CompletionContext [out]
If this callback routine returns FLT_PREOP_SUCCESS_WITH_CALLBACK or FLT_PREOP_SYNCHRONIZE, this parameter is an optional context pointer to be passed to the corresponding post-operation callback routine. Otherwise, it must be NULL.
CompletionContext [in, optional]
A context pointer that was returned by the minifilter driver's pre-operation callback (PFLT_PRE_OPERATION_CALLBACK) routine. The CompletionContext pointer provides a way to communicate information from the pre-operation callback routine to the post-operation callback routine.