Mixing managed/unmanaged code

Question about mixing managed/unmanaged code. A little background first:

The core product for my company is developed using Borland. It has hooks in it to allow new functions to be added to it on the fly through DLLs (and you can't imagine the interface -- it has pointers to pointers to pointers).

We have a client who wants us to connect to his server using a secure TCP/IP connection. Easily done, using OpenSSL. The client has come back and says they want us to use .Net SSL instead (I'm still looking into that) so they can maintain it.

This implies to me managed code being called from unmanaged code. What I need to know is -- can mix these environments?

If done carefully ;)

The easiest [ but not necessaryly the best ] is to use a COM interop layer.

The biggest issue will be CAS. This is handled by the COM Interop layer pretty well, but you will have to handle it yourself if you decide to go for a more direct interface.