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We all know about file type associations. It's a neat way to "Associate" a particular file extension with your application, so that every time someone double-clicks on a file having that extension, presto! The file automatically opens up in your own application! This article explains exactly how you can do the same for your own application. I've seen a number of similar articles on CodeGuru, but none clearly explain the process of file association. This article explains the process of file association, and also provides ready-to-use functions to associate your application with a particular file extension. Please note that the sample code was developed for VB, but you could easily port it to VC++, or any other language, for that matter, because most of the code consists mostly of standard Win32 API calls for setting Registry key values.
Working with the Associations
Whenever a certain file extension say, ".abd", has to be associated with a particular program, a key named ".abd" has to be created under the HKEY_CLASSES_ROOT root key in the Windows Registry. The default value is then set to a short identifier for the file type; in this case, it could be "abd_ext" (NOTE: The identifier should not have any spaces in it!). Next, another key, by the name of the identifier, is created under the HKEY_CLASSES_ROOT root key, and its default value is set to a short description of the file type (for example, "My own file type"). Under the identifier key, a key called "Defaulticon" is created, and its default value is set to the complete path of the icon that is to be shown for files having that extension. Also, under the identifier key, another key by the name of "shell" is created. This key contains all the file commands that are registered for the given file extension (as in "Open", "Print", "Preview", and so forth). Within the "shell" key, one key for each file command is created, using the name of the command, say, for example, "Open". Now, within the "Open" key (or whatever command is being set), another key, called "command" is created. The default value of this key is set to the complete path of the program to be used, along with whatever command line parameters are neccessary; for example "c:\windows\notepad.exe" "%1".
Coding the Process
The entire process of associating a file extension with a certain program is automatically performed by the "Associate_File" public function, defined in the "File_Commands.bas" module in the included source code. The function declaration is as follows:
Public Function Associate_File(Extension As String, _ Application As String, Identifier As String, _ Description As String, Icon As String)
|Extension:||This is the extension that you want to associate your program with, for example, ".doc"|
|Application:||This is the entire path of your program that is to be associated with the file extension, for example, "C:\Program Files\MyApp.exe"|
|Identifier:||The identifier of the file type, as explained above, for example, "doc_file". NOTE: This parameter should not have any spaces in between!|
|Description:||A short description of the file type, for example, "My Text Document"|
|Icon:||The complete path of the icon that is to be associated with the file type, for example, "C:\Icons\MyIcon.ico"|
The following VB pseudo-code of the Associate_File() function demonstrates the entire process. For the entire code listing, see the accompanying source code.
Public Function Associate_File(Extension As String, _ Application As String, Identifier As String, _ Description As String, Icon As String) CreateRegistryKey HKEY_CLASSES_ROOT\Extension SetRegistryValue of HKEY_CLASSES_ROOT\Extension, _ use default value, value= Identifier CreateRegistryKey HKEY_CLASSES_ROOT\Identifier SetRegistryValue HKEY_CLASSES_ROOT, Identifier, "", _ REG_SZ, Description CreateRegistryKey HKEY_CLASSES_ROOT\Identifier\DefaultIcon SetRegistryValue of HKEY_CLASSES_ROOT\Identifier\DefaultIcon, _ use default value,value= Icon Identifier = Identifier + "\shell" CreateRegistryKey HKEY_CLASSES_ROOT\Identifier Identifier = Identifier + "\open" CreateRegistryKey HKEY_CLASSES_ROOT\Identifier Identifier = Identifier + "\command" CreateRegistryKey HKEY_CLASSES_ROOT\Identifier SetRegistryValue of HKEY_CLASSES_ROOT\Identifier, _ use default value,value= ("Application" "%1") End Function
Additionally, I coded the "File_Command" function to create new file commands for a particular file type, also defined in the "File_Commands.bas" module in the included source code. The function declaration is as follows:
Public Function File_Command(Extension As String, _ Action As String, Command As String)
|Extension:||This is the extension of the file type to which you want to add a file command, for example, ".doc"|
|Action:||This is the file command that you want to add, for example, "Preview"|
|Command:||The complete path of the program to be used for this command, along with the appropriate command line parameters, for example, "c:\Program Files\MyApp.exe /pre %1"|
The following VB pseudo-code of the File_Command() function demonstrates the entire process. For the entire code listing, see the accompanying source code.
Public Function File_Command(Extension As String, _ Action As String, Command As String) Dim ident as String ident=ReadRegistryKeyValue of HKEY_CLASSES_ROOT\Extension, _ use default value ident=ident+"\shell\"+Action CreateRegistryKey HKEY_CLASSES_ROOT\ident ident=ident+"\command" CreateRegistryKey HKEY_CLASSES_ROOT\ident SetRegistryValue of HKEY_CLASSES_ROOT\ident, use default value, _ value= Command End Function
Using the Included Source Code
To utilize the included source code, all you have to do is add the "File_Commands.bas" module to your project, and call the preceding functions, passing the appropriate parameters. This will associate your application with the specified file extension. Now, every time anyone double-clicks on a file having the specified extension, it will start up your application. Your application must inspect the command line parameters passed to it in the startup module; then accordingly load the file. To access the command line parameters passed to your application, you should inspect the value of Command$ in your startup module. The following code snippet demonstrates this. For more clarity, I suggest you download the included sample project, and go through the source code.
Using the Sample Source Code
Private Sub Form_Load() Dim path As String On Error GoTo nofile If Command$ <> Empty Then Form1.Print Form1.Print Form1.Print Form1.Print Form1.Print "Command Line Parameter passed: " + Command$ If Left$(Command$, 1) = Chr$(34) And Right$(Command$, 1) _ = Chr$(34) Then path = Mid(Command$, 2, (Len(Command$) - 2)) Else path = Command$ End If Form1.Print "Path of File passed: " + path Form1.Print Form1.Print "File contents: " + vbCrLf Form1.Print GetFile(path) Else MsgBox ("No Parameters passed!") End If Exit Sub nofile: MsgBox ("ERROR: No valid text file!") End Sub
Private Sub Form_DblClick() Dim app_path As String Dim Icon As String app_path = App.path + "\" + App.EXEName + ".exe" Icon = App.path + "\" + "book.ico" retval = Associate_File(".abd", app_path, "abd_ext", _ "My own extension", Icon) MsgBox ("Associated the file extension .abd with this _ application!") End Sub
That's it! if you've done everything right, it should work like a breeze. For any comments or queries, you could e-mail me at Joydeep_B@Hotmail.com.
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