Class and Module Enhancements

twinBASIC provides several enhancements for classes and modules.

Parameterized Class Constructors

Classes now support a New sub with ability to add arguments, called as the class is constructed prior to the Class_Initialize event.

Example

For example a class can have:

[ComCreatable(False)]
Class MyClass
Private MyClassVar As Long
Sub New(Value As Long)
MyClassVar = Value
End Sub
End Class

then created by Dim mc As MyClass = New MyClass(123) which sets MyClassVar on create. Note: Classes using this must be private, have the [ComCreatable(False)] attribute, or also contain Class_Initialize(). Class_Initialize() will replace New in callers of a compiled OCX. Within the project, only New will be used if present.

Private/Public Modifiers for Modules and Classes

A private module or class won’t have its members entered into the type library in an ActiveX project.

ReadOnly Variables

In a class, module-level variables can be declared as ReadOnly, e.g. Private ReadOnly mStartDate As Date. This allows more complex constant assignments: you can use a function return to set it inline, Private ReadOnly mStartDate As Date = Now(), or ReadOnly constants can be set in Class_Initialize or Sub New(...) (see parameterized class constructors above), but everywhere else, they can only be read, not changed.

Get/Let/Set Notification for Class Public Variables

When have a variable in a class such as Public myVar As Long, it’s treated as a property you can use the standard syntax to get or set. tB adds an optional notification event for when the Get/Let/Set occurs that’s accessed through the regular Handles syntax:

Class MyClass
    Public myVar As Long
    Public myOtherVar As Long
    Private Sub OnChangeMyVars() Handles myVar.OnPropertyLet, myOtherVar.OnPropertyLet, _
                                         myVar.OnPropertySet, myOtherVar.OnPropertySet
    ...
    End Sub
    Private Sub OnGetMyVar() Handles myVar.OnPropertyGet, myOtherVar.OnPropertyGet
    ...
    End Sub

These are notifications only, you can’t change the OnPropertyGet method to a function and override the return.

Exported Functions and Variables

It’s possible to export a function or variable from standard modules, including with CDecl.

Examples

[DllExport]
Public Const MyExportedSymbol As Long = &H00000001

[DllExport]
Public Function MyExportedFunction(ByVal arg As Long) As Long

[DllExport]
Public Function MyCDeclExport CDecl(ByVal arg As Long)

This is primarily used to create Standard DLLs (see Project Types), but this functionality is also available in Standard EXE and other compiled project types.

Create classes without IDispatch

By default, the compiler creates a default implementation of IDispatch in all VBx/twinBASIC classes. This allows late-binding and other features. Sometimes however you want a more limited class that only implements IUnknown. This is possible in twinBASIC via the NotDispatchable keyword, used like this:

NotDispatchable Class MyClass
'...
End Class

With the above, MyClass will not implement IDispatch. This means it will not be available for late-binding– i.e. you cannot use it with a variable declared As Object. If you attempt to Set an Object (or IDispatch) variable to such a class, it will raise an E_NOINTERFACE error.