Interface

Defines a COM interface using twinBASIC syntax. An interface is a contract: a named set of method and property prototypes, with no implementation. Classes provide implementations of interfaces by using the Implements statement.

Note

The Interface block is a twinBASIC extension. In classic VBA there is no interface keyword — interfaces could only be defined indirectly via a referenced type library (IDL/C++) or by using a class with no implementation.

Syntax:

[ attributes ]
[ Public | Private ] Interface name [ Extends baseinterface [ , baseinterface ] … ]
     [ attributes ]
     member-prototype
     …
End Interface

attributes: optional Interface- and member-level attributes. See Available attributes below.
name: The identifier naming the interface. By convention an interface name begins with an uppercase I (IFoo, ICalculator, …).
baseinterface: optional One or more interfaces that name extends. An implementing class is required to provide bodies for the inherited methods as well; in twinBASIC, a class can Implements name and rely on the inherited interfaces being satisfied automatically.
member-prototype: A header-only declaration. May be a Sub, Function, Property Get, Property Let, or Property Set signature, with arguments and return type. Public/Private/Friend modifiers are not allowed on members. There is no End Sub / End Function / End Property — the prototype ends at end of line.

Interface blocks are valid only in .twin source files (not legacy .bas or .cls files), and must appear before the Class or Module statement in the file. Interfaces always have project-wide scope.

Available attributes

Interface-level attributes:

[InterfaceId("...")] — fixes the IID for the interface (a string GUID). Set this on any public/exported interface so consumers in other projects bind to a stable identity.
[Description("text")] — exposed as the helpstring in the type library.
[Hidden] — hides the interface from IntelliSense and similar lists.
[Restricted] — restricts the interface methods from being called in most contexts.
[OleAutomation(True/False)] — controls whether the attribute is applied in the type library. True by default.
[ComImport] — declares the interface as an import from an external COM library (e.g., the Windows shell).
[ComExtensible(True/False)] — controls whether dynamically-added members can be invoked through IDispatch. False by default.

Member-level attributes:

[Description("text")]
[PreserveSig] — keeps the raw COM signature (returning HRESULT) instead of having the runtime translate negative results into errors. Use this when the literal return value is required, or when negative values mean acceptable failure (e.g. an enumerator running out of items).
[DispId(number)] — fixes the dispatch ID associated with the member.

Example

[InterfaceId("E7064791-0E4A-425B-8C8F-08802AAFEE61")]
[Description("Defines the IFoo interface")]
[OleAutomation(False)]
Interface IFoo Extends IUnknown
    Sub MySub(Arg1 As Long)
    Function Clone() As IFoo
    [PreserveSig]
    Function MyFunc([TypeHint(MyEnum)] Arg1 As Variant) As Boolean
End Interface

A class that implements IFoo provides bodies for every member:

Class FooImpl
    Implements IFoo

    Private Sub IFoo_MySub(Arg1 As Long) Implements IFoo.MySub
        Debug.Print "MySub called with"; Arg1
    End Sub

    Private Function IFoo_Clone() As IFoo Implements IFoo.Clone
        Set IFoo_Clone = New FooImpl
    End Function

    Private Function IFoo_MyFunc(Arg1 As Variant) As Boolean Implements IFoo.MyFunc
        IFoo_MyFunc = True
    End Function
End Class

Interface

Available attributes

Example

See Also