How do I find out the path of the file triggered by opening a file with a custom file extension?

Question

How do i get the location of the file that i used to open my programs with?

Example: if i create a new extention ".xyz" say and i tell windows that i want to open the file type .xyz with myapplication, then it starts my aplication. Great, but how does my application get a handle on the file path of the file that was used to start it?

Also, is there a way to keep just one version of my app running and new files that are opened to just call a method in my application? For example if your using a torrent and you open 5 .torrent files they all just get passed to one application.

Side question: are all file extensions 3 letters long and is there a list of ones that are publicly used? If im creating a file extension I don't want to use one that is already used.

Answer 1

File extension databases:

• http://filext.com/
• http://www.cybertechhelp.com/fileextensions/
• http://extensions.pndesign.cz/
• And of course: http://www.google.com/search?q=file+extension+database

EDIT: removed MSDN link

Answer 2

When you created your file association, you specified the command line that Explorer should run to activate your program. The shell puts the name of the document file on the command line, too, so in your program, check the command-line arguments. How you do that depends on your language and development environment. In Delphi, use the ParamCount and ParamStr functions.

When you create the file association, you can specify exactly where on the command line the document file name should go. Use %1 somewhere on the command line, and the shell will replace it with the file name. Since Windows file names frequently contains spaces, you should put quotation marks around the file name, so the command line in the file association would look like this:

ArthurApp.exe "%1"

With that association, double-clicking another document file will start another instance of your program. If you'd prefer to have the document opened in another window of the already-running instance, then you can write code to make your program look for already-running instances when it starts up. If it finds one, then it can communicate with that instance to tell it what file to open. You can effect that communication any number of ways, including mailslots, sockets, named pipes, memory-mapped files, and DDE.

The shell's file-association mechanism already has a way of communicating via DDE, so a second instance of your program wouldn't be started at all. Instead, the shell would start a DDE conversation with the already-running instance and tell it the new file name that way. However, DDE seems to be falling out of favor nowadays, so check out some of the other options first.

For your side question, no, extensions are not always three characters long. Look around, and that should be obvious: C code goes in .c files, Adobe Illustrator graphics go in .ai files, and new Microsoft Word documents go in .docx files.

But beware. If you ask for *.doc, the results will include .docx files as well. That's because FindFirstFile matches both short and long file names, and long file names with long file extensions have three-character extensions in their short-file-name versions.

Answer 3

Rob covered the answer to your question(s) beautifully.

As to the last part, whether there is a public list of file extensions - not as such, but there is shell.windows.com, the web service Explorer uses to locate handlers for unknown file extensions. You can make up an extension then query shell.windows.com to see whether it's been registered. For example, to check whether the extension .blah has been registered by anyone on shell.windows.com, just open this URL in any browser:

http://shell.windows.com/fileassoc/0409/xml/redir.asp?ext=blah

Of course, replace the trailing blah with your extension.

You can find more details about this in KB929149 and in Raymond Chen's post Where does shell.windows.com get information about file extensions, and how do I get in on that action?.