Quick question: Best way to write data to local json file?

I'm just wondering what the recommended way for me to write content to a local json file from an ASP.NET 5 MVC6 web api controller is?

I had no luck using a streamwriter and File.WriteAllText doesn't seem to work neither. Perhaps I'm just failing on adding the needed dependencies or something. I would also like to know what the best replacement for Server.MapPath is in ASP.NET 5's current state?

Thanks in advance!