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: 
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.