• Create a class
• Define and use constructors
• Use a class in a program

Custom Classes

• Select Project...Add Class to add a class to your project
• Define properties and methods
• Define constructors

Constructors

• Special Sub that instantiates the object
• Called when instantiation statement is used
  • x = New Thing()
• Written as

    Public Sub New([parameters here])
      [code goes here]
    End Sub

OverLoading

• Multiple methods with same name but different argument lists
• Example

Defining a Class in a Project

• Add Class file to a project
• Write code to define class
• Declare and instantiate objects in other forms and modules in project

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In this demo we will create a Class for a rectangle called MyRectangle.  Then we will instantiate objects of this class in another form.  

1.  Open your Unit 10 project.  Add a new class file by selecting Project...Add Class.  Name your class MyRectangle.

2. Now open the class file and copy and paste the following code into it. 

Private iWidth As Int16
Private iHeight As Int16

Public Sub New() 
   iWidth = 0
   iHeight = 0
End Sub

Public Sub New(ByVal width As Int16, ByVal height As Int16)
   iWidth = width
   iHeight = height
End Sub

Public Sub New(ByVal WidthandHeight As Int16)
   iWidth = WidthandHeight
   iHeight = WidthandHeight
End Sub

Public Function getWidth() As Int16
   Return iWidth
End Function

Public Sub setWidth(ByVal width As Int16)
   If width < 200 then 
      iWidth = width
   Else
      iWidth = 0
   End If
End Sub

Public Function getHeight() As Int16
   Return iHeight
End Function

Public Sub setHeight(ByVal height As Int16)
   iHeight = height
End Sub

Public Function calcArea()
   Return iHeight * iWidth
End Function

Public Sub DisplayRectangle(ByVal surface As Form, ByVal x As Int16, ByVal y As Int16)
   surface.CreateGraphics.DrawRectangle(New Pen(Color.Red), x, y, iWidth, iHeight)
End Sub

3.  Study the code carefully.  Notice that there are three different constructors.  Recall that this is an example of overloading.  The first contructor assigns values to the width and height of 0. The second constructor allows the user to enter values for the width and height.  The last constructor creates a square since it assigns the same value to both the width and height.  Notice the Get and Set procedures for assigning and retrieving the properties.  In the setWidth method we have included a condition to limit the size of the width.  We did this just to show how you can use Set methods to control what goes into the properties.  Finally notice the DisplayRectangle method which accepts a Form argument and then uses the built-in DrawRectangle method to draw a rectangle on the Form that is passed in.

4.  Now we will use this class in another form.  Create a new form and add a button (btnNewRectangle) and a textbox (txtSide ).  At the top of the form code window under the Inherits statement add the following code to declare a rectangle object.  We are declaring it at the top since we will use it in several different events.

Dim rectX As MyRectangle

5.  In the click event for the button add the following code.  This code creates an instance of MyRectangle and displays the area of it.  We are going to create a rectangle with equal sides so we will call the constructor that accepts the one parameter.  Also we will pass the value of the textbox into the constructor.

rectX = New MyRectangle(txtSide.Text)
MessageBox.Show(rectX.calcArea)

6. Now add the following to the MouseDown event for the form.  This code will cause the rectangle to be drawn on the form where the user clicks. 

rectX.DisplayRectangle(Me, e.X, e.Y)

7.  Run the program and experiment. 

1.  We can create classes for almost anything.  Create your own class.  Make sure that it has the following: 

1. At least one property
2. Get and Set methods to return the value of the property and assign a value to the property.
3. Two constructors.  One should accept no parameters and the other should allow the user to specify the value(s) of the properties.
4. One method that does something with the properties.

2.  Create a class for a "BullsEye".  The class will be used for drawing a "BullsEye" target on a form.  The class will be similar to the Rectangle class created in the demonstration.  Your BullsEye will consist of two concentric circles (one inside the other with same centers).  Your class should provide the following features:

1. Should allow the user to set the radius of the inner circle and the radius of the outer circle.
2. Should allow the user to set the colors of the two circles (you can declare a variable as a color type).
3. Should provide one default constructor which accepts no parameters and then creates a "default" BullsEye (you determine the colors and radii).
4. Should provide a constructor that enables the user to specify the radii and colors. You should make sure that the inner radius is smaller then the outer radius.
5. Should provide a method for displaying the BullsEye.  The method should accept a form and the x and y locations of where to display it.

3.  Create a program that will use your class and allow the user to create and display BulleEyes.

4. What is "Overloading"?

5.  What is a constructor?

• Search for "Overloaded Properties and Methods"
• Search for "Procedure Overloading"
• Search for "7.2 Constructors"
• Search for "Using Constructors and Destructors"