Question

Is there anyway this code can be refactored? The only difference is the order by part.

Idealy I’d like to use a delegate/lambda expression so the code is reusable but I don’t know how to conditionally add and remove the query operators OrderBy and OrderByDescending

var linq = new NorthwindDataContext();

        var query1 = linq.Customers
            .Where(c => c.ContactName.StartsWith("a"))
            .SelectMany(cus=>cus.Orders)
            .OrderBy(ord => ord.OrderDate)
            .Select(ord => ord.CustomerID);

        var query2 = linq.Customers
            .Where(c => c.ContactName.StartsWith("a"))
            .SelectMany(cus => cus.Orders)
            .OrderByDescending(ord => ord.OrderDate)
            .Select(ord => ord.CustomerID);

Answer 1

You can create your own reusable extension method which will do this:

public static IOrderedQueryable<TSource> OrderBy<TSource, TKey>
    (this IQueryable<TSource> source,
     Expression<Func<TSource, TKey>> keySelector,
     bool ascending)
{
     return ascending ? source.OrderBy(keySelector)
          : source.OrderByDescending(keySelector);
}

and similarly for ThenBy:

public static IOrderedQueryable<TSource> ThenBy<TSource, TKey>
    (this IOrderedQueryable<TSource> source,
     Expression<Func<TSource, TKey>> keySelector,
     bool ascending)
{
     return ascending ? source.ThenBy(keySelector)
          : source.ThenByDescending(keySelector);
}