Making Shell Popup ToolTips for File Items

You may have noticed that when mouse is placed over .doc files (shown in explorer or any where by shell) generated from Word 97 a tooltip showing the author name popups as shown in the screen shot below (The mouse pointer disappeared from the screen shot but it can be understood that it is over offer.doc)

The aim of this article is to show how this could be done for your file types. In particular I would show how this could be done for .cpp file popping out some useful information for the file.

How Shell Pops up a ToolTip

There was not much documentation available for how this could be done so nor any samples are available. All I knew was that Shell uses COM  for supporting all such extensions and such extensions can be implemented by implementing some interfaces and registering appropriate entries at appropriate places. In such a situation Regmon utility developed by Mark Russinovich and Bryce Cogswell came to rescue. You can download this utility from http://www.sysinternals.com . It shows all the queries which are being done on registry by different applications.

All that I did was to start regmon to monitor all the Registry related calls from explorer.exe. I placed the mouse over a doc file and found out what is being queried in the registry. Soon it became clear that explorer queries for HKEY_CLASSES_ROOT\.doc\ShellEx\{00021500-0000-0000-C000-000000000046} and it creates an object of CLSID specified as the deafult value for this key. In this case it was {83799FE0-1F5A-11d1-95C7-00609797EA4F}. It was not hard to figure out that the registry entry was for a inproc-server of type "DocFile InfoTip Handler". The next step was to observe what are the interfaces supported by this object through OLEView.exe. Here is a screen shot of what OLEView showed.

Since IPersistFile is derived from IPersist it can be concluded that essentially "DocFile InfoTip Handler" implements IPersistFile and IQueryInfo. Luckily documentation for both these interfaces were available and soon it was clear how shell displays tooltips for files. Here is how it all happens :

1. When mouse is placed over a file item (let's say file extension is .xxx) displayed in explorer. Shell queries for for the registry entry
   HKEY_CLASSES_ROOT\.doc\ShellEx\{00021500-0000-0000-C000-000000000046}
2. It then checks the default value present at this key. The value should be CLSID is string form of a COM class. If it is a valid value shell creates an object of that class.
3. Once the object is created Shell queries for IPersistFile and calls the Load method of it passing the filename of the item over which mouse is placed. Obviously the intention is that that the object would find out information about the file show that it can display the tooltip.
4. After that IQueryInfo is queried for and the method GetInfoTip is called. GetInfoTip has an out parameter *ppwszTip of type LPWSTR  which recieves the address of the tool tip buffer.
5. Finally shell displays the tooltip.

With this much information in hand it was fairly trivial to develop an ATL project with an object implementing the interfaces.

The Sample Application

The sample application is an ATL project that implements these interfaces. To make it useful the sample pops up tooltips for .c, cpp, .h files. When the mouse is placed over any of these files shell creates the object. The application gets the first 256 characters of the file and checks whether it begins with '//' or '/*'. If it begins with '//' the first line of comment is displayed as a tooltip otherwise for C-Style comments the text before the comment ends (subject to 256 characters) is displayed as tooltip. Here is a screen shot:

Download demo project - 27 KB    infotip.zip