VB Shell Programming

Data Handler Interfaces

Data handlers implement two interfaces: IPersistFile andIDataObject. Everything that needs to be said about IPersistFile has already been said (see Section 5.2.1 in Chapter 5). Conversely, we know almost nothing about IDataObject. And even when we are done with this chapter, there will still be much that has not been said about this interface. The world of IDataObject is huge. It is one of the fundamental interfaces involved in OLE data transfers.

IDataObject

Table 8.1 shows all nine methods of IDataObject, but only three are required when writing data handlers in Visual Basic: QueryGetData, EnumFormatEtc, and GetData.

Table 8-1. The IDataObject Interface

Method	Description
GetData	Retrieves data from a data object, as defined by a FORMATETC structure. The data is then transferred through a STGMEDIUM structure.
GetDataHere	Similar to GetData except the caller is responsible for allocating and freeing all memory associated with FORMATETC and STGMEDIUM.
QueryGetData	Given a FORMATETC structure, this method determines if a resulting call to GetData will be successful.
GetCanonicalFormatEtc	Determines if two different FORMATETC structures would produce the same data, providing a means to eliminate a second call to GetData.
SetData	Allows another object to send data to the data object.
EnumFormatEtc	Provides a means to enumerate all the ways a data object can describe data in a FORMATETC structure.
DAdvise	Creates an event sink ...