Question

So, I’m beggining to use EF, and I’m developing an application using it as ORM. The thing is I haven’t had much time to dig into the documentation (I Plan to, in due time)and I’m kind of lost in some things. For example, I have these two queries:

public static int GetNextPadlockNumber()
        {
            LockersDBEntities1 entities = new LockersDBEntities1();
            var query = (from p in entities.PadLocks select p.PadlockNumber).DefaultIfEmpty(0).Max();
            return (int)query + 1;
        }

        public static Data.PadLock GetPadLockByNumber(int number)
        {
            Data.LockersDBEntities1 entities = new LockersDBEntities1();
            var query = (from p in entities.PadLocks where p.PadlockNumber == number select p).FirstOrDefault();
            return query;
        }

and

public static int GetNextLockerNumber()
        {
            LockersDBEntities1 entities = new LockersDBEntities1();
            var query = (from l in entities.Lockers select l.LockerNumber).DefaultIfEmpty(0).Max();
            return (int)query+1;
        }

        public static Data.Locker GetLockerByNumber(int number)
        {
            Data.LockersDBEntities1 entities = new LockersDBEntities1();
            var query = (from l in entities.Lockers where l.LockerNumber == number select l).FirstOrDefault();
            return query;
        }

And the thing is They’re Exactly the same query. Isn’t there Any way to do this without having to specify that I want a locker or a padlock? Thanks in advance heroes.

Answer 1

One of the great things with EF is that you can do things like that. It’s not trivial, and it will take some trial and error, but you can abstract a lot of the database complexities away.

Now, assuming that LockersDBEntities1 is a DbContext, you could something like this:

public static class LockersDBEntities1Extensions
{
   public static int GetNextNumber<T>(
        this LockersDBEntities1 context, 
        Expression<Func<T, int>> getNumberExpression)
   {
     var query = (from item in context.Set<T> 
                  select getNumberExpression(item))
                 .DefaultIfEmpty(0)
                 .Max();
     return (int)query + 1;
    }
}

and use it like:

int nextPadlockNumber = new LockersDBEntities1()
                                    .GetNextNumber<Padlock>(p => p.PadlockNumber)

and

int nextPadlockNumber = new LockersDBEntities1()
                                    .GetNextNumber<Locker>(l => l.LockerNumber)

The getNumberExpression expression is needed because there is no common way to access the number across all entities. It might be overdesign, but if that is an issue I would do something like this:

//this should be a better name, to reflect the real scenarion
interface ILockNumberProvider 
{
   int Number {get; }
}

and implement that interface on Locker, Padlock and other classes that need to provide lock numbers. Then, in the method above the expression can be omitted, and a generic constraint can be used, like:

public static class LockersDBEntities1Extensions
{
   public static int GetNextNumber<T>(this LockersDBEntities1 context)
                     where T:ILockNumberProvider 
   {
     var query = (from item in context.Set<T> 
                  select item.Number)
                 .DefaultIfEmpty(0)
                 .Max();
     return (int)query + 1;
    }
}