The ProcessOSUser property of the LogMessage2 object is a Read-Only string that represents the O/S user associated with the process.
The ProcessOSUser is automatically added to all messages sent with MessageLog2.SendLogMessage. This is typically the authenticated user of the application which is logging the message however this is not always the case. For applications that do impersonation, the identity of the impersonating thread posting a message may differ from the process identity. This is because different threads may have different identities and also because the request to log may be invoked on one thread but is executed on the thread where the MessageLog2 object was instantiated (for single threaded apartment objects such as the MessageLog and MessageLog2). For this reason only the process identity is automatically recorded. Even without impersonation it is also possible that an application is doing work, and logging a message in that regard, on behalf of a different user (application servers behave this way). For both of these cases, the application can specify the OriginatingOSUser field when sending a message to provide a more accurate picture of the message context.
The object placeholder is an object expression that evaluates to a LogMessage2 object.