How Do I Sort IList?

Question

There\'s no Sort() function for IList. Can someoene help me with this? I want to sort my own IList.

Suppose this is my IList:

Answer 1

var sorted = myObj.OrderBy(x => x.marker);

Answer 2

I would go against using OrderBy with a list because it's a LINQ extension method, therefore:

• It wraps the list in an enumerable, then enumerates it and fills a new temporary list, then sorts this new list.
• It wraps the sorted list inside another enumerable.
• Then when you call ToList(), it iterates on it and fills another new list with the items.

In essence: it creates and fills 2 new lists and 2 enumerables in addition to the actual sorting. In comparison, List.Sort() sorts in place and create nothing so it's way more efficient.

My recommendation would be:

• If you know the underlying type, use List.Sort() or Array.Sort(array)
• If you don't know the underlying type, copy the List to a temporary array and sort it using Array.Sort(array) and return it.

Answer 3

Use OrderBy

Example

public class MyObject() 
{
    public int number { get; set; }
    public string marker { get; set; }
}

IList<MyObject> myobj = new List<MyObject>();
var orderedList = myobj.OrderBy(x => x.marker).ToList();

For a case insensitive you should use a IComparer

public class CaseInsensitiveComparer : IComparer<string>
{
    public int Compare(string x, string y)
    {
        return string.Compare(x, y, StringComparison.OrdinalIgnoreCase);
    }
}

IList<MyObject> myobj = new List<MyObject>();
var orderedList = myobj.OrderBy(x => x.marker, new CaseInsensitiveComparer()).ToList();

Answer 4

To sort in-place you would essentially see these two approaches:

        IList<T> list = .... // your ilist
        var sorted = list.ToArray();
        Array.Sort(sorted);

        for (int i = 0; i < list.Count; i++)
        {
            list[i] = sorted[i];
        }

and

        IList<T> list = .... // your ilist
        ArrayList.Adapter((IList)list).Sort();

The second one might look simpler but won't be great for value type collections since it incur boxing penalties. Furthermore there is no guarantee your IList<T> will be implementing IList. First one is better IMO.

---

You can also use the first approach to sort an ICollection<T> in-place but it is questionable if you should expose such a functionality since ICollection<T> contract doesn't guarantee an order (think hash structures). Anyway to show you code example:

    ICollection<T> collection = .... // your icollection
    var sorted = collection.ToArray();
    Array.Sort(sorted);

    collection.Clear();
    foreach (var i in sorted)
    {
       collection.Add(i);
    }

---

A note on sort stability, .NET's Array/List sorting algorithms are unstable. For a stable sort you will have to use:

        IList<T> list = .... // your ilist
        var sorted = list.OrderBy(i => i).ToArray();

        for (int i = 0; i < list.Count; i++)
        {
            list[i] = sorted[i];
        }

This can't be as fast as unstable sorts.

---

Finally, for a complete answer, perhaps a composite approach taken by watbywbarif is better:

    public static void Sort<T>(this IList<T> list, IComparer<T> comparer, bool stable)
    {
        if (stable)
        {
            list.StableSort(comparer);
        }
        else
        {
            list.UnstableSort(comparer);
        }
    }

    static void StableSort<T>(this IList<T> list, IComparer<T> comparer)
    {
        list.OrderBy(x => x, comparer).CopyTo(list);
    }

    static void UnstableSort<T>(this IList<T> list, IComparer<T> comparer)
    {
        switch (list)
        {
            case List<T> l:
                l.Sort(comparer);
                break;

            case T[] a:
                Array.Sort(a, comparer);
                break;

            default:
                T[] sortable = list.ToArray();
                sortable.UnstableSort(comparer);
                sortable.CopyTo(list);
                break;
        }
    }

    static void CopyTo<T>(this IEnumerable<T> source, IList<T> target)
    {
        int i = 0;
        foreach (T item in source)
        {
            target[i++] = item;
        }
    }

That's as far as built-in approaches go. For faster implemenation you will have to roll out your own, see: https://stackoverflow.com/a/19167475

Answer 5

For explanation why not to use OrderBy or similar check Christophe's answer.

Here is one attempt to make fast Sort:

public static void Sort<T>(this IList<T> ilist)
{
    switch(ilist)
    {
        case List<T> lst:
            lst.Sort();
            break;
        case Array arr:
            Array.Sort(arr);
            break;
        default:
            throw new NotImplementedException();
            // or add slow impl if you don't want this to fail!!
    }
}

Answer 6

OrderBy definitely gets the job done, but I personally prefer the syntax of List.Sort because you can feed it a Comparison<T> delegate instead of having to write a class that implements IComparer<T>. We can accomplish that goal with an extension method, and if that's something you're interested in, check out SortExtensions:

http://blog.velir.com/index.php/2011/02/17/ilistt-sorting-a-better-way/