<T>. This includes the System.Array type which implements IEnumerable<T>.

The note on the linked page explains why the docs don’t show System.Array implementing IEnumerable<T>

In the .NET Framework version 2.0, the Array class implements the System.Collections.Generic.IList<T>, System.Collections.Generic.ICollection<T>, and System.Collections.Generic.IEnumerable<T> generic interfaces. The implementations are provided to arrays at run time, and therefore are not visible to the documentation build tools. As a result, the generic interfaces do not appear in the declaration syntax for the Array class, and there are no reference topics for interface members that are accessible only by casting an array to the generic interface type (explicit interface implementations). The key thing to be aware of when you cast an array to one of these interfaces is that members which add, insert, or remove elements throw NotSupportedException.