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CHAPTER 7 CLASSES AND INHERITANCE
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So far in this text, every method you ve seen has been associated with the class in which it is declared. The extension method feature of C# 3.0 extends that boundary, allowing you to write methods associated with classes other than the class in which they are declared. To see how you might use this feature, take a look at the following code. It contains class MyData, which stores three values of type double, and contains a constructor and a method called Sum, which returns the sum of the three stored values. class MyData { private double D1; private double D2; private double D3; public MyData(double d1, double d2, double d3) { D1 = d1; D2 = d2; D3 = d3; } public double Sum() { return D1 + D2 + D3; } } This is a pretty limited class, but suppose it would be more useful if it contained another method, which returned the average of the three data points. With what you know so far about classes, there are several ways you might implement the additional functionality: If you have the source code and can modify the class, you could, of course, just add the new method to the class. If, however, you can t modify the class for example, if the class is in a third-party class library then, as long as it isn t sealed, you could use it as a base class and implement the additional method in a class derived from it. If, however, you don t have access to the code, or the class is sealed, or there is some other design reason that neither of these solutions will work, then you will have to write a method in another class that uses the publicly available members of the class.
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// Fields
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// Constructor
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// Method Sum
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CHAPTER 7 CLASSES AND INHERITANCE
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For example, you might write a class like the one in the following code. It contains a static class called ExtendMyData, which contains a static method called Average, which implements the additional functionality. Notice that the method takes an instance of MyData as a parameter. static class ExtendMyData Instance of MyData class { public static double Average( MyData md ) { return md.Sum() / 3; } } Use the instance of MyData.
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class Program { static void Main() { Instance of MyData MyData md = new MyData(3, 4, 5); Console.WriteLine("Average: {0}", ExtendMyData.Average(md)); } } Call the static method. This code produces the following output:
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Average: 4
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Although this is a perfectly fine solution, it would be more elegant if you could call the method on the class instance itself, rather than creating an instance of another class to act on it. The following two lines of code illustrate the difference. The first uses the method just shown invoking a static method on an instance of another class. The second shows the form we would like to use invoking an instance method on the object itself. Extension methods allow you to use the second form, even though the first form would be the normal way of writing the invocation. ExtendMyData.Average( md ) md.Average(); // Static invocation form // Instance invocation form
CHAPTER 7 CLASSES AND INHERITANCE
By making a small change in the declaration of method Average, you can use the instance invocation form. The change you need to make is to add the keyword this before the type name in the parameter declaration as shown following. Adding the this keyword to the first parameter of the static method of the static class changes it from a regular method of class ExtendMyData into an extension method of class MyData. You can now use both invocation forms. Must be a static class static class ExtendMyData { Must be public and static Keyword and type public static double Average( this MyData md ) { ... ) } The important requirements for an extension method are the following: The extension method must be declared static. The class in which the extension method is declared must also be declared static. The extension method must contain as its first parameter type the keyword this, followed by the name of the class it is extending. Figure 7-21 illustrates the structure of an extension method.
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