CoGetClassObject

Provides a pointer to an interface on a class object associated with a specified CLSID. CoGetClassObject locates, and if necessary, dynamically loads the executable code required to do this.

Call CoGetClassObject directly when you want to create multiple objects through a class object for which there is a CLSID in the system registry. You can also retrieve a class object from a specific remote machine. Most class objects implement the IClassFactory interface. You would then call IClassFactory::CreateInstance to create an uninitialized object. It is not always necessary to go through this process. To create a single object, call instead the either the CoCreateInstanceEx function, which allows you to create an instance on a remote machine. This replaces the CoCreateInstance function, which can still be used to create an instance on a local machine. Both functions encapsulate connecting to the class object, creating the instance, and releasing the class object. Two other functions, CoGetInstanceFromFile and CoGetInstanceFromIStorage, provide both instance creation on a remote system, and object activation. OLE also provides many other ways to create an object in the form of numerous helper functions and interface methods whose function is to create objects of a single type and provide a pointer to an interface on that object.

STDAPI CoGetClassObject(
  REFCLSID rclsid,  //CLSID associated with the class object
  DWORD dwClsContext,
                    //Context for running executable code
  COSERVERINFO * pServerInfo,
                    //Pointer to machine on which the object is to 
                    // be instantiated
  REFIID riid,      //Reference to the identifier of the interface
  LPVOID * ppv      //Address of output variable that receives the 
                    // interface pointer requested in riid
);
 

Parameters

rclsid

[in] CLSID associated with the data and code that you will use to create the objects.

dwClsContext

[in] Context in which the executable code is to be run. To enable a remote activation, CLSCTX_REMOTE_SERVER must be included. For more information on the context values and their use, see the CLSCTX enumeration.

pServerInfo

[in] Pointer to machine on which to instantiate the class object. May be NULL, in which case the class object is instantiated on the current machine or at the machine specified under the class's RemoteServerName key in the registry, according to the interpretation of the dwClsCtx parameter (see the CLSCTX documentation for details).

riid

[in] Reference to the identifier of the interface, which will be supplied in ppv on successful return. This interface will be used to communicate with the class object. Typically this value is IID_IClassFactory, although other values – such as IID_IClassFactory2 which supports a form of licensing – are allowed. All OLE-defined interface IIDs are defined in the OLE header files as IID_interfacename, where interfacename is the name of the interface.

ppv

[out] Address of pointer variable that receives the interface pointer requested in riid. Upon successful return, *ppv contains the requested interface pointer.

Return Values

S_OK

Location and connection to the specified class object was successful.

REGDB_E_CLASSNOTREG

CLSID is not properly registered. Can also indicate that the value you specified in dwClsContext is not in the registry.

E_NOINTERFACE

Either the object pointed to by ppv does not support the interface identified by riid, or the QueryInterface operation on the class object returned E_NOINTERFACE.

REGDB_E_READREGDB

Error reading the registration database.

CO_E_DLLNOTFOUND

In-process DLL or handler DLL not found (depends on context).

CO_E_APPNOTFOUND

EXE not found (CLSCTX_LOCAL_SERVER only).

E_ACCESSDENIED

General access failure (returned from LoadLib/CreateProcess).

CO_E_ERRORINDLL

EXE has error in image.

CO_E_APPDIDNTREG

EXE was launched, but it didn't register class object (may or may not have shut down).

Remarks

A class object in OLE is an intermediate object that supports an interface that permits operations common to a group of objects. The objects in this group are instances derived from the same object definition represented by a single CLSID. Usually, the interface implemented on a class object is IClassFactory, through which you can create object instances of a given definition (class).

A call to CoGetClassObject creates, initializes, and gives the caller access (through a pointer to an interface specified with the riid parameter) to the class object. The class object is the one associated with the CLSID that you specify in the rclsid parameter. The details of how the system locates the associated code and data within a given machine are transparent to the caller, as is the dynamic loading of any code that is not already loaded.

If the class context is CLSCTX_REMOTE_SERVER, indicating remote activation is required, the COSERVERINFO structure provided in the pServerInfo parameter allows you to specify the machine on which the server is located. For information on the algorithm used to locate a remote server when pServerInfo is NULL, refer to the CLSCTX enumeration.

There are two places to find a CLSID for a given class:

• The registry holds an association between CLSIDs and file suffixes, and between CLSIDs and file signatures for determining the class of an object.
• When an object is saved to persistent storage, its CLSID is stored with its data.

To create and initialize embedded or linked OLE document objects, it is not necessary to call CoGetClassObject directly. Instead, call one of the OleCreate or OleCreateXxx helper functions. These functions encapsulate the entire object instantiation and initialization process, and call, among other functions, CoGetClassObject.

The riid parameter specifies the interface the client will use to communicate with the class object. In most cases, this interface is IClassFactory. This provides access to the IClassFactory::CreateInstance method, through which the caller can then create an uninitialized object of the kind specified in its implementation. All classes registered in the system with a CLSID must implement IClassFactory.

In rare cases, however, you may want to specify some other interface that defines operations common to a set of objects. For example, in the way OLE implements monikers, the interface on the class object is IParseDisplayName, used to transform the display name of an object into a moniker.

The dwClsContext parameter specifies the execution context, allowing one CLSID to be associated with different pieces of code in different execution contexts. The CLSCTX enumeration, defined in Compobj.H, specifies the available context flags. CoGetClassObject consults (as appropriate for the context indicated) both the registry and the class objects that are currently registered by calling the CoRegisterClassObject function.

To release a class object, use the class object's Release method. The function CoRevokeClassObject is to be used only to remove a class object's CLSID from the system registry.

Windows CE: Windows CE supports only the CLSCTX_INPROC_SERVER value for the dwClsContext parameter. For any other values, the function returns E_NOTIMPL. Since no remote server is allowed, pServerInfo must point to null.

Passing into this function any invalid and, under some circumstances, NULL pointers will result in unexpected termination of the application. For more information about handling exceptions, see Programming Considerations.

QuickInfo

  Windows NT: Use version 3.1 or later.
  Windows: Use Windows 95 or later.
  Windows CE: Use version 2.0 or later.
  Header: Declared in objbase.h.
  Import Library: Included as a resource in ole32.dll.

See Also

CoCreateInstanceEx, CoRegisterClassObject, CoRevokeClassObject, OleLoad, CLSCTX, Creating an Object through a Class Object