CODEWITHSUNDEEP
vb.netwinforms
Chandrahas Balleda
Chandrahas Balleda

Reputation: 2832

Object declaration in VB

I have a small doubt:

What is the difference between str and str_obj ; cls and cls_obj.

Are all of them are objects..??

(Got this doubt as they are declared in different syntaxes).

Can someone please elaborate ?

Thanks in advance.

Public Class Form1

    Public Structure dayplan_str

        Public a As Integer
        Public b As String
        Public c As Boolean

    End Structure

    Public Class dayplan_cls
        Public a As Integer
        Public b As String
        Public c As Boolean
    End Class

    Public str As dayplan_str   'structure
    Public cls As dayplan_cls  'class
    Dim str_obj As dayplan_str = New dayplan_str  'structure
    Dim cls_obj As dayplan_cls = New dayplan_cls  'class


    Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load

        str.a = 5
        cls.a = 6
        str_obj.a = 7
        cls_obj.a = 8

    End Sub

End Class

Upvotes: 1

Views: 1165

Answers (3)

Cody Gray
Cody Gray

Reputation: 244991

Public str As dayplan_str   'structure

This declares a variable named str that can hold a reference to an object of type dayplan_str. dayplan-str happens to be a structure, but it could also be a class.

The variable str has not been initialized. It currently does not hold a reference to any particular object. If you try to use the variable str in an expression, you will get a NullReferenceException.

Public str_obj As dayplan_str = New dayplan_str  'structure

This declares and initializes a variable named str_obj.

Like the previous declaration, it also declares a variable that can hold a reference to an object of type dayplan_str, but the second part of the statement (the part after the equal sign) initializes the variable by calling the dayplan_str structure's constructor and creating a new object.

If you use the variable str_obj in an expression, it will work fine (no NullReferenceException) because it has been initialized to point to a valid dayplan_str object.

Presumably, this is where the name comes from. The suffix _obj suggests that str_obj refers to an actual object, whereas str does not.

However, you can easily initialize str so that it does point to a valid object:

str = New dayplan_str

This has the same effect as the second form, it just spreads the declaration and initialization across two lines, instead of keeping it all in a single line.

Generally, you want to initialize variables at the point of their declaration, so the second (combined) form is to be preferred. You would only use this separated form if you had a particular reason that the variable needed to be declared before it could be meaningfully initialized.

There is another difference between the two declarations in your code, though, and that is one of scope. More specifically, it is a difference in the accessibility of the variables.

When you declare a class-level variable as Public, it is accessible from both inside of the class and outside of the class.

When you declare a class-level variable as Private, it is only accessible from inside of the class.

Variables in a class can also be declared Friend, Protected, and Protected Friend. Consult the documentation for more details on what those access specifiers mean.

The Dim keyword is very old, coming into VB.NET from the classical BASIC programming language. It just means "dimension" and is a generic way of declaring a variable. Because of its history, it does not have any intrinsic access specification. The accessibility of a variable declared with Dim in VB.NET depends on the context in which that variable is declared. When it is declared at class scope, as in your case, its accessibility will be private. So in that sense:

Dim str_obj As dayplan_str = New dayplan_str  'structure

is equivalent to:

Private str_obj As dayplan_str = New dayplan_str  'structure

which is another way that it is different from:

Public str_obj As dayplan_str = New dayplan_str  'structure

Upvotes: 1

Magisch
Magisch

Reputation: 7352

Public str As dayplan_str   'structure

This is a declaration of a dayplan_str Object called str

Dim str_obj As dayplan_str = New dayplan_str  'structure

This is a declaration and initialization of a dayplan_str Object called str_obj

Upvotes: 1

Vityata
Vityata

Reputation: 43595

The difference between "cls" and "cls_obj" is in the way of declaration. They are all objects, but in "new cls_obj" the new object is also initialized. The difference is like in the following:

Dim k as long
Dim i as long
i = 0

In your case cls_obj is i.

Edit: Concerning the public and dim - the value declared with dim is only visible from the module, where it is declared. The value with public is visible in all modules.

Upvotes: 1

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