Linq To SQL：防止创建重复项

Question

Consider the following simple User table

I want to write Linq To SQL Code that returns a user by some predicate, eg the email address, or creates the user if he does not yet exists. The code might look like

User GetUser(String name, String email) {
    using (var context = new DataContext()) {
        User user = context.Users.FirstOrDefault(u => u.email.Equals(email));
        if (user != null)
            return user;
        user = new User() { name = name, email = email };
        context.Users.InsertOnSubmit(user);
        context.SubmitChanges();
        return user;
    }
}

The code might get executed in parallel, but should not create duplicate users.

I would prefer to not create a unique constraint for the table for several reasons. Do you see any alternative that could be realized using Linq To SQL and without touching the database?

Answer 1

Use TransactionScope (Serializable isolation level is by default):

User GetUser(String name, String email) 
{
    using (var tran = new TransactionScope())
    {
        using (var context = new DataContext()) 
        {
            User user = context.Users.FirstOrDefault(u => u.email.Equals(email));
            if (user != null)
                return user;
            user = new User() { name = name, email = email };
            context.Users.InsertOnSubmit(user);
            context.SubmitChanges();

            tran.Complete();

            return user;
        }
    }
}

More info on Transactions support in LINQ to Sql: http://msdn.microsoft.com/en-us/library/bb386995