Public Class Circle
    Public rad As Double
End Class

File: Exercise.vb

Module Exercise

    Public Function Main() As Integer
        Dim circ As Circle = New Circle
        circ.rad = 25.84

        Console.WriteLine(" -=- Circle Characteristics -=-")
        Console.WriteLine("Radius:        {0}", circ.rad)

        Return 0
    End Function

End Module

When you create the member variables of a class or of a structure, such as the above Radius of the Circle class, it is a good idea not to directly expose them to other parts of the program so that those other parts would not be able to easily change the values of the members and retrieve their values anyhow. This technique makes sure that a member variable is not accessed outside the class (or structure) so that the clients of the class (or structure) cannot directly influence the value of the member variable. To avoid this type of access, you can make the member variable(s) private. This would transform the above Circle class to the following:

Public Class Circle
    Private rad As Double

End Class

If you create a member variable as private but still want other classes (or structures) or procedures to access or get the value of such a member variable, you should then provide a means for members of the class to access that private member.

A property is a member of a class that acts as an intermediary to a member variable of the class. For example, if you have a member variable of a class and that member represents the salary of an employee, a property can be the "door" that other procedures or classes that need the salary must present their requests to. As such, these external procedures and classes cannot just change the salary or retrieve it as they wish. A property can be used to validate their request, to reject or to accept them.

As mentioned already, a property is used to "filter" access to a member variable of a class. Therefore, you start by declaring a (private (if you don't make it private, you may be deceiving the purpose of creating a property)) member variable as we did for the rad member variable of the above Circle class.

Obviously, as we learned in Lesson 21, this private member variable cannot be accessed by a procedure or class outside of its class. Therefore, to let outside classes access this variable, you would/can create a property. To create a property, you use the Property keyword. With regards to their role, there are two types of properties.

1. Start your programming environment
2. Create a Console Application named DeptStore3
3. To create a new class, on the main menu, click Project -> Add Class...
4. Set the Name to DepartmentStore and click Add
5. From what we know so far, type the following:
    

   Public Class DepartmentStore Private pItemNo As String Private pCat As String Private pName As String Private pSize As String Private pPrice As Double End Class

6. Save the file

A property is referred to as read-only if its role is only to make available the value of the member variable it represents. To create a read-only property, use a formula as follows (this formula takes into consideration only the keywords we have learned so far; there are other options that we choose to ignore at this time):

Public | Private | Protected ] [ ReadOnly ] [ Overloads | Overrides ] _
[ Overridable ] | Shadows | Shared ] Property PropName As PropType
   Get
      
   End Get
End Property

The optional Public, Private, or Protected keywords allow you to specify the level of access of the property. As introduced in the Lesson 11, the Public keyword would indicate that the property can be accessed outside of its class. The Private keyword would show that the property is available only to members of its class. The Protected keyword would indicate that the property can be accessed by either the members of its class or only the members of classes derived from it.

The optional Shared keyword would allow you to use the property without declaring an instance of its class.

The ReadOnly keyword is used to indicate that the property's value can be accessed but it cannot be changed. If you are creating a read-only property, you must include the ReadOnly keyword.

The Property keyword is required. It is followed by the name of the property. The name essentially follows the rules of Visual Basic object names. The Get keyword, the End Get and the End Property lines are also required.

Here is an example:

Public Class Circle
    ' This is a new property
    Public ReadOnly Property Radius()
        Get

        End Get
    End Property
End Class

Notice that we omitted the As keyword and the data type of the property. If you don't specify the data type, the property is treated as Object. Otherwise, you can specify the necessary data type of the property. Here is an example:

Public Class Circle
    ' This is a new property
    Public ReadOnly Property Radius() As Double
        Get

        End Get
    End Property
End Class

Between the Get and the End Get lines, you can implement the behavior that would be used to make the member variable's value available outside. The simplest way consists of just returning the corresponding member variable. To do this, type the Return keyword, followed by the hidden member variable whose value would be accessed through this property. Here is an example:

Public Class Circle

    Private rad As Double

    ' This is a new property
    Public ReadOnly Property Radius() As Double
        Get
            Return rad
        End Get
    End Property

End Class

When the clients of a class access a read-only property, they can only retrieve the value of the property but they cannot change it. Therefore, if you create a read-only property, you should provide the users with the ability to primarily specify the value of the member variable. To do this, you can create an appropriate method whose role would only be used to initialize the property. Most of the time, you would use a constructor to do this. Here is an example of  such a constructor used to initialize a read-only property:

Public Class Circle

    Private rad As Double

    Public Sub New(ByVal r As Double)
        rad = r
    End Sub

    ' This is a new property
    Public ReadOnly Property Radius()
        Get
            Return rad
        End Get
    End Property

