FILE: How to Call CoGetClassObjectFromURL() in an MFC App

• Microsoft Visual C++, 32-bit Editions, version 5.0
• Internet Client SDK, versions 4.0, 4.01

SUMMARY

This sample, Urlobj.exe, demonstrates how to call the CoGetClassObjectFromURL() function to download, register, and instantiate an object. The sample implements two interfaces necessary to allow the CoGetClassObjectFromURL() function to work properly: IBindStatusCallback and IWindowForBindingUI.

The following file is available for download from the Microsoft Software Library:

 ~ Urlobj.exe (size: 51878 bytes) 

   ARTICLE-ID: Q119591
   TITLE     : How to Obtain Microsoft Support Files from Online Services

MORE INFORMATION

The CoGetClassObjectFromURL() function returns a class factory object for a given CLSID. If the desired object is installed on the system, it is instantiated. Otherwise, the necessary code is downloaded and installed from the specified URL. The sample uses a CFormView to get the value of the CLSID, the URL pointing to the code for the ActiveX object, the major and minor version, and the size of the object for displaying (it assumes that the control will display itself in some manner).

The bulk of the relevant code is found in the OnButton1() handler, which starts the download process, and the OnStopBind() handler, which handles creating the object after it has finished downloading.

The IBindStatusCallback and IWindowForBindingUI interfaces are implemented in the file Download.cpp by using a CCmdTarget derived class and the MFC interface map macros.

After the CoGetClassObjectFromURL() has completed the asynchronous download, the OnStopBind() handler is called to display the control. In this handler the sample demonstrates instantiating the actual object by using the CWnd::CreateControl() function. This is assuming that the object is an OLE control. The handler also contains some code that has been commented out that demonstrates calling IClassFactory::CreateInstance() in the case that the object is not an OLE control.

To maintain simplicity, the sample does not implement much error checking or verification of user input.