Lesson 7: Introduction to Classes

This lesson introduces you to C# Classes. Our objectives are as follows:

• Implement Constructors.
• Know the difference between instance and static members.
• Understand Destructors.
• Familiarization with Class Members.

Since the beginning of this tutorial, you have been using classes. By now, you should have a sense of what a class is for and how to specify one. This lesson will build upon what you already know and introduce the various class members.
Classes are declared by using the keyword class followed by the class name and a set of class members surrounded by curly braces. Every class has a constructor, which is called automatically any time an instance of a class is created. The purpose of constructors is to initialize class members when an instance of the class is created. Constructors do not have return values and always have the same name as the class. Listing 7-1 is an example of a class.

Listing 7-1. Example C# Classes: Classes.cs

// Namespace Declaration
using System;

// helper class
class OutputClass
{
    string myString;

    // Constructor
    public OutputClass(string inputString)
    {
        myString = inputString;
    }

    // Instance Method
    public void printString()
    {
        Console.WriteLine("{0}", myString);
    }

    // Destructor
    ~OutputClass()
    {
        // Some resource cleanup routines
    }
}

// Program start class
class ExampleClass
{
    // Main begins program execution.
    public static void Main()
    {
        // Instance of OutputClass
        OutputClass outCl = new OutputClass("This is printed by the output class.");

        // Call Output class' method
        outCl.printString();
    }
}

Listing 7-1 shows two classes. The top class, OutputClass, has a constructor, instance method, and a destructor. It also had a field named myString. Notice how the OutputClass constructor is used to initialize data members of the class. In this case, the OutputClass constructor accepts a string argument, inputString. This string is copied to the class field myString.
Constructors are not mandatory, as indicated by the implementation of ExampleClass. In this case, a default constructor is provided. A default constructor is simply a constructor with no arguments. However, a constructor with no arguments is not always useful. To make default constructors more useful, you can implement them with initializers. Here is an example:
    public OutputClass() : this("Default Constructor String") { }
Imagine this constructor was included in class OutputClass from Listing 7-1. This default constructor is followed by an initializer. The colon, ":", marks the beginning of the initializer, followed by the this keyword. The this keyword refers to this particular object. It effectively makes a call to the constructor of the same object it is defined in. After the this keyword is a parameter list with a string. The action taken by the initializer above is to invoke the OutputClass constructor that takes a string type as an argument. The initializer helps you to ensure your class fields are initialized when a class is instantiated.
The example above illustrates how a class can have multiple constructors. The specific constructor called depends on the number of parameters and the type of each parameter.
In C#, there are two types of class members, instance and static. Instance class members belong to a specific occurrence of a class. Every time you declare an object of a certain class, you create a new instance of that class. The ExampleClass Main() method creates an instance of the OutputClass named outCl. You can create multiple instances of OutputClass with different names. Each of these instances are separate and stand alone. For example, if you create two OutputClass instances as follows:
    OutputClass oc1 = new OutputClass("OutputClass1");
    OutputClass oc2 = new OutputClass("OutputClass2");
You create two separate instances of OutputClass with separate myString fields and separate printString() methods. On the other hand, if a class member is static, you can access it simply by using the syntax .. The instance names are oc1 and oc2.
Suppose OutputClass had the following static method:
    public static void staticPrinter()
    {
        Console.WriteLine("There is only one of me.");
    }
Then you could call that function from Main() like this:

OutputClass.staticPrinter();

You must call static class members through their class name and not their instance name. This means that you don't need to instantiate a class to use its static members. There is only ever one copy of a static class member. A good use of static members is when there is a function to be performed and no intermediate state is required, such as math calculations. Matter of fact, the .NET Frameworks Base Class Library includes a Math class that makes extensive use of static members.
Another type of constructor is the static constructor. Use static constructor to initialize static fields in a class. You declare a static constructor by using the keyword static just in front of the constructor name. A static constructor is called before an instance of a class is created, before a static member is called, and before the static constructor of a derived class (covered in a later chapter). They are called only once.
OutputClass also has a destructor. Destructors look just like constructors, except they have a tilde, "~", in front of them. They don't take any parameters and do not return a value. Destructors are places where you could put code to release any resources your class was holding during its lifetime. They are normally called when the C# garbage collector decides to clean your object from memory.
Note: You've probably noticed the use of the public modifier (an access modifier), meaning that a class member can be accessed from other classes. When used on a class, it means that the class can be accessed by DLLs outside of the Assembly (which is commonly a *.exe or *.dll file). Lesson 19: Encapsulation discusses access modifiers in more depth.
So far, the only class members you've seen are Fields, Methods, Constructors, and Destructors. Here is a complete list of the types of members you can have in your classes:

• Constructors
• Destructors
• Fields
• Methods
• Properties
• Indexers
• Delegates
• Events
• Nested Classes

Those items not covered in this lesson will be covered in later lessons.
In summary, you can declare instance and static constructors. You know how to initialize class fields. When there is no need to instantiate an object, you can create static class members. You can also declare destructors for cleaning up resources.