PRB: Dynamic Creation of Redistributable Control Fails

• Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1, 4.2, 5.0

SYMPTOMS

Attempting to dynamically create an instance of one of the redistributable ActiveX controls supplied with Visual C++ fails on machines where Visual C++ has not been installed. However, creating an instance of the same control on a dialog box succeeds.

CAUSE

The redistributable controls supplied with Visual C++ are licensed.

RESOLUTION

Specify a valid License string when dynamically creating the control.

STATUS

This behavior is by design.

MORE INFORMATION

When Visual C++ is installed on a machine, the entire machine becomes licensed to use the redistributable ActiveX controls as part of the installation process. If an instance of one of the redistributable controls is created dynamically and no License key is specified, the creation will succeed because the control is able to determine that the entire machine is licensed and so will allow itself to be created. However, if the same code that performs the dynamic creation of the control is executed on a machine that has not had Visual C++ installed on it (such as Visual Basic or another application that provides the same control), the creation will fail. In this case the control will not allow itself to be created if a License key is not specified and the machine has not been licensed.

The CWnd::CreateControl method can be used to dynamically create an instance of an ActiveX control. When an ActiveX control is inserted into a Visual C++ project via the Component Gallery, a wrapper class is created that can be used to work with the control. The wrapper class provides two overridden versions of the Create method. One of the versions allows a string to be passed that will be used as the License key when creating the control; the other version does not.

Dynamic creation of one of the redistributable ActiveX controls will fail on a machine that has not had Visual C++ installed on it if any of the following methods are used to create the control:

• The version of the Create method in the wrapper class that does not allow a License key to be specified is used.
• A NULL string is passed to the Create method in the wrapper class that does accept a License key.
• The CWnd::CreateControl method is called with a value of NULL for the License key string.
• IClassFactory::CreateInstanceLic is used with a NULL value as the License key parameter.

To allow a licensed control to be dynamically created on a machine that is not licensed requires passing a License key. An application that performs dynamic creation of a control must have the control's License key cached in its own code. This must be done while the application is being designed. For sample code illustrating how to request a License key from an object, refer to the following article in the Microsoft Knowledge Base:

  ARTICLE ID: Q151771
  TITLE     : SAMPLE: LicReqst Shows Requesting a License Key from an
              Object

   Control File       Description
   -------------------------------------------
   anibtn32.ocx       Animated button Control
   keysta32.ocx       Key Status Control
   mci32.ocx          Multimedia MCI Control
   mscomm32.ocx       Communications Control
   msmask32.ocx       Masked Edit
   grid32.ocx         Grid Control
   picclp32.ocx       Picture Clip Control

REFERENCES

For more information, please see:

The Visual C++ documentation on the CWnd::CreateControl method.

The Visual C++ documentation on licensing an OLE control.

The Visual C++ documentation on using controls in a non-dialog container.

The OLE Programmer's Reference documentation on the IClassFactory2 interface.