This IMessageFilter interface allows applications to handle incoming or outgoing messages selectively while waiting for responses from synchronous calls. The ability to filter messages is often useful when a dialog box is being displayed or a lengthy operation is in progress. Performance can be improved when applications handle some messages and defer others.

When to Implement

Implement IMessageFilter in place of OLE's default message handling when there is a need to selectively handle incoming or outgoing messages, while at the same time waiting for responses from synchronous calls.

Although not as significant an issue as with 16-bit applications, you may still need to implement IMessageFilter as a way of solving deadlocks. IMessageFilter is still needed to find out if an application is blocking, so that you can task-switch to that application and give the user an opportunity to deal with the blocking application. For example, if you have Word talking to Excel, with Excel running in the background in formula mode, Excel won't check all calls, thereby blocking further action. IMessageFilter would put up a dialog indicating which task is blocking and provide the user with an opportunity to deal with the deadlock.

For applications that do not implement IMessageFilter, incoming and outgoing calls to OLE are handled automatically by OLE's default message handling procedures. OLE provides default message handling for each of the conditions that would be handled by an IMessageFilter method. For incoming calls, OLE dispatches all calls, regardless of their logical thread ID. For incoming Windows messages or Macintosh events, the default behavior is as though IMessageFilter::MessagePending had returned PENDINGMSG_WAITDEFPROCESS. This means that task-switching and window-activation messages are dispatched, as well as WM_TIMER messages. Other input messages are discarded and OLE continues to wait for the reply.

When to Use

You don't call this interface directly. It's provided by the application and called by the system.

Methods in Vtable Order

IUnknown Methods
Returns a pointer to a specified interface.
Increments the reference count.
Decrements the reference count.

IMessageFilter Methods
Provides a single entry point for incoming calls.
Provides application with opportunity to display dialog offering retry or cancel options or to switch tasks.
Indicates a Windows message has arrived while OLE is waiting to respond to a remote call.

See Also

In the WIN32 Programmer's Reference: "Messages and Message Queues"

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