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Right, let s go back to our FireStation class for a minute, and imagine an interface we could create to formalize the contract for clocking in: our billing system might define this contract for us so that we can plug into it.

As it happens, our FireStation provides an implementation which can ClockIn named people, but our billing system s IClockIn contract is much more generic it can clock in anything of type Object, as we had in our original implementation:

interface IClockIn { void ClockIn(object item); }

We can now implement IClockIn on our FireStation, as shown in Example 4-27.

In a JavaScript object, the purpose of the prototype object is to hold all the properties that will be inherited by all the instances. The prototype object defines the structure of an object, in a manner similar to what is done with classes in many object-oriented languages. In the previous section, you saw how a function the constructor can be used to create custom objects and to add properties to the instances. Listing 3.3 shows how you can use the constructor s prototype object to add additional properties and methods to instances.

For example, the following code shows method MyMethod, again, but this time the parameters are reference parameters rather than value parameters. class MyClass { public int Val = 20; }

class FireStation : IClockIn { List<INamedPerson> clockedInStaff = new List<INamedPerson>(); public void ClockIn(INamedPerson staffMember) { if (!clockedInStaff.Contains(staffMember)) { clockedInStaff.Add(staffMember); Console.WriteLine("Clocked in {0}", staffMember.Name); } } public void RollCall() { foreach (INamedPerson staffMember in clockedInStaff) { Console.WriteLine(staffMember.Name); } } public void ClockIn(object item) { // What to do here } }

Our original ClockIn method is unchanged, and we ve added a new overload that takes an object, and therefore matches the requirement in our interface. But how do we implement that new method We want to check that the person being clocked in is an INamedPerson, and if it is, perform our usual operation. Otherwise, we want to tell the user that we can t clock him in. In other words, we need a manual check for the type of the object.

C# provides us with a couple of keywords for checking the type of an object: as and is. Here s how we can use them in our ClockIn implementation:

public void ClockIn(object item) { if (item is INamedPerson) { ClockIn(item as INamedPerson); } else { Console.WriteLine("We can't check in a '{0}'", item.GetType()); } }

class Program ref modifier ref modifier { static void MyMethod(ref MyClass f1, ref int f2) { f1.Val = f1.Val + 5; // Add 5 to field of f1 param. f2 = f2 + 5; // Add 5 to second param. } static void Main() { MyClass A1 = new MyClass(); int A2 = 10; MyMethod(ref A1, ref A2); ref modifiers // Call the method.

Notice how we are using the type name to check if the item is of that type. And then we call our other overload of ClockIn by explicitly converting to a reference of our INamedPerson type, using as. It checks to see if our object would be accessible through a reference of the specified type. It looks at the whole inheritance hierarchy for the object (up and down) to see if it matches, and if it does, it provides us a reference of the relevant type. What if you don t bother with the is check and just use as Conveniently, the as operation just converts to a null reference if it can t find a suitable type match:

public void ClockIn(object item) { INamedPerson namedPerson = item as INamedPerson; if(namedPerson != null) { ClockIn(namedPerson); } else { Console.WriteLine("We can't check in a '{0}'", item.GetType()); } }

This is the form in which you most often see a test like this, because it is marginally more efficient than the previous example. In the first version, the runtime has to perform the expensive runtime type checking twice: once for the if() statement and once to see whether we can actually perform the conversion, or whether null is required. In the second case, we do the expensive check only once, and then do a simple test for null.

Summary

So far, we ve seen how to create classes; to model relationships between instances of those classes through association, composition, and aggregation; and to create relationships between classes by derivation. We also saw how virtual functions enable derived classes to replace selected aspects of a base class. We saw how to use protected and protected internal to control the visibility of members to derived classes. Then, we saw how we can use either abstract classes and methods or interfaces to define public contracts for a class. Finally, we looked at a means of examining the inheritance hierarchy by hand, and verifying whether an object we are referencing through a base class is, in fact, an instance of a more derived class. In the next chapter, we are going to look at some other techniques for code reuse and extensibility that don t rely on inheritance.

function Cat() { this._name; this._age; } Cat.prototype.speak = function() { alert("Meeeeooow!"); }

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