Asp – Problem with JsonResult exception

You should never catch exceptions and ignore them during a NHibernate transaction.

I try to explain why.

There could be exceptions for instance caused by constraints in the database. (it could also be caused by mapping problems, exceptions thrown by properties or anything else.) NHibernate tries to synchronize the state in memory with the database. This is done on commit - and sometimes before queries to make sure that queries are done on actual data. When this synchronization fails, the state in the database is something random, some changes are persisted, others are not. The only thing you can do in such a case is closing the session.

Consider that decisions and calculations in your code are based on values in memory. But - in case of an ignored exception, this values are not the values in the database, they will never be there. So your logic will decide and calculate on 'fantasy-data'.

By the way, it is never a good idea to catch any exception (untyped) and ignore them. You should always know the exceptions you handle, and be sure that you can continue.

What you're doing here is swallowing programming errors. Believe me, the system will not be more stable. The question is only: do you notice the error when it occurs, or do you ignore it there and even persist the result of the error to the database? When you do the latter, you don't have to be surprised when your database is inconsistent and other error arise when you try to get the data from the database. And you will never ever find the code that is the actual cause of the error.

I think this confusing exception occurs when you use a variable in a lambda which is a null-reference at run-time. In your case, I would check if your variable calculationViewModel is a null-reference.

Something like:

public ActionResult MNPurchase()
{
    CalculationViewModel calculationViewModel = (CalculationViewModel)TempData["calculationViewModel"];

    if (calculationViewModel != null)
    {
        decimal OP = landTitleUnitOfWork.Sales.Find()
            .Where(x => x.Min >= calculationViewModel.SalesPrice)
            .FirstOrDefault()
            .OP;

        decimal MP = landTitleUnitOfWork.Sales.Find()
            .Where(x => x.Min >= calculationViewModel.MortgageAmount)
            .FirstOrDefault()
            .MP;

        calculationViewModel.LoanAmount = (OP + 100) - MP;
        calculationViewModel.LendersTitleInsurance = (calculationViewModel.LoanAmount + 850);

        return View(calculationViewModel);
    }
    else
    {
        // Do something else...
    }
}