Can I send a message to another process?

I have a program “A” with C++, which communicates with COM PORT.
And the main program with Unity3d should send message to “A” program.
The reason why I am doing like this is that the protocol of COM i am using is totally complicated.
Even i don’t have enough time. So, Im trying to communicate between two process.

Is it possible to do that?

Unity can talk to a COM port: SerialPort Class (System.IO.Ports) | Microsoft Learn

But if I understand you, you want to have a C++ program (compiled exe) communicate with a Unity program, and not via a DLL, right? I would use a websocket. Google ‘unity3d web socket script’ there’s a lot about that out there.