In object-oriented programming, when creating an object from a given class, it is necessary to call a special class method known as a constructor.
The constructor is a special method of a class created with a New keyword and does not have a return type. The main purpose of the constructor is to initialize the memory, allocated for the object, where its fields will be stored.
There are various types of constructors.
If you don't provide a constructor for your class, VB.NET creates one by default that instantiates the object and sets member variables to the default values.
Here is an example of the most simplified parameterless constructor in the CustomerInfo class.
Public Sub New()
Console.WriteLine("Default Constructor Called")
End Sub
The following code shows how to call a default parameterless constructor.
Dim customer As CustomerInfo = New CustomerInfo()
In the above example, using the New keyword, we call the constructor of the class CustomerInfo. The memory is allocated for the newly created object of the CustomerInfo type.
Similar to the methods, if we need extra data to create an object, the constructor gets it through a parameter list. The following example passes two strings variables name and address, as a parameter to the constructor.
Public Sub New(ByVal name As String, ByVal address As String)
Me.Name = name
Me.Address = address
End Sub
Similarly, the call of a constructor with parameters is done in the same way as the call of a method with parameters. The required values are supplied as a list separated with commas.
Dim customer1 As CustomerInfo = New CustomerInfo("John", "11 wall street")
There is no limit to parameters. You can have as many parameters as you want.
The constructor which creates an object by copying variables from another object is called a copy constructor. The purpose of a copy constructor is to initialize a new instance to the values of an existing instance.
Public Sub New(ByVal customer As CustomerInfo)
Me.Name = customer.Name
Me.Address = customer.Address
End Sub
The copy constructor is called by passing the object as a parameter of the same class.
Dim customer1 As CustomerInfo = New CustomerInfo("John", "11 wall street")
Dim customer2 As CustomerInfo = New CustomerInfo(customer1)
customer1.Print()
customer2.Print()
Let's execute the above code, and you will see the same values for both objects.
Name: John, Address: 11 wall street
Name: John, Address: 11 wall street
A private constructor is declared by specifying the Private access modifier. It is used to prevent creating instances of a class when we have only Shared members.
Public Class Counter
Private Sub New()
End Sub
Public Shared currentCount As Integer
Public Shared Function IncrementCount() As Integer
currentCount += 1
Return currentCount
End Function
End Class
You can call the members using the class name instead of creating its object.
Counter.currentCount = 9
Counter.IncrementCount()
Console.WriteLine("New count: {0}", Counter.currentCount)
Let's execute the above code and you will see the following output.
New count: 10