What does "public ClassName this[ AnEnumType key]" mean in C#?

Question

I stumble upon a piece of C# code that I don't understand, like this:

public class ClassA {
public ClassB this[EnumA key] {
    get {
            ClassB b;
            dictionaryB.TryGetValue(key, out b);
            return b;
    }

}

... }

I don't understand the syntax of "ClassB this[EnumA key] {...", what is it?

Answer 1

This is the indexing operator. It's used by collections most usually to give a callee a direct reference to an object in its storage.