how to get the application path in asp.net?

Server.MapPath("~/bin")

You could also use the HostingEnvironment.ApplicationPhysicalPath property.

Gets the ASP.NET application's virtual application root path on the server.

Request.ApplicationPath;

http://msdn.microsoft.com/en-us/library/system.web.httprequest.applicationpath.aspx

ResolveUrl("~/bin");