How to check if a DLL file is registered?

You need to find out the GUID of a COM object defined in the DLL. Then look at this registry key:

HKEY_CLASSES_ROOT\CLSID\{xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}\InprocServer32

Replace the x's with the GUID.

It should have a default value that contains the full path to the DLL.

If you mean registered in GAC, here is my consideration: to be registered in GAC, an assembly must be signed with a strong name (have a public key token in it's name).

So you can try load it using Assembly.Load(string), if you got FileNotFoundException - assembly was not registered in GAC.

If got no error, but result Assembly.GetName().GetPublicKeyToken() is null or empty -- this mean you found assembly in application directory, not in GAC.

You can use this:

My.Computer.Registry.ClassesRoot.GetSubKeyNames.Contains("gEncrypt.clsEncrypt")

Where "gEncrypt.clsEncrypt" is ComName.ClassName.

If you know the CLSID of the COM dll, you can just check if there's a key with that CLSID on HKEY_CLASSES_ROOT\CLSID\{CLSID-of-your-COM-component} or HKEY_CLASSES_ROOT\Wow6432Node\CLSID\{CLSID-of-your-COM-component} (Wow6432Node => 32-bit COM registered on a 64-bit machine)

Here is an example:

private bool IsAlreadyRegistered()
{
    using (var classesRootKey = Microsoft.Win32.RegistryKey.OpenBaseKey(
           Microsoft.Win32.RegistryHive.ClassesRoot, Microsoft.Win32.RegistryView.Default))
    {
        const string clsid = "{12345678-9012-3456-7890-123456789012}";

        var clsIdKey = classesRootKey.OpenSubKey(@"Wow6432Node\CLSID\" + clsid) ??
                        classesRootKey.OpenSubKey(@"CLSID\" + clsid);

        if (clsIdKey != null)
        {
            clsIdKey.Dispose();
            return true;
        }

        return false;
    }
}