End Class

Once a read-only property has been created, other classes or procedures can access it, for example they read its value as follows:

Module Exercise

    Public Function Main() As Integer
        Dim circ As Circle = New Circle(25.84)

        Console.WriteLine(" -=- Circle Characteristics -=-")
        Console.WriteLine("Radius:        {0}", circ.Radius)

        Return 0
    End Function

End Module

This would produce:

-=- Circle Characteristics -=-
Radius:        -64.25

We described a property as serving as a door from outside to its corresponding member variable, preventing those outside classes, structures, or procedures to mess with the member variable. Notice that the Square class was given a negative value for the member variable, which is usually unrealistic for the side of a square. In this case and others, while still protecting the member variable as private, you can use the read property to reset the value of the member variable or even to reject it. To provide this functionality, you can create a conditional statement in the property to perform a checking process. Here is an example:

File: Circle.vb

Public Class Circle

    Private rad As Double

    Public Sub New()
        rad = 0
    End Sub

    Public Sub New(ByVal r As Double)
        rad = r
    End Sub

    Public ReadOnly Property Radius()
        Get
            If rad < 0 Then Return 0
            ' else is implied
            Return rad
        End Get
    End Property

End Class

File: Exercise.vb

Module Exercise

    Public Function Main() As Integer
        Dim circ As Circle = New Circle(-64.25)

        Console.WriteLine(" -=- Circle Characteristics -=-")
        Console.WriteLine("Radius:        {0}", circ.Radius)
        Console.WriteLine()

        circ = New Circle(38.18)

        Console.WriteLine(" -=- Circle Characteristics -=-")
        Console.WriteLine("Radius:        {0}", circ.Radius)
        Console.WriteLine()

        Return 0
    End Function

End Module

This would produce:

-=- Circle Characteristics -=-
Radius:        0

 -=- Circle Characteristics -=-
Radius:        38.18

1. To create read-only properties, change the contents of the DepartmentStore file as follows:
    

   Public Class DepartmentStore Private pItemNo As String Private pCat As String Private pName As String Private pSize As String Private pPrice As Double Public Sub New(ByVal nbr As String, ByVal ctg As String, _ ByVal nme As String, ByVal siz As String, _ ByVal prc As Double) pItemNo = nbr pCat = ctg pName = nme pSize = siz pPrice = prc End Sub ' A property for store number of a merchandise Public ReadOnly Property ItemNumber() As String Get If pItemNo = "" Then Return "Invalid Item" Else Return pItemNo End If End Get End Property ' A property for type of a merchandise Public ReadOnly Property Category() As String Get If pCat= "" Then Return "Unknown Category" Else Return pCat End If End Get End Property ' A property for the name of a merchandise Public ReadOnly Property ItemName() As String Get If pName = "" Then Return "Item no Description" Else Return pName End If End Get End Property ' A property for size of a merchandise Public ReadOnly Property Size() As String Get If pSize = "" Then Return "Unknown Size or Fits All" Else Return pSize End If End Get End Property ' A property for the marked price of an item Public ReadOnly Property UnitPrice() As Double Get If pPrice.Equals(0) Then Return 0.0 Else Return pPrice End If End Get End Property End Class

2. In the Solution Explorer, right-click Module1.vb and click Rename
3. Type it DeptStore.vb and press Enter.
   If asked whether you want to change the file, click Yes
4. In the Solution Explorer, double-click DeptStore.vb and change its file as follows:
    

   Module DeptStore Public Function Main() As Integer Dim store As DepartmentStore = _ New DepartmentStore("53564", "Men", _ "Khaki Pants Sahara", "34", 24.95) Dim quantity As Integer = 4 Dim totalPrice As Double = store.UnitPrice * quantity Console.WriteLine(" =#=#= Customer Invoice =#=#=") Console.WriteLine("Item #: {0}", store.ItemNumber) Console.WriteLine("Category: {0}", store.Category) Console.WriteLine("Description: {0}", store.ItemName) Console.WriteLine("Item Size: {0}", store.Size) Console.WriteLine("Unit Price: {0}", store.UnitPrice.ToString("C")) Console.WriteLine("Quantity: {0}", quantity) Console.WriteLine("Total Price: {0}" & vbCrLf, _ totalPrice.ToString("C")) Return 0 End Function End Module

5. Execute the program:
    

   =#=#= Customer Invoice =#=#= Item #: 53564 Category: Men Description: Khaki Pants Sahara Item Size: 34 Unit Price: $24.95 Quantity: 4 Total Price: $99.80

6. Return to your programming environment