How to create and use shared members by using Visual Basic .NET or Visual Basic 2005 (308371)

SUMMARY

This article describes how to implement shared members within a class, as well as how to reference other class members from shared methods and shared properties.

Visual Basic .NET or Visual Basic 2005 provides the necessary language features to enable object-oriented programming. Central to this approach is the class that implements its functionality using methods (functions and procedures), properties, and fields. These are referred to as members.

Shared members (which are called static or class members in some other languages) are not associated with a specific instance of a class and are thus shared by all objects that are instantiated from that class. To call a shared member, you can qualify it either with the class name or with an object of that class. Because shared members are not associated with object instances, they do not have access to non-shared members (which are accessed through "Me," which represents the current object instance).

Non-shared members are called instance members because they are associated with individual object instances. Think of shared members as belonging to the class and instance members belonging to instances of the class (that is, objects).

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Requirements

This article assumes that you are familiar with the following topics:

• Object-oriented concepts
• Creating classes in Visual Basic .NET or in Visual Basic 2005
• Visual Basic .NET or Visual Basic 2005 properties

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Create a New Visual Basic .NET or Visual Basic 2005 Project and Class

The code in this project demonstrates how to implement shared methods, shared properties, and instance methods. Some of the following code contains deliberate errors to illustrate key points about how to access other members from shared members. As a result, the code compiles only after you comment out or remove the lines that cause the error.

1. Start Visual Studio .NET or Visual Studio 2005, and create a new Visual Basic Console Application project named SharedMethod.
2. From the Project menu, click Add Class to create a new class.
3. Add two private instance fields as follows:

       'Instance variables
       Private myInt As Integer
       Private myStr As String
   					

4. Use the Shared keyword to add two private shared fields as follows:

       'Class variables
       Private Shared sharedInt As Integer
       Private Shared sharedPropValue As String
   					

5. Save the project.

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Create the Instance Method, Shared Method, and Shared Property

1. Add the following instance method named mySub:

      'An instance method
       Sub mySub()
           myInt = 1       ' Ok, same as Me.myInt = 1
           sharedInt = 1   ' Ok, same as Class1.sharedInt = 1
       End Sub
   					

2. Use the Shared keyword to add the following shared method named sharedSub. Remember that shared methods can only access other shared members and not instance members.

       'A class method
       Shared Sub sharedSub()
           myInt = 1       ' Error, cannot access Me.myInt
           sharedInt = 1   ' Ok, same as Class1.sharedInt = 1
       End Sub
   					

3. Use the Shared keyword to add a shared, read-write string property named sharedProp as follows:

       'A class property
       Shared Property sharedProp() As String
           Get
               'can only refer to shared variables
               Return myStr            'Error, cannot access Me.myStr
               Return sharedPropValue  'Ok, same as Return Class1.sharedPropValue
   		
           End Get
           Set(ByVal Value As String)
               'can only refer to shared variables
               myStr = Value              'Error, cannot access Me.myStr
               sharedPropValue = Value    'Ok, same as Class1.sharedPropValue = Value		
           End Set
       End Property
   					

4. Save Class1.vb.

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Compile the Class

1. Build the class, and notice that three error messages appear in the Task List window.
   
   The first error occurs at the following line of code in the SharedSub method:

   myInt = 1       ' Error, cannot access Me.myInt
   						

   Because SharedSub is a shared member, it is associated with the class and can only access shared members. You cannot use the "Me" keyword here because instance members are not accessible from within a shared member. Remove the offending line of code.
2. The second error occurs at the following line of code in the sharedProp property:

   Return myStr      'Error, cannot access Me.myStr
   						

   Remove the offending line of code.
3. The third error occurs at the following line of code in the sharedProp property:

   myStr = Value              'Error, cannot access Me.myStr
   						

   Remove the offending line of code.
4. Rebuild the class.

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Create the Test Module

1. Open Module1.vb.
2. Add the following code, which uses the class name to reference Class1 members, to the sub Main procedure:

   Dim outStr As String
   
           'Use class to refer to members
           Class1.sharedSub()              'Ok
           Class1.sharedProp= "Class"      'Ok
           outStr = Class1.sharedProp      'Ok
           Class1.mySub()                  'Error only available to an 
                                           'instance
   					

3. Add the following code, which uses an object reference to reference Class1 members:

   'Use an object reference to refer to members
           Dim MyObject As Class1
           MyObject.sharedSub()               'Ok
           MyObject.sharedProp = "Obj Ref"    'Ok
           outStr = MyObject.sharedProp       'Ok
           MyObject.mySub()                   'This will fail at runtime as
                                              'MyObject is Nothing
   					

4. Save the project.

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Compile the Test Module

1. Build the project.
2. Notice that an error message appears in the Task List window. This error occurs in the following line of code:

           Class1.mySub()                  'Error only available to an 
                                           'instance
   						

   mysub is an instance method and thus is only available to an instance of a Class1 object. Remove the offending line of code.
3. From the Debug menu, click Start Without Debugging to run the application outside of the debugger. Note that the Just-In-Time (JIT) debugger displays the "System.NullReferenceException" error. Click No in the Just-In-Time dialog box to return to the source code.
4. This exception occurs because the following line of code creates an object reference instead of the instantiated object, which is required to execute an instance method.

           Dim MyObject As Class1
   					

5. To create an instantiated object, modify the offending code as follows:

           Dim MyObject As New Class1()
   					

6. Save, rebuild, and run the project.

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Troubleshoot

• You can only invoke instance methods and properties on an object instance. Within the instance method, you can reference both instance and shared members.
• To invoke shared members, you can use the class itself, an object reference, or an object instance. Within the shared member, you can only reference other shared members.
• You cannot refer to an instance member of a class from within a shared method or within a shared member initializer without an explicit instance of the class. Me is valid only within an instance method.

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