Is it possible? GUI in c# , app in C++

You have several options for doing it, including:

1. Use P/Invoke to call into the C++ DLL from C#.
2. Expose a COM interface from the native code, and call it from C# using COM interop.
3. Write a native Windows service and call into it from managed code.
4. Use C++/CLI to write a managed library in C++, which you can easily link to from C#.

If you're starting from scratch, option 4 is probably your best option. (Aside from just writing the whole thing in C#, that is.) The first three options all involve some additional wrangling and overhead, and probably aren't worth the hassle if you don't have a compelling reason such as needing to interact with an existing native library or having some need for a service-oriented architecture.

write the app logic in c++ dll, then use pinvoke from c# to talk to the dll.