Wsrtjtyk

I'm implementing the filter (criteria) pattern and I'm not sure what the best approach is to create a generic TypeOfCriteria.

To give some context, part of the filtering system is specific, for example I have an EmailAddressContains filter inside a UserCriteria folder. The user class is abstract and there are several different usertypes inheriting from that base class, for example AdminUser and FullControlUser. Other parts of the system have similar inheritance trees. At first I implemented specific TypeOf criteria like TypeOfAdminUser, but I soon figured I could make this more generic.

I came up with three approaches and I'm not sure which is "the best". Or perhaps there's another approach which I haven't thought of yet.

Approach 1

public interface ICriteria<T> where T : BaseEntity

{

    IEnumerable<T> MeetCriteria(IEnumerable<T> entities);

}



public class TypeOfCriteria<T> : ICriteria<T> where T : BaseEntity

{

    private readonly Type _type;



    public TypeOfCriteria(Type type)

    {

        _type = type;

    }



    public IEnumerable<T> MeetCriteria(IEnumerable<T> entities)

    {

        return from T entity in entities

               where entity.GetType().Equals(_type)

               select entity;

    }

}

Usage:

ICriteria<User> criteria = new TypeOfCriteria<User>(typeof(AdminUser));

admins = criteria.MeetCriteria(users);

Approach 2

public interface ICriteria<T> where T : BaseEntity

{

    IEnumerable<T> MeetCriteria(IEnumerable<T> entities);

}



public class TypeOfCriteria<T, TResult> : ICriteria<T>

    where T : BaseEntity

    where TResult : BaseEntity

{

    public IEnumerable<T> MeetCriteria(IEnumerable<T> entities)

    {

        return entities.OfType<TResult>().Cast<T>();

    }

}

Usage:

ICriteria<User> criteria = new TypeOfCriteria<User, AdminUser>();

admins = criteria.MeetCriteria(users);

Approach 3

public interface ICriteria<T> where T : BaseEntity

{

    IEnumerable<T> MeetCriteria<TResult>(IEnumerable<T> entities);

}



public class TypeOfCriteria<T> : ICriteria<T> where T : BaseEntity

{

    public IEnumerable<T> MeetCriteria<TResult>(IEnumerable<T> entities)

    {

        return entities.OfType<TResult>().Cast<T>();

    }

}

Usage:

ICriteria<User> criteria = new TypeOfCriteria<User>();

admins = criteria.MeetCriteria<AdminUser>(users);

Pros & Cons

Approach 1 feels the most like my other Criteria classes, where I pass the actual criteria in the constructor (new EmailAddressContains(".com")).

The MeetCriteria implementation of Approach 2 feels a bit cleaner. I'm not sure about performance, this looks like it could be faster, but that's not really a metric that I worry about for this project.

I like Approach 3 the least, because it requires me to change the interface.