Answer:
You can set the reference for RF‑COM5 in C# as follows (see also Image 01):
In the project of the Project Explorer, right-click on "References" and select "Add Reference..."
In the new window, enter the path of Dlubal.RFEM5.dll using "Browse".
The path depends on the version: "C:\Windows\Microsoft.NET\assembly\GAC_MSIL\Dlubal.RFEM5\...". Then, select the folder with the highest version number.
Finally, it is necessary set the embedding of the Interop types to "False" in the properties of the link.
