Use own IComparer with Linq OrderBy

Question

I have a generic

List<MyClass>

where MyClass has a property InvoiceNumber which contains values such as:

200906/1
200906/2
..
200906/10
200906/11
200906/12

My list is bound to a

BindingList<T>

which supports sorting with linq:

protected override void ApplySortCore(
           PropertyDescriptor property, ListSortDirection direction)
{

    _sortProperty = property;
    _sortDirection = direction;

    var items = this.Items;

    switch (direction)
    {
        case ListSortDirection.Ascending:
            items = items.OrderByDescending(x => property.GetValue(x)).ToList();
            break;
        case ListSortDirection.Descending:
            items = items.OrderByDescending(x => property.GetValue(x)).ToList();
            break;
    }

    this.Items = items;

}

However the default comparer sorts (as supposed) like this:

200906/1
200906/10
200906/11
200906/12
200906/2

which is nasty in this case.

Now I want to use my own IComparer<T> with this. It looks like this:

public class MyComparer : IComparer<Object>
{

    public int Compare(Object stringA, Object stringB)
    {
        String[] valueA = stringA.ToString().Split('/');
        String[] valueB = stringB.ToString().Split('/');

        if(valueA .Length != 2 || valueB .Length != 2)
             return String.Compare(stringA.ToString(), stringB.ToString());

        if (valueA[0] == valueB[0]) 
        {
          return String.Compare(valueA[1], valueB[1]);
        }
        else
        {
          return String.Compare(valueA[0], valueB[0]);
        }

    }

}

and changed the ApplySortCore code to use this IComparer:

case ListSortDirection.Ascending:
    MyComparer comparer = new MyComparer();
    items = items.OrderByDescending(
              x => property.GetValue(x), comparer).ToList();
    break;

When I debug my code, I see that MyComparer.Compare(object, object) is called multiple times and returns the right values (-1, 0, 1) for a compare method.

But my list is still sorted the "wrong" way. Am I missing something? I have no clue.

Answer 1

You can use The Alphanum Algorithm:

  (...)
    items.OrderBy(x => property.GetValue(x), new AlphanumComparator())
  (...)

AlphanumComparator

/*
 * The Alphanum Algorithm is an improved sorting algorithm for strings
 * containing numbers.  Instead of sorting numbers in ASCII order like
 * a standard sort, this algorithm sorts numbers in numeric order.
 *
 * The Alphanum Algorithm is discussed at http://www.DaveKoelle.com
 *
 * Based on the Java implementation of Dave Koelle's Alphanum algorithm.
 * Contributed by Jonathan Ruckwood <[e-mail]>
 *
 * Adapted by Dominik Hurnaus <[e-mail]> to
 *   - correctly sort words where one word starts with another word
 *   - have slightly better performance
 *
 * Released under the MIT License - https://opensource.org/licenses/MIT
 *
 * Permission is hereby granted, free of charge, to any person obtaining
 * a copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included
 * in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
 * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
 * DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
 * OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
 * USE OR OTHER DEALINGS IN THE SOFTWARE.
 *
 */
using System;
using System.Collections;
using System.Text;

/*
 * Please compare against the latest Java version at http://www.DaveKoelle.com
 * to see the most recent modifications
 */
namespace AlphanumComparator
{
    public class AlphanumComparator : IComparer
    {
        private enum ChunkType {Alphanumeric, Numeric};
        private bool InChunk(char ch, char otherCh)
        {
            ChunkType type = ChunkType.Alphanumeric;

            if (char.IsDigit(otherCh))
            {
                type = ChunkType.Numeric;
            }

            if ((type == ChunkType.Alphanumeric && char.IsDigit(ch))
                || (type == ChunkType.Numeric && !char.IsDigit(ch)))
            {
                return false;
            }

            return true;
        }

        public int Compare(object x, object y)
        {
            String s1 = x as string;
            String s2 = y as string;
            if (s1 == null || s2 == null)
            {
                return 0;
            }

            int thisMarker = 0, thisNumericChunk = 0;
            int thatMarker = 0, thatNumericChunk = 0;

            while ((thisMarker < s1.Length) || (thatMarker < s2.Length))
            {
                if (thisMarker >= s1.Length)
                {
                    return -1;
                }
                else if (thatMarker >= s2.Length)
                {
                    return 1;
                }
                char thisCh = s1[thisMarker];
                char thatCh = s2[thatMarker];

