Force Entity Framework to return a new instance

Question

We have a scenario in our code when only a few properties of an entity are allowed to be changed. To guarantee that, we have code similar to this:

public void SaveCustomer(Customer customer)
{
    var originalCustomer = dbContext.GetCustomerById(customer.Id);

    if (customer.Name != originalCustomer.Name)
    {
        throw new Exception("Customer name may not be changed.");
    }

    originalCustomer.Address = customer.Address;
    originalCustomer.City = customer.City;

    dbContext.SaveChanges();
}

The problem with this code is that the call to dbContext.GetCustomerById does not always gives me a new instance of the Customer class. If the customer already has been fetched from the database, Entity Framework will keep the instance in memory and return it on every subsequent call.

This leads us to the actual problem - customer and originalCustomer may refer to the same instance. In that case, customer.Name will be equal to originalCustomer.Name and we will not be able to detect if it differs from the database.

I guess the same problem exists with most other ORMs as well, because of the identitymap design pattern.

Any ideas how this can be solved? Can I somehow force EF to always give me a new instance of the customer class?

Or should we refactor the code instead? Does anyone know of any good design patterns for this scenario?

Answer 1

you can try by detaching the entity from the context, this will remove all the references to the context (as well as the identitymap behaviour). So, before passing the Customer to your method you can detach it:

yourContext.Detach(customer);