CODEWITHSUNDEEP
linqnhibernatelinq-to-sqllinq-to-nhibernate
Baz1nga
Baz1nga

Reputation: 15579

Linq queries on dB and using custom Comparers

Whats the use of using custom comparers in Linq queries? Are they beneficial or are they just an overload on the server.

So i am talking about queries like 

IEnumerable<Class> GetMatch(Class comparerObject)
{
  return  Session.Linq<Class>().Where(x=>new StringComparer<Class>().Equals(x,comparerObject))
}

and this is how my stringcomparer class looks like

public class StringComparer<T> : IEqualityComparer<T>
    where T : class, IStringIdentifiable
{
    public bool Equals(T x, T y)
    {
        if (x == null && y == null)
            return true;

        if (x == null || y == null)
            return false;

        return x.Id.Equals(y.Id, StringComparison.OrdinalIgnoreCase);
    }

So I was wondering how is this query run against the db? I think linq handles this internally where in it sends a request to the db only after all the cases in the comparere are run.

Edit:

Well if you are finding it hard to believe that the above will not work then take a simple example like

return  Session.Linq<Class>().Where(x=>x.Id.Equals(comparerObject,StringComparison.InvariantCultureIgnoreCase))

Then what do you think is the expected behavior?

Thanks.

Upvotes: 0

Views: 121

Answers (1)

Jon Skeet
Jon Skeet

Reputation: 1500835

For LINQ to SQL, I'd expect that to fail at execution time - the query translator won't know what to do with your StringComparer<T> class.

In the case you've given, you should just use:

string idToMatch = comparerObject.Id;
return Session.Linq<Class>().Where(x => x.Id == idToMatch);

More complicated cases may or may not be feasible depending on exactly what you want to achieve.

Upvotes: 1

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