How two different projects, two different language one solution

Hello Arijeet Ghosh, For more clarity refer below articles / blogs which includes step by step explaination with example.

Unmanaged C++ Dll Call From Managed C# Application

https://www.c-sharpcorner.com/uploadfile/tanmayit08/unmanaged-cpp-dll-call-from-managed-C-Sharp-application/

managed C# Dll Call From Unmanaged C++ Application

https://bountify.co/call-c-dll-from-c-application

https://www.codeproject.com/Tips/695387/Calling-Csharp-NET-methods-from-unmanaged-C-Cplusp

https://stackoverflow.com/questions/13293888/how-to-call-a-c-sharp-library-from-native-c-using-c-cli-and-ijw