Classes are a complex data type. Simple data types store values, such as integers and Booleans. Composite data types, such as structures that are capable of storing more than one value, aren't more sophisticated than integers and Booleans. The values that you can store by using classes, however, are beyond mere integers or sets of integers.

A class is a data type that's capable of storing data and actions. When you declare and initialize a variable by using a class data type, you create an object that stores data and action values defined by the class data type. Objects are used to accommodate the potential complexity of values stored for class data types. An object is often referred to as an instance of the class data type.

Classes act as a template for defining the data and actions of an object. You create objects by using the class as a template. Each object created from the class is like a blank form into which you can place data. The source of that data may be a database, data typed on a screen by a user, or another object.

Some people find it helpful to think of classes as the blueprints for objects — or to think that classes are abstract and that objects are concrete.

All classes inherit from the .NET class System.Object. Inheriting from System.Object provides all the basic services your classes need to function as objects and get along with other objects.

System.Object is the base class of your class, and your class is a derived class of System.Object.

Any time your classes derive from a base class, you should check the documentation of the base class to determine whether there are any methods you should override. When you override a member, you write your own code to implement the member.

For example, the System.Object class has a method called ToString that you should override. By overriding ToString in the classes you define, you can get a string representation of objects created from your class. See Chapter 5 of Book III for more information on overriding.

Several overridden methods are available for System.Object. Table 3-3 lists all the methods your class inherits from System.Object and identifies those that you should override.

Because classes are data types, you use them like you use any data type. They're reference types, so you use the new operator to create a new instance of a class.

The new operator calls the class's constructor method. A class may have more than one constructor. If no constructor method is defined in the class, the object is initialized by using the default constructor. The default constructor is the New() method defined in the System.Object class. Recall that all classes derive from System.Object.

To create a new instance of the Message class created in the preceding section, you type the following:

Message m = new Message("Hello");

You use dot notation to access the members of the class. The member's visibility is determined by the modifier set on the member's declaration. You can always see public members, but not private members. To call the ReverseContents member, type the following:

string s = m.ReverseContents();

Here's the equivalent code, using Visual Basic syntax:

Dim m As New Message("Hello")
Dim s As String = m.ReverseContents()

Hopefully, this syntax looks familiar to you. If you've used any features in the .NET Framework, you've encountered this syntax.

Class Designer is capable of designing more than just classes. You can use Class Designer to design or view any kind of data type, including

Class Designer can also create Visual Basic modules, which you can use to encapsulate your code. See the Visual Basic documentation for more information on the differences between classes and modules.

You add a new class diagram to your project using the Add New Item window. A project may have multiple class diagrams. To add a new class diagram

Class diagrams use the .cd filename extensions. Like most files in Visual Studio, the class diagram file is an XML file.

Class diagrams are visual representations of classes in a project. The diagrams are similar to diagrams created using the Unified Modeling Language (UML) specification.

Class Designer consists of the

The Designer provides a toolbar and shortcut menus to access additional features. The Designer works with Solution Explorer, Class View, and OTB.

Class Designer provides several commands you can use for working with your diagram. The commands that you can use depend on what you have selected in the diagram. You have three levels of commands for working with class diagrams:

Figure 3-1 shows the Class Designer toolbar. The first column of Table 3-4 lists the commands of the Class Designer toolbar, as they appear on the toolbar from left to right.

Figure 3-1. Use the Class Designer toolbar to control the diagram's layout.

The Class Designer menu provides you the opportunity to act upon the shapes that represent data types on the diagram. Table 3-5 lists the options available for data types you click in the class diagram.

You can access most of these commands from a shortcut menu when you right-click a shape in a class diagram. The shape's shortcut menu includes commands to delete the shape from the diagram and view the type's underlying source code.

One of the more exciting features of Class Designer is the ability to design classes. You can drag and drop data types on the class diagram; in the background, Visual Studio is generating the code to create the data type.

Creating a new class

Class Designer can create all kinds of data types. The Toolbox includes all the shapes you need to create data types and show relationships between data types.

To create a new class with Class Designer, follow these steps:

1. Add a new class diagram, following the steps in the previous section.

2. Press Ctrl+Alt+X to open the Toolbox.

   The Toolbox pane displays icons representing items you can add to the Class Designer. Open the Class Designer tab, if necessary, to see the controls you can use.

3. Drag and drop a Class icon from the Toolbox onto Class Designer.

   The New Class dialog box appears.

4. Type a name for the class in the Name text box.

   The example uses Message as the name.

5. Select an access modifier from the Access drop-down list.

   The public access modifier is accepted by default. The example uses public as the access modifier.

6. Type a new filename for the class or select an existing file to add the class.

   The example uses the Create New File option and Message.cs as the filename.

7. Click OK.

   The Class shape appears on Class Designer.

   Visual Studio generates the class declaration and places it in the file you created or selected in Step 6.

Figure 3-2 shows Class Designer with a class called Message. You can see the Message.cs source code that Visual Studio created in the pane next to the class diagram.

Figure 3-2. Visual Studio generates the class declaration for the new class.

To add more types to the class diagram, drag the type from the Toolbox onto the design surface. A dialog box, similar to the one in Step 3, appears.

