C# .NET - object reference not set to an instance of an object

I have a fairly large object that also has a fairly large object within it. among other small objects like a user object, etc. I carry this object in a session. The page for this object is used in order to buy product in options per contract.

functionality on the page...

the functionality is the user comes in to a contract by default (alphabetically set up) and then chooses what products he wants to buy. Then the user buys the product for that day. The user can then either go to another day in the future by pressing the "next day" button to by product for that days as well. The user can do this for as many days out as they want. The user may also be able to go to the next contract by pressing on the "next contract" button. so the user goes through each contract, buys product for as many days out that they wants and then go to the next contract until they have made all of there purchases.

what my code does....

when i load the page for the first time, I instantiate the object, use the object and save data in it and then I put it in a session variable to keep the data that I need to know which contract I need to go to next. When the page reloads...not postback, I check to see if the session variable is null. If the session variable is not null, then i make the object equal to the session variable and use the object and data and then I save the object again in the session variable.

the problem...

The problem is that when I go through 7-20 days in the future, I get the error "object reference not set to an instance of an object" within the product object that is stored in the session. The weird thing is that this error is given on diferent days depending on the contract, but it always eventually errors out. I have noticed that if the contract has less data in it, I can go through more days before I get the error. If I go through a contract with more data, I can go through less days before I get the error.

Does anyone know why this is happening? I have tried to recreate this issue on my local in debug mode, but I never get the errors.

Are you expecting the object to stay in session past the predefined timeout period?  By default it is 20 minutes but this dump of all session (and other things) will also take place any time the app pool in IIS recycles.

I don't think it's a time issue. I cannot even get into the functionality for two minutes before I lose the object. I even tried slowing down the speed at which I am moving from page to page and that doesn't seem to matter. So I am positive it's not a timeout issue.

You'd need to post your code for us to look at.  Remember my previous comments though, if the app pool recycles or a timeout happens, session is lost.  I'm also assuming you are only running this on one web server.

yes one web server. If I instantiate another object (a public one) within an object, will the instantiated object within that object be kept in memory as long as the main object is kept in a session? Maybe that is my problem.

Not exactly sure what are trying to do.  Remember, the object gets serialized in and out of session and a reference is not maintained "inside" session.  Think of session as a bucket, when you pull the apple out and bite into it, the apple that was in session will not show teeth marks until you put the biten apple back into session.