Preventing race condition of if-exists-update-else-insert in Entity Framework

Question

I\'ve been reading other questions on how to implement if-exists-insert-else-update semantics in EF, but either I\'m not understanding how the answers work, or they are in f

Answer 1

Just to add my way, not that it really deals with the annoyance of exceptions being thrown and transactions not quite cutting it as a scalable solution but it does avoid race conditions from causing problems where lock type solutions are not possible (easily managed) such as in distributed systems.

I very simply use the exception and try the insert first. I use a modification of your original code as an example:

using(var context = new MyEntities())
{
    EntityEntry entityUser = null;
    try 
    {
        user = new User
        {
             // etc
        };
        entityUser = context.Users.Add(user);
        context.SaveChanges(); // Will throw if the entity already exists
    } 
    catch (DbUpdateException x)
    when (x.InnerException != null && x.InnerException.Message.StartsWith("Cannot insert duplicate key row in object"))
    {
        if (entityUser != null)
        {
            // Detach the entity to stop it hanging around on the context
            entityUser.State = EntityState.Detached;
        }
        var user = context.Users.Find(userId);
        if (user != null) // just in case someone deleted it in the mean time
        {
            // update the user
            user.property = newProperty;
            context.SaveChanges();
        }
    }
}

It's not pretty, but it works and might be of use to someone.