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:
("", "Hello", Microsoft.Win32.RegistryValueKind.String)
("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:
|Code ||What it does |
|Creates a registry key in |
ClassesRoot for the .hello extension. Note that you must include the beginning period.
- "" (Or
Nothing) sets the default value of the key.
- "Hello" is like the "txtfile" we saw earlier, it tells which registry key contains the information about the .hello extension.
CreateSubKey("Hello" & _ "\shell\open\command")
|This creates the "|
Hello" sub-key and the "
store\open\command" subkey that is needed to store the path to the application that will open this file type.
.SetValue("", Application.ExecutablePath & _ " ""%l"" ",...
- Again, "" tells the application to set the key's default value.
Application.ExecutablePath tells the code to associate the currently running executable with this file type.
" ""%1"" " passes the opened file's location to your program. The quotes around it are optional, but if you have more than one argument, you must put them around each.
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:
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
msgbox("Hello, " & My.Computer.FileSystem.ReadAllText_
(My.Application.CommandlineArgs(0)) & " World!")
- Fixed up article so it fits on one page
- Updated demo project
|6/3/07 ||Added this tip|
|6/8/07||Fixed problems in the DLL|
|6/26/07||Changed the wordings in some phrases and fixed some errors in the examples|
- Fixed links
- No more sidescrolling!
|6/19/09||Removed buggy library|
- Rewrote the sample applications so that they do not rely on the removed library and are fully open source
- Updated article