When I build my project, which references the System.IO.Compression namespace, the build succeeds without issue inside Visual Studio. However, inside the Unity editor, I see the following error: error CS1069: The type name 'ZipArchive' could not be found in the namespace 'System.IO.Compression'. This type has been forwarded to assembly 'System.IO.Compression, Version=188.8.131.52, Culture=neutral, PublicKeyToken=b77a5c561934e089' Consider adding a reference to that assembly. I added a csc.rsp file inside my Assets folder, with the following line: -r:System.IO.Compression.FileSystem.dll Unfortunately, that did not resolve the issue. For reference, here are the properties of the System.IO.Compression DLL that I reference inside Visual Studio: How can I fix this error?