MTrinder wrote:
Sorry, I still can't see in that article where the entities are implementing any interfaces at all - we're still on entirely clear what you are trying to acheive.
You said in your original post that you had tried something that didn't work - perhaps you could show us what you tried and why it failed...?
Matt
Here is the section:
Generic Predicates
Suppose every table in your database has ValidFrom and ValidTo columns as follows:
create table PriceList
(
ID int not null primary key,
Name nvarchar(50) not null,
ValidFrom datetime,
ValidTo datetime
)
To retrieve rows valid as of DateTime.Now (the most common case), you'd do this:
from p in PriceLists
where (p.ValidFrom == null || p.ValidFrom <= DateTime.Now) &&
(p.ValidTo == null || p.ValidTo >= DateTime.Now)
select p.Name
Of course, that logic in bold is likely to be duplicated across multiple queries! No problem: let's define a method in the PriceList class that returns a reusable expression:
public static Expression<Func<PriceList, bool>> IsCurrent()
{
return p => (p.ValidFrom == null || p.ValidFrom <= DateTime.Now) &&
(p.ValidTo == null || p.ValidTo >= DateTime.Now);
}
OK: our query is now much simpler:
var currentPriceLists = db.PriceLists.Where (PriceList.IsCurrent());
And with PredicateBuilder's And and Or methods, we can easily introduce other conditions:
var currentPriceLists = db.PriceLists.Where (
PriceList.IsCurrent().And (p => p.Name.StartsWith ("A")));
But what about all the other tables that also have ValidFrom and ValidTo columns? We don't want to repeat our IsCurrent method for every table! Fortunately, we can generalize our IsCurrent method with generics.
The first step is to define an interface:
public interface IValidFromTo
{
DateTime? ValidFrom { get; }
DateTime? ValidTo { get; }
}
Now we can define a single generic IsCurrent method using that interface as a constraint:
public static Expression<Func<TEntity, bool>> IsCurrent<TEntity>()
where TEntity : IValidFromTo
{
return e => (e.ValidFrom == null || e.ValidFrom <= DateTime.Now) &&
(e.ValidTo == null || e.ValidTo >= DateTime.Now);
}
The final step is to implement this interface in each class that supports ValidFrom and ValidTo. If you're using Visual Studio or a tool like SqlMetal to generate your entity classes, do this in the non-generated half of the partial classes:
public partial class PriceList : IValidFromTo { }
public partial class Product : IValidFromTo { }
