Is it possible to use a C++ .lib file from within a C# program?

There are plenty of ways. Read about "interop" in MSDN..

One way is to expose the lib as a DLL, and then use pinvoke to call these functions from a C# project. That limits you to a C-style interface, though.

If your interface is more complex (e.g. object oriented) you can create a C++/CLI layer that will expose the lib's class structure to your C# program. This means you'll have to create a managed C++ (or C++/CLI as it's now called) project; then design an interface in managed code that will be implemented by calls to native C++ (i.e. your lib).

Another way of doing this is by wrapping your lib with a COM interface. But COM's a pain, so I wouldn't...

Not directly. You can create a C++/CLI assembly that consumes the lib and then access that from C#, or you can wrap the lib as a DLL.

What you need is a managed wrapper (C++/CLI) around the native C/C++ library that you are working with.

If you are looking for any C++/CLI book I'd recommend Nishant Sivakumar's C++/CLI in Action

Already answered to wrap it but here's an example . Good luck!

I would take a look at swig, we use this to good effect on our project to expose our C++ API to other language platforms.

It's a well maintained project that effectively builds a thin wrapper around your C++ library that can allow languages such as C# to communicate directly with your native code - saving you the trouble of having to implement (and debug) glue code.

That depends, do you have any limitations on this scenario?

If you have a lib file, it should be possible to first compile it into a DLL file, secondly exposing the functions you want to call in the DLL interface, and thirdly, call them using C# native methods (have a look at pinvoke.net on how to do this bit).

you can't use a lib, but like the others said, you can use it if you wrap it into a dll.

swig can take the headers of your .lib, and if they are not too complex it can generate the dll for you which you would then call with a pinvoke from c# which would also be generated by swig.

if your library is complex and has reference counted smart pointers everywhere, you should find an alternative.