This may happen if your application has never stored anything in the session state. In this case, a new session state (with a new ID) is created in every request, but is never saved because it contains nothing. However, there are two exceptions to this same session ID behavior: If the user has used the same browser instance to request another page that uses the session state, you will get the same session ID every time. For details, see "Why does the SessionID remain the same after the Session times out?"If the Session_OnStart event is used, ASP.NET will save the session state even when it is empty.

When using cookie-based session state, ASP.NET does not allocate storage for session data until the Session object is used. As a result, a new session ID is generated for each page request until the session object is accessed. If your application requires a static session ID for the entire session, you can either implement the Session_Start method in the application’s Global.asax file and store data in the Session object to fix the session ID, or you can use code in another part of your application to explicitly store data in the Session object.

When using cookie-based session state, ASP.NET does not allocate storage for session data until the Session object is used. As a result, a new session ID is generated for each page request until the session object is accessed. If your application requires a static session ID for the entire session, you can either implement the Session_Start method in the application’s Global.asax file and store data in the Session object to fix the session ID, or you can use code in another part of your application to explicitly store data in the Session object.