Class. Programs are complex. A class is one part of a program. It is self-contained. When we modify a class, other parts of the program are not affected.
A program in VB.NET may also contain Modules and Namespaces. But the Class is the core unit—things like Strings and Lists are special classes.
First example. This program introduces an Example Class. In the Class, we have a Private field of type Integer. We also have a constructor—the New() Sub.
Value Finally we have the Value() Function, which returns an expression based on a field. It is Public, so can be called from Main.
Step 1 In the Main Sub, control flow begins. We create an instance of the Example Class—an Example now exists on the managed heap.
Step 2 We access the Example instance (called "x") and invoke its Value Function. This returns 2, and we print the result.
VB.NET program that uses Class
Class Example
Private _value As Integer
Public Sub New()
_value = 2
End Sub
Public Function Value() As Integer
Return _value * 2
End Function
End Class
Module Module1
Sub Main()
' Step 1: create a new instance of Example Class.
Dim x As Example = New Example()
' Step 2: call Value Function on the Example.
Console.WriteLine(x.Value())
End Sub
End Module
4
Me qualifier. With the Me qualifier, we specify a member directly on the current instance of the class. If the class is derived, any further derived classes are ignored.
Info With "Me," we can qualify whether we mean to indicate a local variable, or a member on the current class.
Here We use Me.species to reference the field on the Bird class. The variable "species" is the parameter to the New Sub.
VB.NET program that uses Me qualifier
Class Bird
Private species As Integer
Public Sub New(ByVal species As Integer)
' Print Me and not-Me variables.
Console.WriteLine("ME: {0}/{1}", Me.species, species)
' The Me qualification makes it clear we are assigning a field.Me.species = species
End Sub
End Class
Module Module1
Sub Main()
Dim bird = New Bird(10)
End Sub
End Module
ME: 0/10
Inherits. With Inherits, one class can inherit from another class. This means it gains all the fields and procedures from the parent class.
Class A Let's begin by looking at Class A: this class contains a field (_value) as well as a Sub (Display()).
Class B, C These 2 classes use the Inherits keyword and are derived from Class A. They provide their own constructors (New).
Info You can see the New B() and New C() will do slightly different things when called.
Display Class B and Class C inherit Display from Class A. When b.Display() and c.Display() are called, the A.Display Sub is invoked.
VB.NET program that uses Inherits keyword
Class A
Public _value As Integer
Public Sub Display()
Console.WriteLine(_value)
End Sub
End Class
Class B : Inherits A
Public Sub New(ByVal value As Integer)
MyBase._value = value
End Sub
End Class
Class C : Inherits A
Public Sub New(ByVal value As Integer)
MyBase._value = value * 2
End Sub
End Class
Module Module1
Sub Main()
Dim b As B = New B(5)
b.Display()
Dim c As C = New C(5)
c.Display()
End Sub
End Module
5
10
MyClass versus Me. The MyClass and Me qualifiers have an important difference. MyClass refers to the current class, and Me refers to the current instance—it could be called "MyInstance."
MyClass With MyClass, a method on the current instance of the class is called. So in the Animal class, Animal.Test is invoked.
Me This references the current instance, not the current class. So Cat.Test is invoked, because we have a Cat object.
VB.NET program that uses MyClass, Me qualifiers
Class Animal
Public Sub Enter()
' Use MyClass and Me to call subroutines.MyClass.Test()
Me.Test()
End Sub
Public Overridable Sub Test()
Console.WriteLine("Animal.Test called")
End Sub
End Class
Class Cat : Inherits Animal
Public Overrides Sub Test()
Console.WriteLine("Cat.Test called")
End Sub
End Class
Module Module1
Sub Main()
Dim cat As Cat = New Cat()
cat.Enter()
End Sub
End Module
Animal.Test called
Cat.Test called
Shared. Some fields in a Class are not tied to a Class instance. Only one instance is needed. Shared is used on fields to make one field shared among all Class instances.
Tip A Public Shared field can be used in the same way as a global variable. This is useful for storing settings.
Class Test
Public Shared _v As Integer
End Class
Module Module1
Sub Main()
Test._v = 1
Console.WriteLine(Test._v)
Test._v = 2
Console.WriteLine(Test._v)
End Sub
End Module
1
2
Shared Sub. These methods are not tied to a Class instance. A Shared Sub can be called with a composite name. Next, the Write Sub inside the Test class is called with "Test.Write()".
VB.NET program that uses Shared Sub
Class Test
Public Shared Sub Write()
Console.WriteLine("Shared Sub called")
End Sub
End Class
Module Module1
Sub Main()
Test.Write()
End Sub
End Module
Shared Sub called
Partial. This modifier specifies that a class is specified in multiple declarations. With Partial, were open a class and add new parts to it. A Class can span multiple files.
VB.NET program that uses Partial
Module Module1
Partial Class Test
Public Sub X()
Console.WriteLine("X")
End Sub
End Class
Partial Class Test
Public Sub Y()
Console.WriteLine("Y")
End Sub
End Class
Sub Main()
' Invoke methods on the partial class.
Dim t As Test = New Test()
t.X()
t.Y()
End Sub
End Module
X
Y
Friend. This modifier makes a member (like a Class, Sub or Function) unavailable outside the present assembly. An assembly is a physical file that contains compiled code.
Here We specify that Display() is a Friend Sub. So it is Public inside the assembly, but not available outside.
VB.NET program that uses Friend
Class Item
Friend Sub Display()
Console.WriteLine("Friend Class used")
End Sub
End Class
Module Module1
Sub Main()
' The Display Sub is public if we are in the same assembly.
Dim local As Item = New Item()
local.Display()
End Sub
End Module
Friend Class used
Object. All classes inherit from Object, the ultimate base class. We thus can cast all variables to an Object. The object class provides some helpful methods.
Equals This compares the contents of two objects. Implementations from derived classes are used.
GetType This returns a Type object based on the type of the current object. GetType is similar to VarType.
GetHashCode Returns a hash code integer based on the content of the instance. Derived implementations are often present.
ToString Converts an Object to a string representation. The exact string returned varies by implementation.
VB.NET program that uses Object
Class Example
' An empty Example class: it automatically inherits from Object.
End Class
Module Module1
Sub Main()
' Create instance of the Example class.
Dim x As Example = New Example()
' The class inherits from Object, so we can cast it to an Object.
Dim y As Object = x
' ... This method is from Object.
Console.WriteLine(y.GetHashCode())
' When we call GetHashCode, the implementation from Object is used.
Console.WriteLine(x.GetHashCode())
End Sub
End Module
46104728
46104728
Interface. Instead of Inherits, we could use an Interface. We use the Implements keyword with Interfaces. An Interface is a contract—a set of demands that compliant types fill.
Meanwhile A base class is a core template of data and functionality. Base classes are not the same as interfaces.
Note With Inherits, we implement complex object models that can closely represent, in a declarative way, a type framework.
MustInherit. This provides an alternative to the Interface type. It modifies a Class so that it can only be used as a base Class. The class no longer can be directly instantiated.
And A MustInherit Class is essentially a template that is part of the classes that inherit from it.
VarType. This function is equivalent to the GetType function. It returns a Type reference for an object instance. Its syntax is different from GetType's.
Modules. A Module has shared data. The Main Subroutine is found in a Module. All fields in a Module are shared, meaning they are not part of an instance. Modules are not types.
Notes, composite names. In programming languages, a name such as "MyBase._value" is called a composite name. We access the base class (A) and the field on the base class (_value).
A summary. Classes are essential building blocks. A class is a reference type: it is allocated on the managed heap. It can have Functions, Subs, and data members.