C# Collections – What is a Collection?

Admitted, we blew it in the first version of the framework with System.Collections.ICollection, which is next to useless. But we fixed it up pretty well when generics came along in .NET framework 2.0: System.Collections.Generic.ICollection<T> lets you Add and Remove elements, enumerate them, Count them and check for membership.

Obviously from then on, everyone would implement ICollection<T> every time they make a collection, right? Not so. Here is how we used LINQ to learn about what collections really are, and how that made us change our language design in C# 3.0.

Collection initializers

With LINQ, the Language INtegrated Queryframework that we’re shipping in Orcas, we’re enabling a moreexpression-oriented style of programming. For instance it should bepossible to create and intialize an object within one expression. Forcollections, initialization typically amounts to adding an initial setof elements. Hence collection initializers in C# 3.0 look like this:

new MyNames { “<st1:personname w:st=”on”>Luke Hoban</st1:personname>”, “<st1:personname w:st=”on”>Karen Liu</st1:personname>”, “<st1:personname w:st=”on”>Charlie Calvert</st1:personname>” }

The meaning of this new syntax is simply to create an instance of MyNamesusing its no-arg constructor (constructor arguments can be supplied ifnecessary) and call its Add method with each of the strings.

So what types do we allow collection initializers on? Easy: collection types. What are those? Obvious: types that implement ICollection<T>. This is a nice and easy design – ICollection<T> ensures that you have an Addmethod so obviously that is the one that gets called for each elementin the collection initializer. It is strongly typed, too – theinitializer can contain only elements of the appropriate element type.In the above new expression, MyNames would be a class that implements ICollection<string> and everything works smoothly from there.

There’s just one problem: Nobody implements ICollection<T>!

LINQ to LINQ

Well, nobodyis a strong word. But we did an extensive study of our own frameworkclasses, and found only a few that did. How? Using LINQ of course. Thefollowing query does the trick:

from name in assemblyNames

select Assembly.LoadWithPartialName(name) into a

from c in a.GetTypes()

where c.IsPublic &&

c.GetConstructors().Any(m => m.IsPublic) &&

GetInterfaceTypes(c).Contains(typeof(ICollection<>))

select c.FullName;<o:p></o:p>

Let.s go through this query a little bit and see what it does. For each name in a list of assemblyNames that we pre-baked for the purpose, load up the corresponding assembly:

from name in assemblyNames

select Assembly.LoadWithPartialName(name)

One at a time, put the reflection objects representing these assemblies into a, and for each assembly a run through the types c defined in there:

from c in a.GetTypes()

Filter through, keeping each type only if it

a) IsPublic<o:p></o:p>

b) has Any constructor that IsPublic

c) implements ICollection<T> for some T:

where c.IsPublic &&

c.GetConstructors().Any(m => m.IsPublic) &&

GetInterfaceTypes(c).Contains(typeof(ICollection<>))

For those that pass this test, select out their full name:

select c.FullName;<o:p></o:p>

Nothing to it, really.

What is a collection?

What did we find then? Only 14 of our own (public) classes (with public constructors) implement ICollection<T>!Obviously there are a lot more collections in the framework, so it wasclear that we needed some other way of telling whether something is acollection class. LINQ to the rescue once more: With modified versionsof the query it was easy to establish that among our public classeswith public constructors there are:

<<<<<< 189 that have a public Add method and implement System.Collections.IEnumerable

<<<<<< 42 that have a public Add method but do not implement System.Collections.IEnumerable

If youlook at the classes returned by these two queries, you realize thatthere are essentially two fundamentally different meanings of the name .Add.:

a) Insert the argument into a collection, or

b) Return the arithmetic sum of the argument and the receiver.

People are actually very good at (directly or indirectly) implementing the nongeneric IEnumerable interface when writing collection classes, so that turns out to be a pretty reliable indicator of whether an Add method is the first or the second kind. Thus for our purposes the operational answer to the headline question becomes:Continues…