Question

So I was looking to play some audio in C# and I found this:
which basically says to reference quartz.dll and use the FilgraphManagerClass.

This is perfect for my application since I would like to be able to start playing the song at specific points within the song. One can do this by setting FilgraphManagerClass.CurrentPosition. Also, I wanted the song to play in the background without any special player popping up, which happens when using this dll.

Unfortunately for me, when I try to do as the example suggests, Visual Studio 2010 Express complains and tells me “Interop type FilgraphManagerClass cannot be embedded.” I am allowed to use the FilgraphManager interface, but that is missing the CurrentPosition property and any seeking ability.

This article suggests just finding the appropriate interface. But there doesn’t appear to be one that really matches the FilgraphManagerClass. It also briefly talks about the safety involved in embedding assemblies

To use the class, I ended up right clicking on the QuartzTypeLib reference and changing the setting Embed Interop Types to false. Now everything works as it’s supposed to.

Ok… finally my questions:

• Is it safe to change the Embed Interop Types setting like I did for the QuartzTypeLib?
• Is that true in general of all COM types?
• What is this embedding and metadata stuff and why would it be safe or not?
• Are there dlls that one CAN trust?

Answer 1

You need to ignore the safety argument, it is just nonsense. The issue is that the Embed Interop Type feature requires types to have a one-to-one match with the interface in the COM server. They must have a [Guid] attribute that matches. That attribute allows them to declare that types loaded from different assemblies are compatible, even though they came from different assemblies. The match on the Guid is the key.

Which is a problem with the classes whose name end in “Class”. They are ‘fake’ classes, synthesized from the real COM coclasses. It solves a problem with .NET not supporting multiple inheritance. Since they are auto-generated, their [Guid] doesn’t match a CLSID in the COM type library and the type cannot be embedded because their is no way to verify type equivalence.

You can very often avoid the multiple inheritance problem and just create an instance of the coclass without the “Class” postfix. Certainly in this case:

        var player = new QuartzTypeLib.FilgraphManager();
        player.RenderFile(@"c:\temp\test.avi");

Compiles and runs, doesn’t play. But that’s a different problem, you should use Windows Media Player instead. Add a reference from the COM tab.

Disabling EIT is not really an issue, you just get an interop.dll file in your build directory that you need to deploy on the user machine. EIT was really designed to solve a problem with PIAs (Primary Interop Assemblies), a problem that you don’t have here since you are not exposing the native COM interfaces to any other assemblies.