Walkthrough: Implementing Inheritance with COM Objects (Visual Basic)
Updated: July 20, 2015
You can derive Visual Basic classes from
Public classes in COM objects, even those created in earlier versions of Visual Basic. The properties and methods of classes inherited from COM objects can be overridden or overloaded just as properties and methods of any other base class can be overridden or overloaded. Inheritance from COM objects is useful when you have an existing class library that you do not want to recompile.
The following procedure shows how to use Visual Basic 6.0 to create a COM object that contains a class, and then use it as a base class.
Your computer might show different names or locations for some of the Visual Studio user interface elements in the following instructions. The Visual Studio edition that you have and the settings that you use determine these elements. For more information, see Personalizing the IDE.
In Visual Basic 6.0, open a new ActiveX DLL project. A project named
Project1is created. It has a class named
In the Project Explorer, right-click Project1, and then click Project1 Properties. The Project Properties dialog box is displayed.
On the General tab of the Project Properties dialog box, change the project name by typing
ComObject1in the Project Name field.
In the Project Explorer, right-click
Class1, and then click Properties. The Properties window for the class is displayed.
In the Project Explorer, right-click
MathFunctions, and then click View Code. The Code Editor is displayed.
Add a local variable to hold the property value:
' Local variable to hold property value Private mvarProp1 As Integer
Public Property Let Prop1(ByVal vData As Integer) 'Used when assigning a value to the property. mvarProp1 = vData End Property Public Property Get Prop1() As Integer 'Used when retrieving a property's value. Prop1 = mvarProp1 End Property
Add a function:
Function AddNumbers( ByVal SomeNumber As Integer, ByVal AnotherNumber As Integer) As Integer AddNumbers = SomeNumber + AnotherNumber End Function
Create and register the COM object by clicking Make ComObject1.dll on the File menu.
Although you can also expose a class created with Visual Basic as a COM object, it is not a true COM object and cannot be used in this walkthrough. For details, see COM Interoperability in .NET Framework Applications.
In the following procedure, you will create an interop assembly, which acts as a bridge between unmanaged code (such as a COM object) and the managed code Visual Studio uses. The interop assembly that Visual Basic creates handles many of the details of working with COM objects, such as interop marshaling, the process of packaging parameters and return values into equivalent data types as they move to and from COM objects. The reference in the Visual Basic application points to the interop assembly, not the actual COM object.
Open a new Visual Basic Windows Application project.
On the Project menu, click Add Reference.
The Add Reference dialog box is displayed.
On the COM tab, double-click
ComObject1in the Component Name list and click OK.
On the Project menu, click Add New Item.
The Add New Item dialog box is displayed.
In the Templates pane, click Class.
The default file name,
Class1.vb, appears in the Name field. Change this field to MathClass.vb and click Add. This creates a class named
MathClass, and displays its code.
Add the following code to the top of
MathClassto inherit from the COM class.
Overload the public method of the base class by adding the following code to
Extend the inherited class by adding the following code to
The new class inherits the properties of the base class in the COM object, overloads a method, and defines a new method to extend the class.
Add a button to your startup form, and then double-click it to view its code.
In the button's
Clickevent handler procedure, add the following code to create an instance of
MathClassand call the overloaded methods:
Dim Result1 As Short Dim Result2 As Integer Dim Result3 As Integer Dim MathObject As New MathClass Result1 = MathObject.AddNumbers(4S, 2S) ' Add two Shorts. Result2 = MathObject.AddNumbers(4, 2) 'Add two Integers. Result3 = MathObject.SubtractNumbers(2, 4) ' Subtract 2 from 4. MathObject.Prop1 = 6 ' Set an inherited property. MsgBox("Calling the AddNumbers method in the base class " & "using Short type numbers 4 and 2 = " & Result1) MsgBox("Calling the overloaded AddNumbers method using " & "Integer type numbers 4 and 2 = " & Result2) MsgBox("Calling the SubtractNumbers method " & "subtracting 2 from 4 = " & Result3) MsgBox("The value of the inherited property is " & MathObject.Prop1)
Run the project by pressing F5.
When you click the button on the form, the
AddNumbers method is first called with
Short data type numbers, and Visual Basic chooses the appropriate method from the base class. The second call to
AddNumbers is directed to the overload method from
MathClass. The third call calls the
SubtractNumbers method, which extends the class. The property in the base class is set, and the value is displayed.
You may have noticed that the overloaded
AddNumbers function appears to have the same data type as the method inherited from the base class of the COM object. This is because the arguments and parameters of the base class method are defined as 16-bit integers in Visual Basic 6.0, but they are exposed as 16-bit integers of type
Short in later versions of Visual Basic. The new function accepts 32-bit integers, and overloads the base class function.
When working with COM objects, make sure that you verify the size and data types of parameters. For example, when you are using a COM object that accepts a Visual Basic 6.0 collection object as an argument, you cannot provide a collection from a later version of Visual Basic.
Properties and methods inherited from COM classes can be overridden, meaning that you can declare a local property or method that replaces a property or method inherited from a base COM class. The rules for overriding inherited COM properties are similar to the rules for overriding other properties and methods with the following exceptions:
If you override any property or method inherited from a COM class, you must override all the other inherited properties and methods.
Properties that use
ByRefparameters cannot be overridden.