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CHAPTER 17 INTERFACES
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Example Using the IComparable Interface
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To understand what this means and why it s useful, let s start by taking a look at the following code, which takes an unsorted array of integers and sorts them in ascending order. The first line creates an array of five integers that are in no particular order. The second line uses the static Sort method of the Array class to sort the elements. The foreach loop prints them out, showing that the integers are now in ascending order. int[] MyInt = new int[5] { 20, 4, 16, 9, 2 }; // Create an array of ints. Array.Sort(MyInt); foreach (int i in MyInt) Console.Write("{0} ", i); This code produces the following output: // Sort elements by magnitude. // Print them out.
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The Sort method works great on an array of ints, but what would happen if you were to try to use it on one of your own classes, as shown here class MyClass { public int TheValue; } ... MyClass[] mc= new MyClass[5]; ... Array.Sort(mc); // Declare a simple class.
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// Create an array of five elements. // Create and initialize the elements. // Try to use Sort--raises exception
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When you try to run this code, it raises an exception. So why did it work for an array of ints, but not for an array of MyClass objects The reason Sort doesn t work with the array of user-defined objects is that it relies on the objects in the array to implement interface IComparable. When Sort is running, it compares one element of the array to another by calling the element s CompareTo method and passing in as a parameter a reference to the other element. The int type implements IComparable, but MyClass does not, so when Sort tries to call the nonexistent CompareTo method of MyClass, it raises an exception.
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CHAPTER 17 INTERFACES
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So how can you make the Sort method work with objects of type MyClass To implement an interface, a class or struct must do two things: It must list the interface name in its base class list. It must provide an implementation for each of the interface s members. For example, the following code updates MyClass to implement interface IComparable. Notice the following about the code: The name of the interface is listed in the base class list of the class declaration. The class implements a method called CompareTo, whose parameter type and return type match those of the interface member. Method CompareTo is implemented following the definition given in the interface s documentation. That is, it returns a negative 1, positive 1, or 0, depending on its value compared to the object passed into the method. Interface name in base class list class MyClass : IComparable { public int TheValue; public int CompareTo(object obj) // Implementation of interface method { MyClass mc = (MyClass)obj; if (this.TheValue < mc.TheValue) return -1; if (this.TheValue > mc.TheValue) return 1; return 0; } } Figure 17-2 illustrates the updated class. The arrow from the grayed interface method to the class method indicates that the interface method does not contain code, but is implemented by the class-level method.
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Figure 17-2. Implementing IComparable in MyClass
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CHAPTER 17 INTERFACES
Now that MyClass implements IComparable, Sort will work on it as well. It would not, by the way, have been sufficient to just declare the CompareTo method it must be part of implementing the interface, which means placing the interface name in the base class list. The following shows the complete updated code, which can now use the Sort method to sort an array of MyClass objects. Main creates and initializes an array of MyClass objects and then prints them out. It then calls Sort and prints them out again to show that they have been sorted. class MyClass: IComparable // Class implements interface { public int TheValue; public int CompareTo(object obj) // Implement the method. { MyClass mc = (MyClass)obj; if (this.TheValue < mc.TheValue) return -1; if (this.TheValue > mc.TheValue) return 1; return 0; } } class Program { static void PrintOut( string s, MyClass[] ma) { Console.Write(s); foreach (MyClass i in ma) Console.Write("{0} ", i.TheValue); Console.WriteLine(""); } static void Main() { int[] MyInt = new int[5] { 20, 4, 16, 9, 2 }; MyClass[] MyMc = new MyClass[5]; for (int i = 0; i < 5; i++) { MyMc[i] = new MyClass(); MyMc[i].TheValue = MyInt[i]; } // Create array of MyClass objs // Initialize the array.
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