Question

I’m trying to make a repository class that has a method to order the results based upon a “sort” parameter. I need to pass it as a parameter since I’m trying to be very strict that my repository doesn’t return IQueryable and only returns List. The problem is that I have no idea how to make it so that it meets the following requirements:

1. Allow for multiple columns.
2. Strongly typed to the entity that’s returned (no strings for columns as parameters).
3. Ability to set a specific column as descending.

Is this even possible or is it useless that a Repository allows returning with ordering? Should the repository be just able to perform CRUD operations? Maybe returning IQueryable would be the best option?

Answer 1

Perhaps you need something like this:

public class Ordering<T>
{
    private readonly Func<IQueryable<T>, IOrderedQueryable<T>> transform;

    private Ordering(Func<IQueryable<T>, IOrderedQueryable<T>> transform)
    {
        this.transform = transform;
    }

    public static Ordering<T> Create<TKey>
        (Expression<Func<T, TKey>> primary)
    {
        return new Ordering<T>(query => query.OrderBy(primary));
    }

    public Ordering<T> ThenBy<TKey>(Expression<Func<T, TKey>> secondary)
    {
        return new Ordering<T>(query => transform(query).ThenBy(secondary));
    }

    // And more for the descending methods...

    internal IOrderedQueryable<T> Apply(IQueryable<T> query)
    {
        return transform(query);
    }
}

Then a client can create an Ordering<T> to be passed into the repository, and the repositry can call Apply with the IQueryable<T>. Does that make sense?

Sample (slightly silly) use:

var ordering = Ordering<FileInfo>.Create(fi => fi.Length)
                                 .ThenBy(fi => fi.Name);