Server.Mappath in C# classlibrary

By calling it?

var path = System.Web.HttpContext.Current.Server.MapPath("default.aspx");

Make sure you add a reference to the System.Web assembly.

You can get the base path by using the following code and append your needed path with that.

string  path = System.AppDomain.CurrentDomain.BaseDirectory;

You should reference System.Web and call:

  HttpContext.Current.Server.MapPath(...)

Maybe you could abstract this as a dependency and create an IVirtualPathResolver. This way your service classes wouldn't be bound to System.Web and you could create another implementation if you wanted to reuse your logic in a different UI technology.