This function supports the standard return value E_INVALIDARG, as well as the following:

S_OK

The emulation was successfully established or removed.

REGDB_E_CLASSNOTREG

The clsidOldparameter is not properly registered in the registration database.

REGDB_E_READREGDB

Error reading from registration database.

REGDB_E_WRITEREGDB

Error writing to registration database.

This function sets the TreatAsentry in the registry for the specified object, allowing the object to be emulated by another application. Emulation allows an application to open and edit an object of a different format, while retaining the original format of the object.

After this entry is set, whenever any function like CoGetClassObjectspecifies the object's original CLSID ( clsidOld), it is transparently forwarded to the new CLSID ( clsidNew), thus launching the application associated with the TreatAsCLSID.

When the object is saved, it can be saved in its native format, which may result in loss of edits not supported by the original format.

Your application will call CoTreatAsClassin two situations if it supports emulation:

• In response to an end-user request (through a conversion dialog box) that a specified object be treated as an object of a different class (an object created under one application be run under another application, while retaining the original format information).
  
  
• In a setup program, to register that one class of objects be treated as objects of a different class.
  
  

An example of the first case is that an end user might want to edit a spreadsheet created by one application using a different application that can read and write the spreadsheet format of the original application.

For an application that supports emulation, CoTreatAsClasscan be called to implement a Treat As option in a conversion dialog box.

An example of the use of CoTreatAsClassin a setup program would be in an updated version of an application.

When the application is updated, the objects created with the earlier version can be activated and treated as objects of the new version, while retaining the previous format information. This would give the user the option to convert when saving, or to save it in the previous format, possibly losing format information not available in the older version.

One result of setting an emulation is that verb enumeration, as in the IOleObject::EnumVerbsmethod implementation in the default handler, operates on the verbs from clsidNewinstead of clsidOld.

To ensure that existing emulation information is removed when you install an application, your setup programs should call CoTreatAsClass, setting the clsidNewparameter to CLSID_NULL to remove any existing emulation for the classes they install.

If there is no CLSID assigned to the AutoTreatAskey in the registry, setting clsidNewand clsidOldto the same value removes the TreatAsentry, so there is no emulation. If there is a CLSID assigned to the AutoTreatAskey, that CLSID is assigned to the TreatAs key.

The CoTreatAsClassfunction does not validate whether an appropriate registry entry for clsidNewcurrently exists.

To determine whether the platform supports this function, see Determining Supported COM APIs.