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.
You should reference System.Web and call:

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

```