A .Net AddIn doesn't get autoloading after I moved to Windows7

I creates an addin which runs in XP.
In XP I simply load the .Net dll only once.
The next time I launched MicroStation the dll gets loadung automatically.

I created a form based on BMW and put some commands on it.
If I call 
- ActiveDesignFile.Close() the addin gets unloaded
- OpenDesignFile(...) the addin gets loaded again

Now I moved to Windows7 and the AddIn doesn't get autoloaded anymore.
Are there anybody who has a hint to solve this issue?

code:

[Bentley.MicroStation.AddIn(MdlTaskID ="MyName"]
internal sealed class Connector : Bentley.MicroStation.AddIn, _ApplicationEvents
{
  ...