Adding members to a class

You use Class Designer to add members to your class. When you add a member using the designer, you specify the following information about a member:

• Name

• Data type

• Access modifier

You can, optionally, supply a descriptive comment and mark whether to hide the member on the class diagram.

Visual Studio uses the information you supply to create a member declaration in your source code. You have two options for adding members to your class with Class Designer:

• Type the members in the Class Details pane. The Class Details pane shows the class's members grouped by methods, properties, fields, and events.

• Add the members to the class in the class diagram.

The class diagram and the source code are synchronized. Any members you declare directly in source code appear in the class diagram.

To add a new member via the Class Details pane

1. Click the section of the Class Details pane for the type of member you wish to add.

   If the Class Details pane is closed, choose View

   

2. Type the member's name in the line immediately below the section's header.

   For example, to add a new field, click the line below the Fields section where you see <add field>.

3. Press the Tab key.

   The cursor stops in the Type cell.

4. Type the member's data type.

   Tip

   IntelliSense works in the Class Details pane.

5. Press Tab and select the member's access modifier.

6. Press Tab and add an optional descriptive comment.

7. Click to place a check mark in the Hide column if you don't want the member to appear on the class diagram.

   Figure 3-3 shows a field (m_strMessage) and a property (Content) in the Class Details pane.

Figure 3-3. Use the Class Details pane to add new members.

Visual Studio generates the member declarations for you in the class's source code file.

The Class Details pane features a toolbar button with shortcuts for adding members. To access the shortcut button, click the arrow on the button, as shown in Figure 3-4.

Adding a new constructor method

A method's declaration is a little more complex than a field or property declaration because a method may accept parameters. In addition, multiple kinds of methods are available, such as constructors and destructors. The Class Details pane has no problems adding methods to your class.

Figure 3-4. Use the Class Details toolbar to add members.

To add a new constructor method, follow these simple steps:

1. Click the shortcut button on the Class Details toolbar.

2. Chose New Constructor.

   A new constructor method appears in the Methods section of the Class Details pane.

   Tip

   Constructors use the same name as the class in C#. In Visual Basic, constructors are named New.

3. Type the constructor's parameters in the lines below the constructor's declaration, as shown in Figure 3-5.

Figure 3-5. Add parameters to the constructor method.

Visual Studio generates the member declarations and stubs out the member's bodies. (The members don't perform any useful task — they simply provide a placeholder for code you add later.) Exceptions are placed inside the generated members to remind you that you need to add your code if you try to use the members. For example, the following code is the constructor generated for the preceding sample:

public Message(string message)
{
  throw new System.NotImplementedException();
}

Tip

Class Designer is a good tool to use for designing a class domain. Designers can lay out the classes and then pass off their implementation to a programmer.

Creating members using the class diagram

You can also create members by using the class diagram. To create a member in the class diagram, do the following:

1. Right-click the class in the class diagram.

2. From the contextual menu that appears, choose Add.

   A list of members appears, which you can add to the class.

3. Select the type of member you wish to add.

   The member appears in the diagram.

4. Type the member's name in the diagram, as shown in Figure 3-6.

   Use the Class Details pane to complete the member's declaration.

Figure 3-6. Type the member's name in the class diagram.

Creating associations

The nature of OOP is that classes interact. You create an interaction between classes when you create member declarations in your class. For example, if you declare a field of the string type, you create an association between your class and the string class.

In Class Designer, you show associations between only classes that are relevant to the model you're creating. You could show every association, but then your diagrams would be too crowded for people to understand. Instead, you should show only the associations that you want to draw attention to.

Here's how to create an association between two types:

1. Create both types on the class diagram.

2. Click the Association icon in the Toolbox.

3. Click the type where you want to draw the association.

   The cursor appears as a line, as shown in Figure 3-7.

4. Drag the line to the class.

   An association between the classes appears. Visual Studio adds the association as a property.

Figure 3-7. Drag the cursor between the two classes to create an association.

You can show associations between classes and collections. To show an association between a class and a strongly typed collection, follow these steps:

1. Add the member to your class that uses the strongly typed collection.

   For example, add a field that uses a generic collection based on another data type in your diagram. The example uses a field named m_Collection, which is of type Int32[] (an array).

2. Right-click the member on the class diagram.

3. From the contextual menu that appears, choose Show as Collection Association.

   An association between the classes appears, as shown in Figure 3-8.

Figure 3-8. Create an association between a class and a collection.

Class Designer is an excellent tool for viewing information about data types. You can view data types within your own project, as well as any types referenced by your project. This means that you can view types from the .NET Framework or a third party.

To view a type in your project, simply drag and drop the source code file from Solution Explorer to the class diagram.

You use Class View to drag referenced types from your project to the class diagram. For example, to add the type NullReferenceException to a class diagram, follow these steps:

Admittedly, viewing a single type on a class diagram isn't very exciting. You can use the Class Details pane to view the type's members. You can also view a type's inheritance hierarchy.

To view a type's base type, follow these steps:

Figure 3-9. Use Class Designer to view a type's inheritance hierarchy.

To view the class diagram for an entire project, follow these steps:

Figure 3-10. Create a class diagram for a project.