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Q141939: HOWTO: Create an OLE Server to Implement "Thunking"

Article: Q141939
Product(s): Microsoft Visual Basic for Windows
Version(s): 4.0
Operating System(s): 
Keyword(s): kbVBp400
Last Modified: 13-JUL-2001

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The information in this article applies to:

- Microsoft Visual Basic Standard Edition, 32-bit, for Windows, version 4.0 
- Microsoft Visual Basic Professional Edition, 16-bit, for Windows, version 4.0 
- Microsoft Visual Basic Professional Edition, 32-bit, for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition, 16-bit, for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows, version 4.0 
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SUMMARY
=======

An out-of-process OLE server (that is, an EXE) may be called by either a 16- bit
or 32-bit program. Hence, it can be used as an interface between a 16- bit
program that requires the services of a 32-bit DLL, or a 32-bit program that
calls functions inside a 16-bit application/DLL. This permits the OLE system
DLLs to handle any necessary "thunking" of the 16-bit to 32-bit or 32-bit to
16-bit translations.

This article demonstrates how the functions contained in a 16-bit DLL can be
called from a 32-bit program using an OLE server as an intermediary.

MORE INFORMATION
================

A key goal of OLE is to establish a standardized way for objects to be created
and to communicate with one another. This communication can take place between
applications even if they are written in different languages. Because OLE
defines the interface, it handles the communication between the two objects.
Please see the REFERENCES section below for more details.

In particular, one useful corollary of the standardized interface that OLE
demands is that an out-of-process OLE server can act as an intermediary to
perform the "thunking" required if a 32-bit application needs to call a function
contained in a 16-bit DLL and vice versa.

Example: A 32-Bit Program Calling a 16-Bit DLL
----------------------------------------------

This example creates a 16-bit OLE server to wrap the functions contained in a
16-bit DLL. In this example, the Mbf2ieee.dll is used. You must have this DLL in
order to be able to complete this example. This DLL converts floating point
numbers in the earlier Microsoft Binary Format (MBF) into IEEE format. For
additional information about how to download Mbf2ieee.dll, click the article
number below to view the article in the Microsoft Knowledge Base:

  Q113439 PRB: VB3: Converting MBF to IEEE in Visual Basic for Windows


The file can be downloaded from the Microsoft Download Center as Mbf2ieee.exe.
This DLL is an example of a 16-bit DLL that is difficult to update to 32-bit
because the DLL calls two C run-time functions that have not been ported over to
32-bit.

Use the steps below to create the OLE server:

1. Start a new Visual Basic project in Visual Basic 16-Bit Edition.

2. Add a class module to the project by clicking the Insert/Class Module menu
  option.

3. Bring up the Property window for the class module and modify the following
  properties:

     Instancing:   2-Creatable MultiUse
     Name:         OLE2MBF2IEEE
     Public:       True

4. In the new class module, place the following code in the General Declarations
  section:

        Private Declare Function cvs Lib "mbf2ieee.dll" _
        (x As String) As Single
        Private Declare Function cvd Lib "mbf2ieee.dll" _
        (x As String) As Double
        Private Declare Function mks Lib "mbf2ieee.dll" _
        (x As Single) As String
        Private Declare Function mkd Lib "mbf2ieee.dll" _
        (x As Double) As String

        Public Function CvsOle(x As String) As Single
            CvsOle = cvs(x)
        End Function

        Public Function cvdole(x As String) As Double
            cvdole = cvd(x)
        End Function

        Public Function mksole(x As Single) As String
            mksole = mks(x)
        End Function

        Public Function mkdole(x As Double) As String
            mkdole = mkd(x)
        End Function

5. Add a module to the project by clicking the Insert/Module menu option. Add a
  single, empty subroutine to the code module:

        Sub Main()
        End Sub

  This is required because the OLE server must either start in a "Sub Main"
  subroutine or in a form. Because a form is not required for this OLE server,
  including it would not be a good use of resources.

6. Add a name for the project. On the Tools/Options menu, click the Project tab,
  and enter the following settings:

     Startup Form:   Sub Main()
     Project Name:   MBFIEEEServer

  Also, in this same tab, select the OLE Server radio button in the StartMode
  frame.

7. Build the server. On the File menu, click Make EXE, name the executable
  Mbfole.exe, and place the EXE into the \vb directory.

8. Exit from Visual Basic 16-bit edition.

Now, it is a simple matter to make a client that makes uses of the server.

1. Create a new project in Visual Basic, using any 32-bit edition. Form1 is
  created by default.

2. On the Tools menu, click References and place an x next to the MBFIEEEServer
  option. Click Okay to close the dialog box.

3. In the Form_Click event for the Form, place the following code:

        Private Sub Form_Click()
            Dim fInput As Single
            Dim CVSString As String
            Dim MBF as New OLE2MBF2IEEE
            Dim newresult As String

            fInput = 1234.6789  'IEEE format

        'Convert from Single to string
            newresult = MBF.mksole(fInput)

        'Convert from String to single
            CVSString = MBF.CvsOle(newresult)
            MsgBox CStr(CVSString)
        End Sub

Run the program by pressing the F5 key. Click on Form1, and a message box will
appear with the original number dimensioned in the routine. The OLE server has
successfully called the Cvs and Mks functions.

REFERENCES
==========

"Microsoft OLE Today and Tomorrow: Technology Overview," created December, 1993.
Found under Backgrounders and White Papers, Operating System Extensions,
Microsoft Developer Network CD-ROM.

For additional information, please see the following article in the Microsoft
Knowledge Base:

  Q140520 PRB: Converting MBF to IEEE in Visual Basic for Windows

Additional query words:

======================================================================
Keywords          : kbVBp400 
Technology        : kbVBSearch kbAudDeveloper kbVB400Search kbVB400 kbVB16bitSearch
Version           : :4.0
Issue type        : kbhowto

=============================================================================

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