67 Chapter 3 Learning Just Enough C# and VB.NET: Types and Members 68 Microsoft Visual Studio 2010: A Beginner’s Guide Key Skills & Concepts ● Create Classes ● Write Methods ● Code Fields and Properties A type is a general term for classes, modules, enums, and more. This chapter will specifically discuss the class type, which allows you to create your own custom types. You’ll also see the value of a class when you learn about class members. You’ll see how the field, method, and property class members can be used. We’ll start with learning how to create and use classes. Creating Classes Previously, you learned about the primitive types, which are built into languages and alias the underlying .NET types. You can also create your own types, via classes, which you can instantiate and create objects with. The following section explains how to create a class and then instantiate an object from it. Class Syntax To create a new custom class definition, right-click the project, select Add | Class, name the class Employee for this example, and type the file extension .cs for C# or .vb for VB; then click Add (VS will add this file extension for you if you don’t). You’ll see a file with the same code as Listing 3-1. Listing 3-1 A new Employee class C#: using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace FirstProgram Chapter 3: Learning Just Enough C# and VB.NET: Types and Members 69 { public class Employee { public string FirstName; } } VB: Public Class Employee Public Dim FirstName As String End Class The C# Employee class is nearly the same as the Program class that you created in the preceding chapter, except that the class name here is Employee. In VB, you’ve only created a module before, and the Employee class is your first class for this book. You can add members to a class, which could be events, fields, methods, and properties. Listing 3-1 shows an example of a field, FirstName, and you’ll learn about events, methods, and properties in later sections of this chapter. A field is a variable in a class that holds information specific to that class. Listing 3-2 shows how to instantiate an object of type Employee, which is your new custom type, and use it. You would put this code inside of Main or another method. You’ll learn more about methods in the later section “Writing Methods.” Listing 3-2 Code that uses a class C#: Employee emp = new Employee(); emp.FirstName = "Joe"; VB: Dim emp As New Employee emp.FirstName = "Joe" In Listing 3-2, you can see that emp is a variable declared as type Employee. The C# new Employee() or VB New Employee clause creates a new instance of Employee, and you can see that this new instance is being assigned to emp. With that new instance, via the emp variable, you can access the Employee object, including its instance members. In Listing 3-2, the FirstName field of that particular instance of Employee is assigned a string value of "Joe". Here you see that an object can contain data. 70 Microsoft Visual Studio 2010: A Beginner’s Guide Now that you can define a new class, create an instance from that class, and use it, the next section shows you another feature of classes called inheritance. Class Inheritance One class can reuse the members of another through a feature known as inheritance. In programming terms, we say a child class can derive from a parent class and that child class will inherit members (such as fields and methods) of the parent class that the parent class allows to be inherited. The following example will create a Cashier class that derives from the Employee class. To create this class, right-click the project, select Add | Class, and name the class Cashier. Listing 3-3 shows the new class and modifications for implementing inheritance. Listing 3-3 Class inheritance C#: using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace FirstProgram { public class Cashier : Employee { } } VB: Public Class Cashier Inherits Employee End Class The C# inheritance relationship is indicated by the colon after the Cashier identifier, followed by the class being derived from, Employee. In VB, you write the keyword Inherits, on a new line, followed by the class being derived from. Essentially, this means that Cashier has all of the same members as Employee. Listing 3-4 demonstrates the benefits of inheritance. Chapter 3: Learning Just Enough C# and VB.NET: Types and Members 71 Listing 3-4 Code using inheritance C#: Cashier cashr = new Cashier(); cashr.FirstName = "May"; VB: Dim cashr As New Cashier cashr.FirstName = "May" According to Listing 3-4, Cashier does not have a field named FirstName. However, Employee does have a FirstName field and Cashier derives from Employee. Because of inheritance, Cashier automatically inherits FirstName, and the code in Listing 3-4 is perfectly legal. Inheritance can be thought of as specialization in the sense that, in this example, Cashier is a specialized kind of Employee. To take advantage of this specialization, you could add a new field to your new Cashier class called “assignedCashRegister” where now, not only does the Cashier class have the fields and methods of Employee, it is able to hold the value for a specific cash register name or number. An instance of the Employee class would not be able to contain this information. The .NET Framework uses inheritance extensively to offer you reusable class libraries. TIP You can often use the phrase “is a” to describe the relationship between inherited classes when starting from the child class. For example, you can say “Cashier is an Employee.” If you apply this phrase technique to your software design and the sentence sounds logically correct, then you’ve probably used inheritance correctly. The class Snippet C# has a class snippet, but VB doesn’t. Before using the class snippet, create a new class file by right-clicking the project, select Add | New Item | Code File, and name the file Manager. You’ll now have a blank file to work with. To use the class snippet, type cl and press TAB, TAB; and you’ll see the snippet template in Figure 3-1. Figure 3-1 The C# class snippet template 72 Microsoft Visual Studio 2010: A Beginner’s Guide Just type in the class name in the field and press ENTER. The carat will locate to the inside of the class block. Now that you know how to create classes, you’ll need to know how to add members, starting with methods. Writing Methods You can divide your algorithms into blocks of code called methods. In different programming languages, methods are called functions, procedures, or subroutines. I’ll use the term method as a generic term, except when I need to be more specific. You’ve already used methods when coding Console.WriteLine, where WriteLine is a method of the Console class. A method contains one or more statements. Reasons for creating methods include the ability to modularize your code, isolate complex operations in one place, or group a common operation that can be reused in multiple places. The following sections show you how to declare and use methods. Declaring and Using a Method To start off, I’ll show you a very simple method so that you can see the syntax and understand the program flow. Listing 3-5 will move the Console.Writeline statement from the Main method discussed in Chapter 2 into a new containing method and then add a statement to the Main method that calls the new method. Listing 3-5 Declaring and calling a method C# (Program.cs) using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace FirstProgram { class Program { static void Main(string[] args) { MessagePrinter msgPrint = new MessagePrinter(); msgPrint.PrintMessageInstance(); } } . 67 Chapter 3 Learning Just Enough C# and VB.NET: Types and Members 68 Microsoft Visual Studio 2010: A Beginner’s Guide Key Skills & Concepts ● Create Classes ● Write Methods ●. FirstName field of that particular instance of Employee is assigned a string value of "Joe". Here you see that an object can contain data. 70 Microsoft Visual Studio 2010: A Beginner’s. the snippet template in Figure 3-1. Figure 3-1 The C# class snippet template 72 Microsoft Visual Studio 2010: A Beginner’s Guide Just type in the class name in the field and press ENTER. The carat