                StringBuilder thisChunk = new StringBuilder();
                StringBuilder thatChunk = new StringBuilder();

                while ((thisMarker < s1.Length) && (thisChunk.Length==0 ||InChunk(thisCh, thisChunk[0])))
                {
                    thisChunk.Append(thisCh);
                    thisMarker++;

                    if (thisMarker < s1.Length)
                    {
                        thisCh = s1[thisMarker];
                    }
                }

                while ((thatMarker < s2.Length) && (thatChunk.Length==0 ||InChunk(thatCh, thatChunk[0])))
                {
                    thatChunk.Append(thatCh);
                    thatMarker++;

                    if (thatMarker < s2.Length)
                    {
                        thatCh = s2[thatMarker];
                    }
                }

                int result = 0;
                // If both chunks contain numeric characters, sort them numerically
                if (char.IsDigit(thisChunk[0]) && char.IsDigit(thatChunk[0]))
                {
                    thisNumericChunk = Convert.ToInt32(thisChunk.ToString());
                    thatNumericChunk = Convert.ToInt32(thatChunk.ToString());

                    if (thisNumericChunk < thatNumericChunk)
                    {
                        result = -1;
                    }

                    if (thisNumericChunk > thatNumericChunk)
                    {
                        result = 1;
                    }
                }
                else
                {
                    result = thisChunk.ToString().CompareTo(thatChunk.ToString());
                }

                if (result != 0)
                {
                    return result;
                }
            }

            return 0;
        }
    }
}

Answer 2

Your comparer looks wrong to me. You're still just sorting in the default text ordering. Surely you want to be parsing the two numbers and sorting based on that:

public int Compare(Object stringA, Object stringB)
{
    string[] valueA = stringA.ToString().Split('/');
    string[] valueB = stringB.ToString().Split('/');

    if (valueA.Length != 2 || valueB.Length != 2)
    {
        stringA.ToString().CompareTo(stringB.ToString());
    }

    // Note: do error checking and consider i18n issues too :)
    if (valueA[0] == valueB[0]) 
    {
        return int.Parse(valueA[1]).CompareTo(int.Parse(valueB[1]));
    }
    else
    {
        return int.Parse(valueA[0]).CompareTo(int.Parse(valueB[0]));
    }
}

(Note that this doesn't sit well with your question stating that you've debugged through and verified that Compare is returning the right value - but I'm afraid I suspect human error on that front.)

Additionally, Sven's right - changing the value of items doesn't change your bound list at all. You should add:

this.Items = items;

at the bottom of your method.

Answer 3

I encountered the issue of general natural sorting and blogged the solution here:

Natural Sort Compare with Linq OrderBy()

public class NaturalSortComparer<T> : IComparer<string>, IDisposable
{
    private bool isAscending;

    public NaturalSortComparer(bool inAscendingOrder = true)
    {
        this.isAscending = inAscendingOrder;
    }

    #region IComparer<string> Members

    public int Compare(string x, string y)
    {
        throw new NotImplementedException();
    }

    #endregion

    #region IComparer<string> Members

    int IComparer<string>.Compare(string x, string y)
    {
        if (x == y)
            return 0;

        string[] x1, y1;

        if (!table.TryGetValue(x, out x1))
        {
            x1 = Regex.Split(x.Replace(" ", ""), "([0-9]+)");
            table.Add(x, x1);
        }

        if (!table.TryGetValue(y, out y1))
        {
            y1 = Regex.Split(y.Replace(" ", ""), "([0-9]+)");
            table.Add(y, y1);
        }

        int returnVal;

        for (int i = 0; i < x1.Length && i < y1.Length; i++)
        {
            if (x1[i] != y1[i])
            {
                returnVal = PartCompare(x1[i], y1[i]);
                return isAscending ? returnVal : -returnVal;
            }
        }

        if (y1.Length > x1.Length)
        {
            returnVal = 1;
        }
        else if (x1.Length > y1.Length)
        { 
            returnVal = -1; 
        }
        else
        {
            returnVal = 0;
        }

        return isAscending ? returnVal : -returnVal;
    }

    private static int PartCompare(string left, string right)
    {
        int x, y;
        if (!int.TryParse(left, out x))
            return left.CompareTo(right);

        if (!int.TryParse(right, out y))
            return left.CompareTo(right);

        return x.CompareTo(y);
    }

    #endregion

    private Dictionary<string, string[]> table = new Dictionary<string, string[]>();

    public void Dispose()
    {
        table.Clear();
        table = null;
    }
}

Answer 4

The sorted list is only bound to the local variable items, not to the Items property of your binding list, hence it remains unsorted.

[Edit] Basically, you're simply throwing away the result of your sorting efforts ;-)

Answer 5

Can't we do like this:

public class MyComparer : IComparer<string>
{

    public int Compare(string stringA, string stringB)
    {
        string small = stringA;
        string big = stringB;
        if (stringA.Length > stringB.Length)
        {
            small = stringB;
            big = stringA;
        }
        else if (stringA.Length < stringB.Length)
        {
            small = stringA;
            big = stringB;
        }
        for (int j = 0; j < small.Length; j++)
        {
            if (Convert.ToInt32(small[j]) > Convert.ToInt32(big[j])) return -1;
            if (Convert.ToInt32(small[j]) < Convert.ToInt32(big[j])) return 1;
        }

        //big is indeed bigger
        if (big.Length > small.Length) return 1;

        //finally they are smae
        return 0;
    }
}

Usage:

string[] inputStrings = {"_abc*&","#almnp","abc" };
//string[] inputStrings = { "#", "_", "_a", "@", "_" };
MyComparer computer = new MyComparer();
var kola = inputStrings.OrderBy(x => x, new MyComparer()).ToArray();

This is same as :

    Array.Sort(inputStrings, StringComparer.Ordinal);