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C H A P T E R 19
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What Are Generics Generics in C# Generic Classes Declaring a Generic Class Creating a Constructed Type Creating Variables and Instances Constraints on Type Parameters Generic Methods Extension Methods with Generic Classes Generic Structs Generic Delegates Generic Interfaces Covariance and Contravariance in Generics
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With the language constructs you ve learned so far, you can build powerful objects of many different types. You do this mostly by declaring classes that encapsulate the behavior you want and then creating instances of those classes. All the types used in the class declarations so far have been specific types either programmerdefined or supplied by the language or the BCL. There are times, however, when a class would be more useful if you could distill or refactor out its actions and apply them not just to the data types for which they are coded but for other types as well. Generics allow you to do just that. You can refactor your code and add an additional layer of abstraction so that, for certain kinds of code, the data types are not hard-coded. This is particularly designed for cases in which there are multiple sections of code performing the same instructions, but on different data types. That might sound pretty abstract, so we ll start with an example that should make things clearer.
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A Stack Example
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Suppose first that you have created the following code, which declares a class called MyIntStack, which implements a stack of ints. It allows you to push ints onto the stack and pop them off. This, by the way, isn t the system stack. class MyIntStack { int StackPointer = 0; int[] StackArray; int int public void Push( int x ) { ... } int public int Pop() { ... } ... } // Stack for ints // Array of int // Input type: int
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// Return type: int
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Suppose now that you would like the same functionality for values of type float. There are several ways you could achieve this. One way is to perform the following steps to produce the subsequent code: Cut and paste the code for class MyIntStack. Change the class name to MyFloatStack. Change the appropriate int declarations to float declarations throughout the class declaration. // Stack for floats // Array of float // Input type: float
class MyFloatStack { int StackPointer = 0; float [] StackArray; float float public void Push( float x ) { ... } float public float Pop() { ... } ... }
// Return type: float
This method certainly works, but it s error-prone, and has the following drawbacks: You need to inspect every part of the class carefully to determine which type declarations need to be changed and which should be left alone. You need to repeat the process for each new type of stack class you need (long, double, string, and so on). After the process, you end up with multiple copies of nearly identical code, taking up additional space. Debugging and maintaining the parallel implementations is inelegant and error-prone.
GENERICS
Generics in C#
With C# 2.0, Microsoft introduced the generics features, which offer more elegant ways of using a set of code with more than one type. Generics allow you to declare type-parameterized code, which you can instantiate with different types. This means you can write the code with placeholders for types and then supply the actual types when you create an instance of the class. By this point in the text, you should be very familiar with the concept that a type is not an object but a template for an object. In the same way, a generic type is not a type but a template for a type. Figure 19-1 illustrates this point.
Figure 19-1. Generic types are templates for types. C# provides five kinds of generics: classes, structs, interfaces, delegates, and methods. Notice that the first four are types, and methods are members. Figure 19-2 shows how generic types fit in with the other types covered.
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