The accepted answer is incorrect because the question is about native C/C++, managed C/C++ is a very different story.

The MS VS does not allow to combine unmanaged and managed projects together in one solution.

But there is some trick: you need to know what a solution file is, how it is formatted and composed. So, you need to create a solution file manually.

The main point is: the VS recognizes a project type by NOT an extension of the file name or even by parsing its content, this is the work for MSBuild. There are some sort of registered project type GUIDs that are accepted by VS, one of them is for simple folder nodes, others for VC++ and C# projects respectively.

This is excerpt from real solution file:

Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "cpp_template.con", "cpp\cpp_template.con.vcxproj", "{00000C13-EEEE-BBBB-6667-000000000020}"
EndProject
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "csh_template.con", "net\csh_template.con.csproj", "{00000C13-EEEE-BBBB-6667-CAA000000000}"
EndProject

C++ native and C# projects can be very friendly to each other. :D

The complete description of the genetic solution file is in this document mix_proj_in_vs_sln.doc, but if you do not want to waste your time reading this file there is a solution that includes simple projects: C# and C++ console apps. It was created in VS 2017, can be downloaded and built.