problem while working on sessions

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C#

ASP.NET

hi freinds

I have created some public properties in one class and accesing in session_start event of Global.asax file.
in that event i'm assigning some values into that public properties.

now when i access those properties in class i don't get any value

code of class file:

public string username
      {
          set
          {
              userName = value;
          }
          get
          {
              return userName;
          }
      }

while in global.asax code is :
obj.username = "some user name"; // here obj is object of class
please tell me where i'm getting wrong.

The purpose for doing this is whenever application starts i'll initialize this value...