Programmatically determine which iFilters are installed

Question

I have a problem whereby the Adobe PDF iFilter doesn\'t work consistently for us. As such, we like to use the one from Foxit. The problem is, if we install the Foxit iFilter a

Answer 1

Since the foxit IFilter implements IPersistStream interface, I think you can try get this interface from the IFilter, and query for its CLSID to see if it is the one from foxit. Foxit IFilter has a CLSID of {987f8d1a-26e6-4554-b007-6b20e2680632}, which is the "Persistent Handlers Addins Registered" column in IFilter Explorer.

Adobe's IFilter doesn't seem to be implementing this interface.

Answer 2

A little strange answer ;) but as alternative way can be used external console app Filtreg.exe from Windows 7 SDK to delegate this job to it.

Answer 3

I'm using this small function to give out a list. It just uses the extension NOT the document type! In the most cases this is ok and could be easily changed here.

/// <summary>
/// Implements a Function to get all available IFilters currently registered in this system
/// </summary>    
public string GetFilterList()
{
    //Our resulting string. We give back a ';' seperated list of extensions.
    string result = @"";
    string persistentHandlerClass;

    RegistryKey rk = Registry.LocalMachine.OpenSubKey(@"Software\Classes");
    if (rk == null)
        return null;

    using (rk)
    {
        foreach(string subKeyName in rk.GetSubKeyNames())
        {
            if (subKeyName[0] == '.') //possible Extension
            {
                RegistryKey sk = Registry.LocalMachine.OpenSubKey(@"Software\Classes\" + subKeyName + @"\PersistentHandler");
                if (sk == null)
                    continue;

                using (sk)
                {
                    persistentHandlerClass = (string)sk.GetValue(null);
                }

                if (persistentHandlerClass != null)
                {
                    string filterPersistClass = ReadStrFromHKLM(@"Software\Classes\CLSID\" + persistentHandlerClass +
                        @"\PersistentAddinsRegistered\{89BCB740-6119-101A-BCB7-00DD010655AF}");
                    string dllName = ReadStrFromHKLM(@"Software\Classes\CLSID\" + filterPersistClass + @"\InprocServer32");

                    // skip query.dll results, cause it's not an IFilter itself
                    if (dllName != null && filterPersistClass != null && (dllName.IndexOf("query.dll") < 0))
                    {
                        //result = result + subKeyName + @"[" + dllName + @"] - persistentHandlerClassAddin: " + persistentHandlerClass + "\r\n";  //[C:\Windows\system32\query.dll]
                        //result = result + subKeyName + @"[" + dllName + @"];";  //[C:\Windows\system32\query.dll]
                        result = result + subKeyName.ToLower() + @";";
                    }
                }

            }
        }

        return result;
    }

}

Answer 4

I would expect that IFilters are stored in the registry, therefore you could use Process Monitor to see what keys IFilter Explorer is checking.

Then check on MSDN that this is consistent with the documentation.

Then do the same thing using .NET's registry types in your application.

Based on hunting for this answer, the registration can exist at both System and User level, so you are likely going to need to enumerate multiple registry keys.