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Programming C# 4.0 by Jesse Liberty, Matthew Adams, Ian Griffiths

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Chapter 5. Composability and Extensibility with Delegates

In the preceding two chapters, we saw how to encapsulate behavior and information with classes. Using the concepts of association, composition, aggregation, and derivation, we modeled relationships between those classes and looked at some of the benefits of polymorphism along with the use and abuse of virtual functions and their implied contracts with derived classes.

In this chapter, we’ll look at a functional (rather than class-based) approach to composition and extensibility, and see how we can use this to implement some of the patterns that have previously required us to burn our one and only base class and override virtual functions; and all with the added benefit of a looser coupling between classes.

Let’s start with another example. This time, we want to build a system that processes incoming (electronic) documents prior to publication. We might want to do an automated spellcheck, repaginate, perform a machine translation for a foreign-language website, or perform one of any other number of operations that our editors will devise during the development process and beyond.

After some business analysis, our platform team has given us a class called Document, which is shown in Example 5-1. This is their baby, and we’re not allowed to mess with it.

Example 5-1. The Document class

public sealed class Document { // Get/set document text public string Text { get; set; } // Date of the document public DateTime DocumentDate { get; ...

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