i am targeting windows and i know what you said but i want to know what C program should i write to wrap a class library correctly? shoul i keep static references to instances of classes or ...?

I guess there a multiple approaches here, depending on what you need. If you just need to access a single manager class, you can just keep a static reference to that in your C program. If you need to expose the use of instances of classes to C#, you could just cast the instance pointer values to ints and pass those to C# and then back to C++.

can you tell me how unity uses physx or openAL? is there any general approach for creating wrappers?

The use of PhysX or OpenAL (now FMOD) in Unity is somewhat different, as these libraries are not wrapped and accessed from the C# code, but are part of Unity's C++ core - so the situation is not really comparable.

But, really, wrapping such libraries to C# should be well possible - for OpenAL, which is in C, you could directly map every function to C# as they are - for C++ libraries like PhysX, you can just pass the object instance as another parameter.

you are right. thank you so much. i'll take a look at wrapper generators like swig and also i should take a look at some open source wrappers. the real library that i love to wrap is a weather library called simulweather. i encourage you to take a look at simulweather you might love to have such an advanced weather system in unity 3 :)