Class Examples
This page was last reviewed on Apr 16, 2023.
Dot Net Perls
Class. VB.NET 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.
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.
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
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.
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.
Start Let's begin by looking at Class A: this class contains a field (_value) as well as a Sub (Display()).
Info These 2 classes use the Inherits keyword and are derived from Class A. They provide their own constructors (New).
Tip You can see the New B() and New C() will do slightly different things when called.
Tip 2 Class B and Class C inherit Display from Class A. When b.Display() and c.Display() are called, the A.Display Sub is invoked.
Important A MustInherit Class is essentially a template that is part of the classes that inherit from it.
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."
Info With MyClass, a method on the current instance of the class is called. So in the Animal class, Animal.Test is invoked.
And The "Me" keyword references the current instance, not the current class. So Cat.Test is invoked, because we have a Cat object.
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()".
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.
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
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.
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.
Info GetHashCode() returns a hash code integer based on the content of the instance.
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.
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.
Namespaces. With namespaces, we have another way to organize programs. Namespaces contain other types like classes, but are not objects.
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.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Apr 16, 2023 (simplify).
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