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You can declare multiple fields of the same type in the same statement by separating the names with commas. You cannot mix different types in a single declaration. For example, you can combine the four preceding field declarations into two statements, with the exact same semantic result: int F1, F3 = 25; string F2, F4 = "abcd";
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A method is a named block of executable code that can be executed from many different parts of the program, and even from other programs. (There are also anonymous methods, which aren t named but I ll cover those in 15.) When a method is called, or invoked, it executes its code and then returns to the code that called it. Some methods return a value to the position from which they were called. Methods correspond to member functions in C++. The minimum syntax for declaring a method includes the following components: Return type: This states the type of value the method returns. If a method doesn t return a value, the return type is specified as void. Name: This is the name of the method. Parameter list: This consists of at least an empty set of matching parentheses. If there are parameters (which I ll cover in the next chapter), they are listed between the parentheses. Method body: This consists of a matching set of curly braces, containing the executable code.
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For example, the following code declares a class with a simple method called PrintNums. From the declaration, you can tell the following about PrintNums: It returns no value; hence, the return type is specified as void. It has an empty parameter list. It contains two lines of code in its method body.
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class SimpleClass { Return type Parameter list void PrintNums( ) { Console.WriteLine("1"); Console.WriteLine("2"); } }
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Note Unlike C and C++, there are no global functions (that is, methods or functions) declared outside of a type declaration. Also, unlike C and C++, there is no default return type for a method. All methods must include a return type or list it as void.
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The class declaration is just the blueprint from which instances of the class are created. Once a class is declared, you can create instances of the class. Classes are reference types, which, as you will remember from the previous chapter, means that they require memory both for the reference to the data and for the actual data. The reference to the data is stored in a variable of the class type. So, to create an instance of the class, you need to start by declaring a variable of the class type. If the variable isn t initialized, its value is undefined.
Figure 4-2 illustrates how to define the variable to hold the reference. At the top of the code on the left is a declaration for class Dealer. Below that is a declaration for class Program, which contains method Main. Main declares variable theDealer of type Dealer. Since the variable is uninitialized, its value is undefined, as shown on the right in the figure.
Figure 4-2. Allocating memory for the reference of a class variable
CLASSES: THE BASICS
Allocating Memory for the Data
Declaring the variable of the class type allocates the memory to hold the reference, but not the memory to hold the actual data of the class object. To allocate memory for the actual data, you use the new operator. The new operator allocates and initializes memory for an instance of any specified type. It allocates the memory from either the stack or the heap, depending on the type. Use the new operator to form an object-creation expression, which consists of the following: The keyword new. The name of the type of the instance for which memory is to be allocated. Matching parentheses, which might or might not include parameters. I ll discuss more about the possible parameters later. Keyword Parentheses are required. new TypeName ( ) Type If the memory allocated is for a reference type, the object-creation expression returns a reference to the allocated and initialized instance of the object in the heap.
This is exactly what you need to allocate and initialize the memory to hold the class instance data. Use the new operator to create an object-creation expression, and assign the value returned by it to the class variable. Here s an example: Dealer theDealer; // Declare variable for the reference. theDealer = new Dealer(); // Allocate memory for the class object. Object-creation expression The code on the left in Figure 4-3 shows the new operator used to allocate memory and create an instance of class Dealer, which is then assigned to the class variable. The memory structure is illustrated in the figure, to the right of the code.
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