Understanding Properties in VB.NET

In VB.NET, properties are nothing but natural extension of data fields. They are usually known as 'smart fields' in VB.NET community. We know that data encapsulation and hiding are the two fundamental characteristics of any object oriented programming language. In VB.NET, data encapsulation is possible through either classes or structures. By using various access modifiers like private, public, protected, internal etc it is possible to control the accessibility of the class members. 

Usually inside a class, we declare a data field as private and will provide a set of public SET and GET methods to access the data fields. This is a good programming practice, since the data fields are not directly accessible out side the class. We must use the set/get methods to access the data fields. 

An example, which uses a set of set/get methods, is shown below.

'SET/GET methods
Imports System
Class [MyClass]
Private x As Integer
Public Sub SetX(ByVal i As Integer)
x = i
End Sub 'SetX
Public Function GetX() As Integer
Return x
End Function 'GetX
End Class '[MyClass]
Class MyClient
Public Shared Sub Main()
Dim mc As New [MyClass]
mc.SetX(10)
Dim xVal As Integer = mc.GetX()
Console.WriteLine(xVal) 'Displays 10
End Sub 'Main
End Class 'MyClient

But VB.NET provides a built in mechanism called properties to do the above. In VB.NET, properties are defined using the property declaration syntax. The general form of declaring a property is as follows. 

<acces_modifier> <return_type> <property_name>
Get
End Get

Set
End Set

Where <access_modifier> can be private, public, protected or internal. The <return_type> can be any valid VB.NET type. Note that the first part of the syntax looks quite similar to a field declaration and second part consists of a get accessor and a set accessor. 

For example the above program can be modifies with a property X as follows.

Class MyClass
Private x As Integer
Public Property X() As Integer
Get
Return x
End Get
Set(ByVal Value As Integer)
x = value
End Set
End Property
End Class 'MyClass

The object of the class MyClass can access the property X as follows.

Dim mc As New [MyClass]
mc.X = 10 ' calls set accessor of the property X, and pass 10 as value of the standard field "value".This is used for setting value for the data member x.
Console.WriteLine(mc.X) ' displays 10. Calls the get accessor of the property X.

The complete program is shown below.

'VB.NET: Property
Imports System
Class MyClass
Private x As Integer
Public Property X() As Integer
Get
Return x
End Get
Set(ByVal Value As Integer)
x = value
End Set
End Property
End Class '[MyClass]
Class MyClient
Public Shared Sub Main()
Dim mc As New MyClass()
mc.X = 10
Dim xVal As Integer = mc.X
Console.WriteLine(xVal) 'Displays 10
End Sub 'Main
End Class 'MyClient 

Remember that a property should have at least one accessor, either set or get. The set accessor has a free variable available in it called value, which gets created automatically by the compiler. We can't declare any variable with the name value inside the set accessor.

We can do very complicated calculations inside the set or get accessor. Even they can throw exceptions. 

Since normal data fields and properties are stored in the same memory space, in VB.NET, it is not possible to declare a field and property with the same name. 

Static Properties

VB.NET also supports static properties, which belongs to the class rather than to the objects of the class. All the rules applicable to a static member are applicable to static properties also. 

The following program shows a class with a static property.

'VB.NET : static Property
Imports System
Class MyClass
Private Shared x As Integer
Public Shared Property X() As Integer
Get
Return x
End Get
Set(ByVal Value As Integer)
x = value
End Set
End Property
End Class 'MyClass
Class MyClient
Public Shared Sub Main()
MyClass.X = 10
Dim xVal As Integer =MyClass .X
Console.WriteLine(xVal) 'Displays 10
End Sub 'Main
End Class 'MyClient 

Remember that set/get accessor of static property can access only other static members of the class. Also static properties are invoking by using the class name. 

Properties & Inheritance 

The properties of a Base class can be inherited to a Derived class.

'VB.NET : Property : Inheritance
Imports System
Class Base
Public Property X() As Integer
Get
Console.Write("Base GET")
Return 10
End Get
Set(ByVal Value As Integer)
Console.Write("Base SET")
End Set
End Property
End Class 'Base
Class Derived
Inherits Base
End Class 'Derived
Class MyClient
Public Shared Sub Main()
Dim d1 As New Derived
d1.X = 10
Console.WriteLine(d1.X) 'Displays 'Base SET Base GET 10'
End Sub 'Main
End Class 'MyClient

The above program is very straightforward. The inheritance of properties is just like inheritance any other member. 

Properties & Polymorphism 

A Base class property can be polymorphicaly overridden in a Derived class. But remember that the modifiers like virtual, override etc are using at property level, not at accessor level.

'VB.NET : Property : Polymorphism
Imports System
Class Base
Public Overridable Property X() As Integer
Get
Console.Write("Base GET")
Return 10
End Get
Set(ByVal Value As Integer)
Console.Write("Base SET")
End Set
End Property
End Class 'Base
Class Derived
Inherits Base
Public Overrides Property X() As Integer
Get
Console.Write("Derived GET")
Return 10
End Get
Set(ByVal Value As Integer)
Console.Write("Derived SET")
End Set
End Property
End Class 'Derived
Class MyClient
Public Shared Sub Main()
Dim b1 = New Derived
b1.X = 10
Console.WriteLine(b1.X) 'Displays 'Derived SET Derived GET 10'
End Sub 'Main
End Class 'MyClient

Abstract Properties 

A property inside a class can be declared as abstract by using the keyword abstract. Remember that an abstract property in a class carries no code at all. The get/set accessors are simply represented with a semicolon. In the derived class we must implement both set and get assessors. 

If the abstract class contains only set accessor, we can implement only set in the derived class. 

The following program shows an abstract property in action.

'VB.NET : Property : Abstract
Imports System
MustInherit Class Abstract
Public MustOverride Property X() As Integer
Get
End Get
Set
End Set
End Property
End Class 'Abstract
Class Concrete
Inherits Abstract
Public Overrides Property X() As Integer
Get
Console.Write(" GET")
Return 10
End Get
Set(ByVal Value As Integer)
Console.Write(" SET")
End Set
End Property
End Class 'Concrete
Class MyClient
Public Shared Sub Main()
Dim c1 As New Concrete
c1.X = 10
Console.WriteLine(c1.X) 'Displays 'SET GET 10'
End Sub 'Main
End Class 'MyClient

The properties are an important features added in language level inside VB.NET. They are very useful in GUI programming. Remember that the compiler actually generates the appropriate getter and setter methods when it parses the VB.NET property syntax.