Use own IComparer<T> 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

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 2

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;     } }