Using a C++ library from C# and linking them in the same solution

Question

Visual Studio 2019 v 16.11.2

I have a solution with two projects.

Project 1: is a c++ DLL
Project 2: is a c# Console App

Project 2 is using functions in the c++ DLL.

I have found that I have to copy the DLL into the same folder where the Console App exe lives. I see that this can be done automatically with a post-build command line in the DLL project options. Or the output folder can be set in the C++ app to place the DLL in the CSharp directory.

I have tried making the DLL project a dependency of the C# project, but this results in yellow triangles and no explanation of the yellow triangles.

I have also tried to add a reference by browsing to the location of the compiled DLL and selecting it, this results in the message:

`The reference is invalid or unsupported`

So two questions:

1. How do I find out or see what those yellow triangles mean?
2. Is copying the DLL manually (or automatically during a post-build script) the correct way of linking within the same solution?