Microsoft Web API: How do you do a Server.MapPath?

string root = HttpContext.Current.Server.MapPath("~/App_Data");

You can use HostingEnvironment.MapPath in any context where System.Web objects like HttpContext.Current are not available (e.g also from a static method).

var mappedPath = System.Web.Hosting.HostingEnvironment.MapPath("~/SomePath");

See also What is the difference between Server.MapPath and HostingEnvironment.MapPath?