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: 
Use this [System.Web.Hosting.HostingEnvironment.MapPath()](https://msdn.microsoft.com/en-us/library/system.web.hosting.hostingenvironment.mappath.aspx).

```
HostingEnvironment.MapPath("~/file")

```

Wonder why nobody mentioned it here.
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.