File Association in VB.NET

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Ever wanted to have your application open when the user opens a certain type of file? This is accomplished fairly easily by editing just a couple of registry keys.

How are Programs Associated?

The registry stores all the file type-app associations. Click on the Start Menu > Run > Type in 'regedit' > Ok. Now expand the HKEY_CLASSES_ROOT node. At the top of the window are all the extensions that your computer recognizes. Scroll down to .txt and click on it. Now look at the default value, it probably is 'txtfile'. Scrolling down the tree on the left, find the txtfile node. This contains all the information about any extensions that have their default value set to txtfile. Right now, we're just interested in opening the file, so open Shell > Open > Command. If your .txt files open with Notepad, then the default value should be "%SystemRoot%\system32\NOTEPAD.EXE %1".
%SystemRoot% is pretty self-explanatory, it's replaced by the folder that contains system32, which contains NOTEPAD.EXE.
%1 is a command-line argument to pass the program when a txtfile is opened. %1 is replaced by the file's location.

Step 1: Running Your Program When a .hello File is Opened

The first step is to get your application to open when a chosen extension (like .mp3) is double clicked in Windows Explorer. For this article, we'll use a file extension that shouldn't exist: .hello. To use this file type, create a new project called 'Hello World'. The idea in the included project article is this: a .hello file contains (in plain text) an adjective. When one is opened, a message box will pop-up and say, "Hello, (file contents) World". If you open the application manually, it will associate itself with the .hello extension. Therefore, before .hello files will be opened with the demo program by default, you must run it on its own.

Now we have to edit the registry just like you saw with the .txt extension. Use the following code to do this:

My.Computer.Registry.ClassesRoot.CreateSubKey(".hello").SetValue_
	("", "Hello", Microsoft.Win32.RegistryValueKind.String)
My.Computer.Registry.ClassesRoot.CreateSubKey_
	("Hello\shell\open\command").SetValue("", Application.ExecutablePath & _
	" ""%l"" ", Microsoft.Win32.RegistryValueKind.String)

What does all this do? If you don't understand My.Computer.Registry, here's a link to it on MSDN, or look below:

Now run your application once. It will edit the registry. Your program is now associated with the .hello file!

Now let's say you want to create a file association for .txt in your program. You create the file association, but it still opens in Notepad. What's going on? There is another value that needs to be deleted located here:

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.txt

The value name is 'Progid'. This will consist of a string value of the default program to open this filetype. If this value is present, you will not be able to associate anything with this particular filetype. You must delete the 'Progid' value in order for the association to work.

Thanks, Computer Masster!

Now to test it out. Open Notepad, type in an adjective, and save it as a .hello file (Make sure you don't accidentally save it as a .hello.txt file). Open the file in Windows Explorer. Your program will run! But nothing happens...

Step 2: Reading the File Contents

When the registry is set correctly and a file is opened in Windows Explorer, the file's path is passed as a command-line argument (if you think of an application as a function, then these are the parameters). To retrieve the arguments, you use My.Application.CommandlineArgs. It returns a ReadOnlyCollection(Of String). You can use My.Application.CommandlineArgs(0) to retrieve the file path.

Now, we have to display the message. More code for the Load event:

  msgbox("Hello, " & My.Computer.FileSystem.ReadAllText_
	(My.Application.CommandlineArgs(0)) & " World!")

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