OLE Custom Control

An OLE custom control is a COM server that implements IOleObject. In practice, they usually implement a whole lot more than that. These sometimes have the file extension OCX.

The historical context is important. OLE custom controls came into being at a time when the industry was changing fast.

• The specs for OLE custom controls were published in ‘94.
• OLE 2.0 is a lot different than OLE 1.0, which was not COM-based, so existing controls required a substantial rewrite.
• Windows went from 16 to 32 bits.
• MFC had been around, but was not yet widely adopted.
• Many ISVs were actually still in the process switching from C to C++.

Control development at that time meant getting an MSDN subscription and copying sample code off the CDs. OLE custom controls were superseded by ActiveX controls in ‘96. ATL did not come out until that year. If there’s OCX on your system, there’s a very good chance it was hand-rolled. Be afraid.

References

Object Linking and Embedding - Wikipedia IOleObject (oleidl.h) - Win32 apps | Microsoft Learn