how to use session variable in class files asp.net 4.0 C#

Question

i have created on web application in Asp.net 4.0 using C#.

i have implemented session in asp pages but i want to access same session data in my class files.

i have searched regarding this, and then i used

   HttpContext.Current.Session["something"] 

but it is always throwing NullException.

 

I am calling this method from page load of aspx page.

 

Thanks in advance....