Question: How can i use the server.mappath method in a C# class library class ,which acts as my BusinessLayer for My ASP.NET WEbsite

Answer: 
By calling it?

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

```

Make sure you add a reference to the System.Web assembly.
Architecturally, System.web should not be referred in Business Logic Layer (BLL). Employ BLL into the solution structure to follow the separate of concern principle so refer System.Web is a bad practice. BLL should not load/run in Asp.net context. Because of the reason you should consider using of `System.AppDomain.CurrentDomain.BaseDirectory` instead of `System.Web.HttpContext.Current.Server.MapPath`