The information in this article applies to:
- Professional and Enterprise Editions of Microsoft Visual Basic
32-bit only, for Windows, version 4.0
- Standard and Professional Editions of Microsoft Visual Basic,
for Windows, version 3.0
SUMMARY
It is possible to call an OLE server created in Visual Basic version 4.0
(both 16-bit as well as 32-bit Servers) from a Visual Basic version 3.0
client application. Note, however, that a 16-bit application cannot call a
32-bit DLL server. Therefore, if the 32-bit server is an OLE DLL server, it
cannot be called. If the 32-bit server is an OLE EXE server, it can be
called from a 16-bit client application.
This functionality is possible because OLE is binary compatible, which is
one of the main benefits of OLE. However, there are certain limitations
that must be kept in mind simply because these are features not supported
in Visual Basic version 3.0.
MORE INFORMATION
The Limitations
- It is not possible to use early binding while creating object instances.
This is because Visual Basic version 3.0 only supports late binding. In
other words, the only way to create an object of type MyClass
implemented by a server called MyServer is:
Dim obj As Object
Set Obj = CreateObject("MyServer.MyClass")
- You cannot call in-process OLE Servers (that is, Visual Basic version
4.0 OLE DLLs) from Visual Basic version 3.0. This is because you can
only create 32-bit OLE DLLs in Visual Basic version 4.0.
- You cannot return an array data type to Visual Basic version 3.0 from a
Visual Basic version 4.0 OLE server because this is not supported in
Visual Basic version 3.0.
- Visual Basic version 3.0 does not support rich Error information like
that provided by Visual Basic version 4.0's Error Object. So, if the
server raises an error, only a default error message is returned back to
the Visual Basic version 3.0 client. One way to work around this is to
implement your own error object in the server, and have the Visual
Basic version 3.0 client call that object in the error handler for the
default error message.
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