Create registry entry to associate file extension

Question

I would like to know the cleanest way of registering a file extension with my C++ application so that when a data file associated with my program is double clicked, the application is opened and the filename is passed as a parameter to the application.

Currently, I do this through my wix installer, but there are some instances where the application will not be installed on ths user's computer, so I also need the option of creating the registry key through the application.

Additionally, will this also mean that if the application is removed, unused entries in the registry will be left lying around?

Answer 1

I don't know why people keep saying that HKEY_CURRENT_USER\Software\Classes\<.ext>'s Default value (which will redirect you into another (software-created) class.

It does work, but it will be overridden by

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\<.ext>\UserChoice

And I believe Microsoft recommends the second practice- because it's what the built-in "open with" is doing. The value of Progid" key is equal to default value of HKEY_CURRENT_USER\Software\Classes\<.ext> in this case.

Answer 2

This is a two step process:

 1. Define a program that would take care of extension: (unless you want to use existing one)
      1.1 create a key in "HKCU\\Software\\Classes\\" for example 
          "Software\\Classes\\YourProgramName.file.ext"
      1.2 create subkey "Software\\Classes\\YourProgramName.file.ext\\DefaultIcon"
        1.2.1 set default value ("") to your application full path to get
              icon from resources
      1.3 create a subkey "Software\\Classes\\YourProgramName.file.ext\\Shell\\OperationName\\Command"
          OperationName = for example Open, Print or Other
        1.3.1 set default value ("") to your application full path +optional runtime params (filename)

2.Associate file extension with program.
  2.1 create a key HKCU\\Software\\Classes\\.ext   - here goes your extension
  2.2 set default value to the program definition key
    ("YourProgramName.file.ext")

Below is part of the program written in c# which associate file extension. It is not c++ but i think it is simple enought to explain itself and AFAIK it is verv simmilar if not identical to the code in c++

1.

    RegistryKey keyPFCTExt0 = Registry.CurrentUser.OpenSubKey("Software\\Classes\\PFCT.file.enc", true);
        if (keyPFCTExt0 == null)
        {
            keyPFCTExt0 = Registry.CurrentUser.CreateSubKey("Software\\Classes\\PFCT.file.enc");
            keyPFCTExt0.CreateSubKey("DefaultIcon");
                RegistryKey keyPFCTExt0ext = Registry.CurrentUser.OpenSubKey("Software\\Classes\\PFCT.file.enc\\DefaultIcon", true);
                    keyPFCTExt0ext.SetValue("", Application.ExecutablePath +",0");
                keyPFCTExt0ext.Close();
            keyPFCTExt0.CreateSubKey("Shell\\PFCT_Decrypt\\Command");
        }
    keyPFCTExt0.SetValue("", "PFCT.file.enc");
    keyPFCTExt0.Close();

2.

    RegistryKey keyPFCTExt1 = Registry.CurrentUser.OpenSubKey("Software\\Classes\\PFCT.file.enc\\Shell\\PFCT_Decrypt\\Command", true);
        if (keyPFCTExt1 == null)
            keyPFCTExt1 = Registry.CurrentUser.CreateSubKey("Software\\Classes\\PFCT.file.enc\\Shell\\PFCT_Decrypt\\Command");
        keyPFCTExt1.SetValue("", Application.ExecutablePath + " !d %1"); //!d %1 are optional params, here !d string and full file path
        keyPFCTExt1.Close(); 

Answer 3

Your basic overview of the process is found in this MSDN article. The key parts are at the bottom of the list:

• Register the ProgID

A ProgID (essentially, the file type registry key) is what contains your important file type properties, such as icon, description, and context menu items including application used when the file is double clicked. Many extensions may have the same file type. That mapping is done in the next step:

• Register the file name extension for the file type

Here, you set a registry value for your extension, setting that extension's file type to the ProgID you created in the previous step.

The minimum amount of work required to get a file to open with your application is setting/creating two registry keys. In this example .reg file, I create an file type (blergcorp.blergapp.v1) and association a file extension (.blerg) with it.

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Classes\blergcorp.blergapp.v1\shell\open\command]
@="c:\path\to\app.exe \"%1\""
[HKEY_CURRENT_USER\Software\Classes\.blerg]
@="blergcorp.blergapp.v1"

Now, you probably want to accomplish this programmatically. To be absolutely kosher, you could check for the existence of these keys, and change your program behavior accordingly, especially if you're assuming control of some common file extension. However, the goal can be accomplished by setting those two keys using the SetValue function.

I'm not positive of the exact C++ syntax, but in C# the syntax looks something like this:

Registry.SetValue(@"HKEY_CURRENT_USER\Software\Classes\blergcorp.blergapp.v1\shell\open\command", null, @"c:\path\to\app.exe \"%1\"");
Registry.SetValue(@"HKEY_CURRENT_USER\Software\Classes\.blerg", null, "blergcorp.blergapp.v1");

Of course you could manually open each sub key, manually create the ProgID and extension subkey, and then set the key value, but a nice thing about the SetValue function is that if the keys or values don't exist, they will automatically be created. Very handy.

Now, a quick word about which hive to use. Many file association examples online, including ones on MSDN, show these keys being set in HKEY_CLASSES_ROOT. I don't recommend doing this. That hive is a merged, virtual view of HKEY_LOCAL_MACHINE\Software\Classes (the system defaults) and HKEY_CURRENT_USER\Software\Classes (the per-user settings), and writes to any subkey in the hive are redirected to the same key in HKEY_LOCAL_MACHINE\Software\Classes. Now, there's no direct problem doing this, but you may run into this issue: If you write to HKCR (redirected to HKLM), and the user has specified the same keys with different values in HKCU, the HKCU values will take precedence. Therefore, your writes will succeed but you won't see any change, because HKEY_CURRENT_USER settings take precedence over HKEY_LOCAL_MACHINE settings.

Therefore, you should take this into consideration when designing your application. Now, on the flip side you can write to only HKEY_CURRENT_USER, as my examples here show. However, that file association setting will only be loaded for the current user, and if your application has been installed for all users, your application won't launch when that other user opens the file in Windows.

That should be a decent primer for what you want to do. For further reading I suggest

• Best Practices for File Association
• File Types and File Association, especially
• How File Associations Work

And see also my similar answer to a similar question:

• Associating file extensions with a program