asp.net mvc 3 - Entity Framework Optimistic Concurrency Exception not occuring

Question

We have an ASP.Net MVC application that uses EF4 as its data access layer and we're seeing unexpected behaviour with regards to OptimisitcConcurrencyExceptions not being thrown when we think they should be.

We have simplified the problem down to the following code...

   using System.Linq;
    using Project.Model;

    namespace OptimisticConcurrency
    {
        class Program
        {
            static void Main()
            {
                Contact firstContact = null;
                using (var firstEntities = new ProjectEntities())
                {
                    firstContact = (from c in firstEntities.Contacts 
                       where c.LastName == "smith" select c).Single();
                }

                using (var secondEntities = new ProjectEntities())
                {
                    var secondContact = (from c in secondEntities.Contacts 
                       where c.LastName == "smith" select c).Single();

                    secondContact.Title = "a";
                    secondEntities.SaveChanges();
                }

                firstContact.Title = "b";

                using (var thirdEntities = new ProjectEntities())
                {
                    var thirdContact = (from c in thirdEntities.Contacts 
                       where c.LastName == "smith" select c).Single();

                    thirdContact.Title = firstContact.Title;

                    //EXPLICITLY SET VERSION HERE
                    thirdContact.Version = firstContact.Version;  

                    thirdEntities.SaveChanges();
                }
            }
        }
    }

This is a rather simple version of what happens in our MVC app, but the same problem occurs.

When we call SaveChanges on the thirdEntities, I expect the exception and nothing is being thrown.

Much more interestingly, when we attach the SQL Profiler, we see that the Version is being used in the where clause but it is thirdEntities Version value (the current one in the DB) being used, not the firstEntities values DESPITE it being explicitly set immediately before SaveChanges is called. SaveChanges is resetting the Version to be the retrieved value not the set value.

In the EDMX, the Version is set to have a StoreGeneratedPattern is set to Computed.

Anyone have any idea what is going on here?

Answer 1

This is a problem. Once the column is set to Computed you can't set its value in the application (you can but the value is not used).

Edit:

If you load entity from database it is by default tracked with the context. The context stores its original values. Original values are for example used for snapshot change tracking but they are also used as the only valid source of Computed properties. If you set Computed property in your entity the value is not used and original value is used insted. The workaround is to modify original value (before you modify anything else):

using (var context = new TestEntities())
{
    var entityToUpdate = context.MyEntities.Single(e => e.Id == someId);
    entityToUpdate.Timestamp = entity.Timestamp;

    ObjectStateEntry entry = context.ObjectStateManager.GetObjectStateEntry(entityToUpdate);
    entry.ApplyOriginalValues(entityToUpdate);

    // set modified properties
    context.SaveChanges();
}

Edit 2:

Btw. once you have both actually loaded timestamp and previously retrieved timestamp you can simply compare them in your application instead of doing it in the database.