How can you reverse traversal through a c# collection?

Question

Is it possible to have a foreach statement that will traverse through a Collections object in reverse order?

If not a foreach statement is there any other way? Thanks.

Answer 1

You can use a normal for loop backwards, like this:

for (int i = collection.Count - 1; i >= 0 ; i--) {
    var current = collection[i];
    //Do things
}

You can also use LINQ:

foreach(var current in collection.Reverse()) {
    //Do things
}

However, the normal for loop will probably be a little bit faster.

Answer 2

If you're using 3.5, looks like there is a Reverse() method in LINQ That won't iterate through in reverse order, but would reverse the entire list, then you could do your foreach.

Or you could use a simple for statement:

for(int i = list.Count -1; i >= 0; --i)
{
   x = list[i];
}

Answer 3

You could just call Reverse() on the collection.

foreach(var item in collection.Reverse()) { ... }

Answer 4

Alternatively, if the collection's an IEnumerable and therefore without random access, use System.Linq's IEnumerable.Reverse() method and apply forearch as usual.

using System.Linq;

foreach (var c in collection.Reverse()) {
}

Answer 5

        List<string> items = new List<string> 
        { 
            "item 1", 
            "item 2",
            "item 3", 
            "item 4", 
        };
        lines.AsEnumerable().Reverse()
                            .Do(a => Console.WriteLine(a), ex => Console.WriteLine(ex.Message), () => Console.WriteLine("Completed"))
                            .Run();           

Reactive Extension for .NET 3.5/4.0