Object Variable Declaration (Visual Basic)

Artykuł
02/04/2013

You use a normal declaration statement to declare an object variable. For the data type, you specify either Object (that is, the Object Data Type) or a more specific class from which the object is to be created.

Declaring a variable as Object is the same as declaring it as System.Object.

When you declare a variable with a specific object class, it can access all the methods and properties exposed by that class and the classes from which it inherits. If you declare the variable with Object, it can access only the members of the Object class, unless you turn Option Strict Off to allow late binding.

Declaration Syntax

Use the following syntax to declare an object variable:

Dim variablename As [New] { objectclass | Object }

You can also specify Public (Visual Basic), Protected (Visual Basic), Friend (Visual Basic), Protected Friend, Private (Visual Basic), Shared (Visual Basic), or Static (Visual Basic) in the declaration. The following example declarations are valid:

Private objA As Object
Static objB As System.Windows.Forms.Label
Dim objC As System.OperatingSystem

Late Binding and Early Binding

Sometimes the specific class is unknown until your code runs. In this case, you must declare the object variable with the Object data type. This creates a general reference to any type of object, and the specific class is assigned at run time. This is called late binding. Late binding requires additional execution time. It also limits your code to the methods and properties of the class you have most recently assigned to it. This can cause run-time errors if your code attempts to access members of a different class.

When you know the specific class at compile time, you should declare the object variable to be of that class. This is called early binding. Early binding improves performance and guarantees your code access to all the methods and properties of the specific class. In the preceding example declarations, if variable objA uses only objects of class System.Windows.Forms.Label, you should specify As System.Windows.Forms.Label in its declaration.

Advantages of Early Binding

Declaring an object variable as a specific class gives you several advantages:

Automatic type checking
Guaranteed access to all members of the specific class
Microsoft IntelliSense support in the Code Editor
Improved readability of your code
Fewer errors in your code
Errors caught at compile time rather than run time
Faster code execution

Access to Object Variable Members

When Option Strict is turned On, an object variable can access only the methods and properties of the class with which you declare it. The following example illustrates this.

' Option statements must precede all other source file lines.
Option Strict On
' Imports statement must precede all declarations in the source file.
Imports System.Windows.Forms
Public Sub accessMembers()
    Dim p As Object
    Dim q As System.Windows.Forms.Label
    p = New System.Windows.Forms.Label
    q = New System.Windows.Forms.Label
    Dim j, k As Integer
    ' The following statement generates a compiler ERROR.
    j = p.Left
    ' The following statement retrieves the left edge of the label in pixels.
    k = q.Left
End Sub

In this example, p can use only the members of the Object class itself, which do not include the Left property. On the other hand, q was declared to be of type Label, so it can use all the methods and properties of the Label class in the System.Windows.Forms namespace.

Flexibility of Object Variables

When working with objects in an inheritance hierarchy, you have a choice of which class to use for declaring your object variables. In making this choice, you must balance flexibility of object assignment against access to members of a class. For example, consider the inheritance hierarchy that leads to the System.Windows.Forms.Form class:

Suppose your application defines a form class called specialForm, which inherits from class Form. You can declare an object variable that refers specifically to specialForm, as the following example shows.

Public Class specialForm
    Inherits System.Windows.Forms.Form
    ' Insert code defining methods and properties of specialForm.
End Class
Dim nextForm As New specialForm

The declaration in the preceding example limits the variable nextForm to objects of class specialForm, but it also makes all the methods and properties of specialForm available to nextForm, as well as all the members of all the classes from which specialForm inherits.

You can make an object variable more general by declaring it to be of type Form, as the following example shows.

Dim anyForm As System.Windows.Forms.Form

The declaration in the preceding example lets you assign any form in your application to anyForm. However, although anyForm can access all the members of class Form, it cannot use any of the additional methods or properties defined for specific forms such as specialForm.

All the members of a base class are available to derived classes, but the additional members of a derived class are unavailable to the base class.

Udostępnij za pośrednictwem

Object Variable Declaration (Visual Basic)

Declaration Syntax

Late Binding and Early Binding

Advantages of Early Binding

Access to Object Variable Members

Flexibility of Object Variables

See Also

Tasks

Reference

Concepts

Dodatkowe zasoby