Google.Apis.Drive.v2.Data and System.IO both define File class, I mean
Both namespaces (Google.Apis.Drive.v2.Data and System.IO) defines class File and, probably, you have both namespaces "included" with using statement. you need to use fully qualified name, like below snippet
using GoogleDataAPI = Google.Apis.Drive.v2.Data;
using SystemFile = System.IO.File
it will resolve the things