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The System.Collections.IComparer interface makes sorting a breeze.

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Have you ever needed to sort a collection that contained data that was not a string or a number?  By simply implementing the System.Collections.IComparer interface you have to power to sort any object anyway you want.

Here is an example of sorting an ArrayList of date strings.

ArrayList arr = new ArrayList();
arr.Add("10/1/2003");
arr.Add("09/4/2002");
arr.Add("1/1/2004");
arr.Add("12/1/1999");
arr.Sort(new DateComparer() );

public class DateComparer : System.Collections.IComparer {
public int Compare(Object x, Object y) {

        if (x == y) return 0;
if (x == null) return -1;
if (y == null) return 1;

        DateTime t1 = DateTime.Parse((string) x);
DateTime t2 = DateTime.Parse((string) y);
        return DateTime.Compare(t1, t2);
}
}

The Compare method returns an int that has the following meanings.
   Return Value                   Condition                
Is less than zero            X is less than y
Zero                            X equals y
Greater than zero           X is greater than y

So how about being able to sort in descending order.   Here is an example that allows you to specify the sortOrder in the constructor.  I simply added an enum for Ascending/Descending, the appropriate constructors, and a condition to the Compare method to check which way to sort.

public class DateComparer : System.Collections.IComparer {
    public enum SortOrder {
        Ascending,
        Descending
    };

    private SortOrder _sortOrder;

    public DateComparer() {
        _sortOrder = SortOrder.Ascending;
    }

    public DateComparer(SortOrder sortOrder) {
        _sortOrder = sortOrder;
    }

    // Note: Always sorts null entries in the front.
    public int Compare(Object x, Object y) {
        if (x == y) return 0;
if (x == null) return -1;
if (y == null) return 1;

        DateTime t1 = DateTime.Parse((string) x);
DateTime t2 = DateTime.Parse((string) y);
if (_sortOrder == SortOrder.Ascending) {
return DateTime.Compare(t1, t2);
} else {
return DateTime.Compare(t2, t1);
}
}

}

IComparer Interface
https://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpref/html/frlrfsystemcollectionsicomparerclasstopic.asp

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