PRB: Unable to Load Control from C:\Path\File.ocx

The information in this article applies to:

• The Microsoft Foundation Classes (MFC) included with:

      - Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1
      - Microsoft OLE Control Developer's Kit (CDK), version 2.0, 2.1, 2.2
  

SYMPTOMS

When you try to load an OLE Control generated by using ControlWizard into a Visual Basic 4.0 form in Visual Basic by on the Tools menu, clicking Custom Controls and then clicking the Browse button, the following error message appears:

  "Unable to load control from: 'x:\path\file.ocx'"

CAUSE

Visual Basic checks an OLE control to see whether it supports self- registration by searching for the presence of an OLESelfRegister entry in the StringFileInfo section of the control's version information resource. ControlWizard-generated OLE controls do not provide the entry by default.

RESOLUTION

Modify the OLE Control to indicate its support for the self-registration feature. This can be achieved by including the OLESelfRegister entry with an empty value in the StringFileInfo section of the control's version resource. The following steps explain the process in detail.

Step-by-Step Procedure

1. Open the .rc file of the OLE Control as a Text file.

2. Find the StringFileInfo block under Version information.

3. Add the following line to the existing list of VALUE statements:

   VALUE "OLESelfRegister", "\0"

4. Build the OLE Control. Now you can load the OLE Control (.ocx file) by clicking Custom Controls on the Tools menu and clicking the Browse.

STATUS

This behavior is by design.

MORE INFORMATION

A COM server typically indicates its support for self-registration by including the OLESelfRegister entry with an empty value in the StringFileInfo of its version resource. This entry is considered optional because the actual self-registration mechanisms do not depend on its presence. This information simply allows a client application to avoid loading a DLL (that is, an in-process server or OLE control) or launching an .exe (that is, a local server) for no gain.

The only requirement for an OCX/DLL to support the self-registration mechanism is to provide the entry points DllRegisterServer and DllUnregisterServer and export them. ControlWizard-generated OLE controls provide these two entry points and export them, though they do not indicate their self-registration support by not including the OLESelfRegister entry by default.

REFERENCES

Please refer to Chapter 5 of Inside OLE, Second Edition, for more information on self-registration.