Question

I end up writing a lot of code that looks like

var ordered = isDescending ? queryable.OrderByDescending(x => x.ID) : queryable.OrderBy(x => x.ID)

but with different expressions such as x => x.DateOfBirth etc. What I would like to do is place that in a generic extension method that I could parse my expression, and a boolean isDescending to, but I’m not sure how to do this. Something like

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

Can anyone tell me how to do this? Preferably with an explanation so that I can understand it.

Answer 1

You’re very nearly there – you’re just missing a type parameter:

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

Your code wasn’t working before because the func parameter’s type used TKey, and that wasn’t one of the type parameters in your method declaration. The above should work fine.