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CHAPTER 1 COUPLING
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Signature Coupling
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As you ve seen, type coupling is related to classes and interfaces, but coupling can occur at a finer level. If you look at the contents of a class, you find fields and methods. It is reasonable to expect that coupling might occur not only at the class level, but also at the field and method level. Fields are really just classes (or scalars) used inside a class, so you can treat coupling to a field exactly like ordinary type coupling. Methods are a completely different kind of entity from classes. They are not described by a formal type, but by their signature. The signature specifies the number and types of parameters that a caller passes to and from a method. When you write a method, you implicitly define its signature. Just as an interface represents the type of an object, so a signature represents the type of a method. As an example, consider the following C# class: public class A { public int GetAge(string theName) { // ... } } The signature for the GetAge method is int f(string theName) There are three things missing from the signature: The type to which the method belongs: Signatures describe methods outside the context of classes and interfaces, so they don t specify a class or interface. The method s access qualifier: The signature doesn t have an access qualifier (e.g., public or private), because qualifiers don t describe the method, but rather the method s accessibility, which is a class-level property. The method name: The signature doesn t have any particular method name, because signatures are abstract: They don t identify a concrete method. A given signature can apply to any number of methods. The letter f is used in the signature above to act as a placeholder for the method name. In the signature, you can use any word as the method name, including the name of a particular method, so you can also write the signature of GetAge like this: int GetAge(string theName) Any method that takes a single string argument as input and returns an integer has the same signature as A.GetAge. Now that you know what a signature is, let s see what signature coupling is. This flavor of coupling occurs when one object makes an indirect call to a method in another object. The caller must have a pointer that is initialized at run time to point directly to a method, bypassing any class interfaces of the callee. Figure 1-18 shows two classes that might be involved in signature coupling.
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A -methodReference
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B +Method1()
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Figure 1-18. Two classes with no relationships between them
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CHAPTER 1 COUPLING
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Class A has a field called methodReference that is used to make indirect calls to a method. Although the class diagram doesn t show the signature used by A.methodReference, assume the signature is the same as that of B.Method1. The class diagram reveals no information about coupling or other relationships between A and B. Classes A and B are completely decoupled from each other. It is perfectly possible that different people implemented the classes, not predicting that the classes would ever be used together. Assume that at run time there are instances of A and B, called respectively object1 and object2, and that object1.methodReference is initialized to point to object2.Method1, as shown in Figure 1-19.
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object2: B object1: A methodReference Method1()
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Figure 1-19. Objects coupled through a method signature At run time, object1 becomes signature-coupled to object2, due to the method reference pointing at object2.Method1. The signature used by object1.methodReference must match the signature of object2.Method1.
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Definition 7
Signature coupling occurs when one object has a reference pointing to a method in another object.
Signature coupling (SC) is designated using the symbol subscripted with an uppercase S, as shown in the previous diagram. When one object is signature-coupled to another, the former requires the latter to be present at run time. The system shown in Figure 1-19 might be implemented using the C# code in Listing 1-7. Listing 1-7. A C# Implementation of the Example Shown in Figure 1-19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 namespace Signature_Coupling.Example1 { // declare a reference-to-method type public delegate void SimpleDelegate(); public class A { // declare a reference-to-method field public SimpleDelegate methodReference; // use the reference public void MakeIndirectCall() { // call the method pointed at by // the method reference methodReference(); } } public class B
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