User2400 User2400 - 1 month ago 19
C# Question

Associate File Extension with Application

I've written a program that edits a specific filetype , and I want to give the user the option to set my application as the default editor for this filetype (since I don't want an installer) on startup.

I've tried to write a re-useable method that associates a file for me (preferably on any OS, although I'm running Vista) by adding a key to HKEY_CLASSES_ROOT, and am using it with my application, but it doesn't seem to work.

public static void SetAssociation(string Extension, string KeyName, string OpenWith, string FileDescription)
RegistryKey BaseKey;
RegistryKey OpenMethod;
RegistryKey Shell;
RegistryKey CurrentUser;

BaseKey = Registry.ClassesRoot.CreateSubKey(Extension);
BaseKey.SetValue("", KeyName);

OpenMethod = Registry.ClassesRoot.CreateSubKey(KeyName);
OpenMethod.SetValue("", FileDescription);
OpenMethod.CreateSubKey("DefaultIcon").SetValue("", "\"" + OpenWith + "\",0");
Shell = OpenMethod.CreateSubKey("Shell");
Shell.CreateSubKey("edit").CreateSubKey("command").SetValue("", "\"" + OpenWith + "\"" + " \"%1\"");
Shell.CreateSubKey("open").CreateSubKey("command").SetValue("", "\"" + OpenWith + "\"" + " \"%1\"");

CurrentUser = Registry.CurrentUser.CreateSubKey(@"HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.ucs");
CurrentUser = CurrentUser.OpenSubKey("UserChoice", RegistryKeyPermissionCheck.ReadWriteSubTree, System.Security.AccessControl.RegistryRights.FullControl);
CurrentUser.SetValue("Progid", KeyName, RegistryValueKind.String);

Any idea why it doesn't work? An example use might be

SetAssociation(".ucs", "UCS_Editor_File", Application.ExecutablePath, "UCS File");

The part of the method that uses "CurrentUser" seems to work if I do the same using regedit, but using my application it doesn't.


The answer was a lot simpler than I expected. Windows Explorer has its own override for the open with application, and I was trying to modify it in the last lines of code. If you just delete the Explorer override, then the file association will work.

I also told explorer that I had changed a file association by calling the unmanaged function SHChangeNotify()

    public static void SetAssociation(string Extension, string KeyName, string OpenWith, string FileDescription)
        // The stuff that was above here is basically the same

        // Delete the key instead of trying to change it
        CurrentUser = Registry.CurrentUser.OpenSubKey("Software\\Microsoft\\Windows\\CurrentVersion\\Explorer\\FileExts\\.ucs", true);
        CurrentUser.DeleteSubKey("UserChoice", false);

        // Tell explorer the file association has been changed
        SHChangeNotify(0x08000000, 0x0000, IntPtr.Zero, IntPtr.Zero);

    [DllImport("shell32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    public static extern void SHChangeNotify(uint wEventId, uint uFlags, IntPtr dwItem1, IntPtr dwItem2);