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Thread: parametrized constructor & copy constructor

  1. #1

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    Jan 2009
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    Question parametrized constructor & copy constructor

    hi friends,

    can anyone tell me ? . why and where we use copy and parametrized constructor in . and the schenario where is no way . rather than using only copy and parametrized constuctor . any help would be highly appreciated .

  2. #2
    You don't want to know.
    Join Date
    Aug 2010

    Re: parametrized constructor & copy constructor

    Are you sure you're taking a VB .NET course? We don't have "copy constructors" as a formal concept in VB .NET.

    But there's sort of an answer, and informally you can make something that looks like a C++ copy constructor and it serves the same function.

    We make objects to associate logical bits of data and operations that can be performed on it. (Some argue that should be 'or' instead of 'and'.) Usually data is represented by properties, and actions are represented by methods.

    One of the 'jobs' of an object is ensuring the data is valid. For example, if we have a Customer object, it probably has a Name property of type String. If that Name is blank or empty, the customer is invalid. So we can try to set up the property to have a default value, but there will be a brief moment after the object is created when it doesn't really have a Name, and we might forget to set it.

    We use a parameterized constructor when we feel like we need some basic data to create an object at all. If you don't use a parameterized constructor, you have to remember to set those properties.

    A 'copy constructor' is a C++ concept. The idea is it's a parameterized constructor that takes the same object as a parameter. It's called when C++ needs to make a copy of an object, which can be very frequently. .NET doesn't do this as frequently so it's not a built-in feature. But sometimes you want to make sure one object can't impact another and this allows it. (In .NET, some people prefer to call this process 'cloning'.)

    So, practical example.

    This is a type with a parameterized constructor that ensures the Name property can NEVER be Nothing and has a copy constructor:
    Public Class Customer
        Private _name As String
        Public ReadOnly Property Name
                Return _name
        End Property
        Public Sub New(ByVal name As String)
            If name Is Nothing Then
                Throw New ArgumentNullException("name", "Name cannot be null.")
            End If
            _name = name
        End Sub 
        Public Sub New(ByVal other As Customer)
            _name = other.Name
        End Sub
    End Class
    This answer is wrong. You should be using TableAdapter and Dictionaries instead.

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