The Old New Thing

Simulating media controller buttons like Play and Pause

September 29, 2014 Sep 29, 2014 09/29/14

Raymond Chen

Today's Little Program simulates pressing the Play/Pause button on your fancy keyboard. This might be useful if you want to write a program that converts some other input (say, gesture detection) into media controller events. One way of doing this is to take advantage of the Def­Window­Proc function, since the default behavior for ...

Marshaling won't get in your way if it isn't needed

September 26, 2014 Sep 26, 2014 09/26/14

Raymond Chen

I left an exercise at the end of last week's article: "Why is the RPC_X_NULL_REF_POINTER error raised only sometimes?" COM subscribes to the principle that if no marshaling is needed, then an interface pointer points directly at the object with no COM code in between. If the current thread is running in a single-threaded apartment, and it ...

If a process crashes while holding a mutex, why is its ownership magically transferred to another process?

September 25, 2014 Sep 25, 2014 09/25/14

Raymond Chen

A customer was observing strange mutex ownership behavior. They had two processes that used a mutex to coordinate access to some shared resource. When the first process crashed while owning the mutex, they found that the second process somehow magically gained ownership of that mutex. Specifically, when the first process crashed, the second ...

Receiving a notification any time the selection changes in an Explorer window

September 22, 2014 Sep 22, 2014 09/22/14

Raymond Chen

Today's Little Program merely prints a message whenever the user changes the selection on the desktop. I chose the desktop for expediency, since it saves me the trouble of coming up with a way for the user to specify which Explorer window they want to track. Also, all I do is print a message saying "Selection changed!"; actually getting the ...

I marked my parameter as [optional], so why do I get an RPC error when I pass NULL?

September 19, 2014 Sep 19, 2014 09/19/14

Raymond Chen

Consider the following interface declaration in an IDL file: The idea here is that you want to be able to call the Cancel method as pFoo->Cancel(NULL) if you don't want to provide a reason. If you try this, you'll find that the call sometimes fails with error 0x800706F4, which decodes to HRESULT_FROM_WIN32(RPC_X_NULL_REF_POINTER). ...

The stream pointer position in IDataObject::GetData and IDataObject::GetDataHere is significant

September 18, 2014 Sep 18, 2014 09/18/14

Raymond Chen

An oft-overlooked detail of the IData­Object::Get­Data and IData­Object::Get­Data­Here methods is the position of the stream pointer when the result is a stream. These rules are buried in the documentation, so I'm going to call them out louder. Let's look at IData­Object::Get­Data first. If IData­Object::...