Implements statement

Specifies an interface or class that will be implemented in the class module in which it appears.

Syntax

Implements [ InterfaceName | Class ]

The required InterfaceName or Class is the name of an interface or class in a type library whose methods will be implemented by the corresponding methods in the Visual Basic class.

Remarks

An interface is a collection of prototypes representing the members (methods and properties) that the interface encapsulates; that is, it contains only the declarations for the member procedures. A class provides an implementation of all the methods and properties of one or more interfaces. Classes provide the code used when each function is called by a controller of the class. All classes implement at least one interface, which is considered the default interface of the class. In Visual Basic, any member that isn't explicitly a member of an implemented interface is implicitly a member of the default interface.

When a Visual Basic class implements an interface, the Visual Basic class provides its own versions of all the Public procedures specified in the type library of the Interface. In addition to providing a mapping between the interface prototypes and your procedures, the Implements statement causes the class to accept COM QueryInterface calls for the specified interface ID.

When you implement an interface or class, you must include all the Public procedures involved. A missing member in an implementation of an interface or class causes an error. If you don't place code in one of the procedures in a class you are implementing, you can raise the appropriate error (Const E_NOTIMPL = &H80004001) so a user of the implementation understands that a member is not implemented.

The Implements statement can't appear in a standard module.

Example

The following example shows how to use the Implements statement to make a set of declarations available to multiple classes. By sharing the declarations through the Implements statement, neither class has to make any declarations itself. The example also shows how use of an interface allows abstraction: a strongly-type variable can be declared by using the interface type. It can then be assigned objects of different class types that implement the interface.

Assume there are two forms, SelectorForm and DataEntryForm. The selector form has two buttons, Customer Data and Supplier Data. To enter name and address information for a customer or a supplier, the user clicks the customer button or the supplier button on the selector form, and then enters the name and address by using the data entry form. The data entry form has two text fields, Name and Address.

The following code for the interface declarations is in a class called PersonalData:

Public Name As String 
Public Address As String

The code supporting the customer data is in a class module called Customer. Note that the PersonalData interface is implemented with members that are named with the interface name PersonalData_ as a prefix.

Implements PersonalData

'For PersonalData implementation
Private m_name As String
Private m_address As String

'Customer specific
Public CustomerAgentId As Long

'PersonalData implementation
Private Property Let PersonalData_Name(ByVal RHS As String)
    m_name = RHS
End Property
 
Private Property Get PersonalData_Name() As String
    PersonalData_Name = m_name
End Property


Private Property Let PersonalData_Address(ByVal RHS As String)
    m_address = RHS
End Property

Private Property Get PersonalData_Address() As String
    PersonalData_Address = m_address
End Property

'nitialize members
Private Sub Class_Initialize()
    m_name = "[customer name]"
    m_address = "[customer address]"
    CustomerAgentId = 0
End Sub

The code supporting the supplier data is in a class module called Supplier:

Implements PersonalData

'for PersonalData implementation
Private m_name As String
Private m_address As String

'Supplier specific
Public NumberOfProductLines As Long

'PersonalData implementation
Private Property Let PersonalData_Name(ByVal RHS As String)
    m_name = RHS
End Property
 Private Property Get PersonalData_Name() As String
    PersonalData_Name = m_name
End Property

Private Property Let PersonalData_Address(ByVal RHS As String)
    m_address = RHS
End Property

Private Property Get PersonalData_Address() As String
    PersonalData_Address = m_address
End Property

'initialize members
Private Sub Class_Initialize()
    m_name = "[supplier name]"
    m_address = "[supplier address]"
    NumberOfProductLines = 15
End Sub

The following code supports the Selector form:

Private cust As New Customer
Private sup As New Supplier
 
Private Sub Customer_Click()
    Dim frm As New DataEntryForm
    Set frm.PD = cust
    frm.Show 1
End Sub
 
Private Sub Supplier_Click()
    Dim frm As New DataEntryForm
    Set frm.PD = sup
    frm.Show 1
End Sub

The following code supports the Data Entry form:

Private m_pd As PersonalData

Private Sub SetTextFields()
    With m_pd
        Text1 = .Name
        Text2 = .Address
    End With
End Sub

Public Property Set PD(Data As PersonalData) 
    Set m_pd = Data
    SetTextFields
End Property

Private Sub Text1_Change()
    m_pd.Name = Text1.Text
End Sub

Private Sub Text2_Change()
    m_pd.Address = Text2.Text
End Sub

Note how, in the data entry form, the m_pd variable is declared by using the PersonalData interface, and it can be assigned objects of either the Customer or Supplier class because both classes implement the PersonalData interface.

Also note that the m_pd variable can only access the members of the PersonalData interface. If a Customer object is assigned to it, the Customer-specific member CustomerAgentId is not available. Similarly, if a Supplier object is assigned to it, the Supplier-specific member NumberOfProductLines is not available. Assigning an object to variables declared by using different interfaces provides a polymorphic behavior.

Also note that the Customer and Supplier classes, as defined earlier, don't expose the members of the PersonalData interface. The only way to access the PersonalData members is to assign a Customer or Supplier object to a variable declared as PersonalData. If an inheritance-like behavior is desired, with the Customer or Supplier class exposing the PersonalData members, public members must be added to the class. These can be implemented by delegating to the PersonalData interface implementations.

For example, the Customer class could be extended with the following:

'emulate PersonalData inheritance
Public Property Let Name(ByVal RHS As String)
    PersonalData_Name = RHS
End Property

Public Property Get Name() As String
    Name = PersonalData_Name
End Property

Public Property Let Address(ByVal RHS As String)
    PersonalData_Address = RHS
End Property

Public Property Get Address() As String
    Address = PersonalData_Address
End Property