Collection classes enable you to store sets of items by using existing data structures, without concern for how they’re implemented. This is a nice example of code reuse. You can code faster and expect excellent performance, maximizing execution speed and minimizing memory consumption. In this chapter, we discuss

• the collection interfaces that declare each collection type’s capabilities

• the implementation classes

• the enumerators that iterate through collections (these are like iterators in languages like C++ and Java).

The .NET Framework provides several namespaces dedicated to collections:

• System.Collections contains collections that store objects—similar to the data structures we defined in Chapter 19. Such collections can store objects of many different types at the same time, because all C# types derive directly or indirectly from object. You might encounter this namespace’s classes, such as ArrayList, Stack and Hashtable, in C# legacy code prior to the introduction of generics in C# 2.0 (2005). Legacy code uses older programming techniques—possibly including language and library features that a programming language no longer supports or that have been superseded by newer capabilities.

• System.Collections.Generic contains generic collections, such as the List<T> (Section 9.4) and Dictionary<K, V> (Section 17.9) classes, that store objects of types you specify when you create the collection. You should use the generic collections—rather than the object-based legacy collections—to take advantage of compile-time type checking of your collection-processing code.

• System.Collections.Concurrent contains so-called thread-safe generic collections for use in multithreaded applications.

• System.Collections.Specialized contains collections that are optimized for specific scenarios, such as manipulating collections of bits.

In Section 14.3.3, we introduced the concept of a delegate—an object that holds a reference to a method. Delegates enable apps to store methods as data and to pass a method as an argument to another method. In event handling, a delegate stores a reference to the event-handler method that will be called when a user interacts with a GUI control. In this chapter, we’ll discuss delegates in more detail and introduce lambda expressions, which allow you to define anonymous methods that can be used with delegates. Here we’ll focus on using lambdas to pass method references to methods that specify delegate parameters.

So far, we’ve demonstrated three programming paradigms:

• structured programming (also known as procedural programming)

• object-oriented programming

• generic programming (which we’ll continue discussing in this chapter).

Sections 21.10–21.11 define and introduce functional programming, showing how to use it with LINQ to Objects to write code more concisely and with fewer bugs than programs written with other techniques. In Section 21.12, with one additional method call, we’ll demonstrate how PLINQ (Parallel LINQ) can improve LINQ to Objects performance substantially on multicore systems. In the exercises, we’ll also ask you to revisit earlier examples and implement them using functional-programming techniques.