Is that right C++ is unmanaged code and C++.NET is managed code.
There’s no such thing as “C++.NET”. There’s C++/CLI, which is basically C++ with Microsoft extensions that allow you to write code targeting the .NET framework. C++/CLI code compiles to CLR bytecode, and runs on a virtual machine just like C#. I’ll assume you’re actually talking about C++/CLI.
With respect to that, one can say standard C++ is unmanaged and C++/CLI is managed, but that’s very much Microsoft terminology. You’ll never see the term “unmanaged” used this way when talking about standard C++ unless in comparison with C++/CLI.
Both standard C++ and C++/CLI can be compiled by the same Visual C++ compiler. The former is the default on VC++ compilers, while a compiler switch is needed to make it compile in latter mode.
I need to program for a project in C++. For better building the GUI, I
would prefer to use C++.NET.
You can build GUI programs in C++ just as well as C++/CLI. It’s just harder because there isn’t a standard library in standard C++ for building GUI like the .NET framework has, but there are lots of projects out there like Qt and wxWidgets which provide a C++ GUI framework.
I also have another plain C++ library (unmanaged C++ dll), will it be
possible to use it as a normal dll library in the C++.NET project?
Yes. It might take some extra work to deal with the different standard C++ data types and .NET data types, but you can certainly make it work.