A minimal example project demonstrating, that CMake's UseSwig module doesn't copy C# support files into the SWIG_SUPPORT_FILES_DIRECTORY directory when using C++ namespaces.
Configure, build and install the project.
The C# wrapper files are then supposed to get copied from the SWIG_SUPPORT_FILES_DIRECTORY to the CMAKE_INSTALL_PREFIX directory. This partially works, but the C# wrapper code for the mylib::my_class_in_namespace is missing. When building the library this code lives in the ${PROJECT_BINARY_DIR}/CMakeFiles/mylib_csharp.dir/mylib.files/mylib/my_class_in_namespace.cs file, but it seems like only the files inside ${PROJECT_BINARY_DIR}/CMakeFiles/mylib_csharp.dir/mylib.files will get copied to SWIG_SUPPORT_FILES_DIRECTORY during the build step.
Tested on MacOS 10.15.4 with CMake 3.17.0, Swig 4.0.1 and using the Ninja generator.