How do I sort ArrayList of DateTime objects in descending order?
Thank you.
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5 Answers
First of all, unless you are stuck with using framework 1.1, you should not be using an ArrayList at all. You should use a strongly typed generic List<DateTime> instead.
For custom sorting there is an overload of the Sort method that takes a comparer. By reversing the regular comparison you get a sort in descending order:
list.Sort(delegate(DateTime x, DateTime y){ return y.CompareTo(x); });
Update:
With lambda expressions in C# 3, the delegate is easier to create:
list.Sort((x, y) => y.CompareTo(x));
Comments
As "Guffa" already said, you shouldn't be using ArrayList unless you are in .NET 1.1; here's a simpler List<DateTime> example, though:
List<DateTime> dates = ... // init and fill
dates.Sort();
dates.Reverse();
Your dates are now sorted in descending order.
Comments
Use a DateTime Comparer that sorts in reverse. Call Sort.
public class ReverseDateComparer:IComparer{
public int Compare(object x, object y){
return -1 * DateTime.Compare(x, y);
}
}
list.Sort(new ReverseDateComparer());
1 Comment
redcalx
...and moderately more efficient (avoids the multiply per comparison).
If you are using .NET 3.5:
// ArrayList dates = ...
var sortedDates = dates.OrderByDescending(x => x);
// test it
foreach(DateTime dateTime in sortedDates)
Console.WriteLine(dateTime);
1 Comment
Marc Gravell
From ArrayList you'd need a Cast<DateTime>() in there too.
List<T>.Sort(YourDateTimeComparer) where YourDateTimeComparer : IComparer<DateTime>
Here is an example of custom IComparer use: How to remove duplicates from int[][]
