ASP.NET - null reference exception after adding dll..?

Hi all

i created one class1.dll from  class1.cs file.i added in refrence in my web project.when i run the project
throwing error at class1 cl=new class1()

1. You need to check for the correct DLL is added in your project.
2. Have you coded something in the class ?

yes,one constructor is there with three argument.and some other methods are present .i used csc command to make dll