Why is there OrderBy and OrderByDescending but not OrderBy(SortOrder)?

Question

Currently if we get direction of ordering as an external dependency we have to use if to apply this direction:

public static IEnumerable<FileInfo> getlist(string directory, string searchPattern, string order)
{
    var files = new DirectoryInfo(directory).EnumerateFiles(searchPattern);

    if (order == "A")
        return files.OrderBy(f => f.CreationTime);

    return files.OrderByDescending(f => f.CreationTime);
}

Why is there no overload of OrderBy that takes order direction as a parameter? In Reflector I see that it's more or less implemented internally but not exposed for some weird reason.

I would much rather prefer writing something like this:

public static IEnumerable<FileInfo> getlist(string directory, string searchPattern, string order)
{
    return new DirectoryInfo(directory)
        .EnumerateFiles(searchPattern)
        .OrderBy(f => f.CreationTime, order == "A" ? SortOrder.Ascending : SortOrder.Descending);
}

Update:

I can write this myself, just was hoping that it's already in the framework:

public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(
    this IEnumerable<TSource> source,
    Func<TSource, TKey> keySelector,
    ListSortDirection order)
{
    switch (order)
    {
        case ListSortDirection.Ascending: return source.OrderBy(keySelector);
        case ListSortDirection.Descending: return source.OrderByDescending(keySelector);
    }

    throw new ArgumentOutOfRangeException("order");
}

Answer 1

Since a SortOrder enumeration can technically take on more than 2 values (think (SortOrder) 35) it wouldn't capture the duality exactly. Having 2 methods ensures there is no ambiguity or need for range-checking (which is missing from your example btw).

That said, here is the method you want:

public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(
    this IEnumerable<TSource> source,
    Func<TSource, TKey> keySelector,
    SortOrder order)
{
    if(order < SortOrder.Ascending || order > SortOrder.Descending)
    {
        throw new ArgumentOutOfRangeException("order");
    }

    return order == SortOrder.Ascending
        ? source.OrderBy(keySelector)
        : source.OrderByDescending(keySelector);
}

Answer 2

I don't know why. Though, in the while, you can do it yourself.

public static class IEnumerableSortExtension
{
    public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(
            this IEnumerable<TSource> source,
            Func<TSource, TKey> keySelector,
            SortOrder order)
    {
        if (order == SortOrder.Ascending)
            return this.OrderBy(keySelector);
        else if (order == SortOrder.Descending)
            return this.OrderByDescending(keySelector);
        throw new InvalidOperationException(); // do something better than this
    }
}

Answer 3

The OrderBy method already has the flexibility that you require, and more, because it can take an optional IComparer<T> argument:

return new DirectoryInfo(directory)
    .EnumerateFiles(searchPattern)
    .OrderBy(f => f.CreationTime, order == "A"
                                      ? Comparer<DateTime>.Default
                                      : new DescendingComparer<DateTime>);

// ...

public DescendingComparer<T> : Comparer<T>
{
    public override int Compare(T x, T y)
    {
        return Comparer<T>.Default.Compare(y, x);
    }
}