using System; using System.Diagnostics; namespace Beginning.CSharp { public class HelloWorld { public HelloWorld() { Console.WriteLine("Hello World"); } } } Compile using "csc /target:library HelloWorld.cs" and hey presto...your very own DLL.....!

Change the web.config as suggested, or if you're able, get onto the box (ie using terminal services or similar) then when you run the application it'll give you the stack trace which should help you locate the error. .NET hides the full details by default unless you're running it on the box where the web application is hosted.