Best method of calling managed code(c#) from unmanaged C++

Question

We have developed a s/w architecture consisting of set of objects developed in C#. They make extensive use of events to notify the client of changes in status, etc.

The intention originally was to allow legacy code to use these managed objects through COM interop services. That was easy to write in the design specification but, I'm seeing that actually implementing it is more problematic. I've searched for many hours looking for a good sample of event handling using this method. Before we march down that path, I want to make sure that COM interop is the best way to allow legacy code to call our new code.

It appears there are several different options: 1) COM interop, 2) write unmanaged wrapper classes 3) use the /clr compiler switch to enable calling of managed objects, 4) use some sort of reverse pInvoke call. Am I missing any?

Each option will have its benefits & drawbacks and I'm wondering what the best way to go is. Here are specific questions/comments for each

COM INTEROP - It appears event handling is a hurdle. We use events that have variable types as parameters. An event parameter may have an event ID and an object. Based on the event ID, the object will be of a certain type. Can this be handled with COM interop? Many of the objects that are exposed have properties. You can't declare properties in an interface so all properties will need a corresponding get/set method.

WRITE UNMANAGED WRAPPER - I assume this means creating a DLL using the /clr option to allow creating and calling managed objects and exposing unmanaged objects. Would the client of these unmanaged. I haven't done this before. What are benefits/drawbacks of this?

USE THE /CLR SWITCH - I understand this means to add support for managed objects. What are the drawbacks of this approach? Does this option support events as described above? Can we say, "here's the managed library. Use the /clr compiler option with your legacy code and have at it?" I don't know the ramifications of this. Is there a good sample of how this works around? (I'm sure there is, I just haven't found it)

USE A REVERSE PINVOKE - I'm not sure exactly how this would work but, from what I've been able to find, this is not a likely valid solution.

So, what does the decision tree look like to find the correct direction? Any help is appreciated.

• DP

Answer 1

I think your initial solution is the best one. COM interop is stable and reasonably well documented. All you need to do is ensure that all the different event objects that might pop out of a given event handler implement the same COM-visible base event object interface (that has the event type id, etc). From there, individual objects can implement whatever other interfaces they want, and your unmanaged code can QI for the right "detail" interface based on whatever criteria you want to define. It's really not that hard. Have a look at this CodeProject article for an end-to-end sample including unmanaged event handlers.