Possible to iterate backwards through a foreach?

c# foreach
JL. picture JL. · Jul 31, 2009 · Viewed 170.8k times · Source

I know I could use a for statement and achieve the same effect, but can I loop backwards through a foreach loop in C#?

Answer

Matt Howells picture Matt Howells · Jul 31, 2009

If you are on .NET 3.5 you can do this:

IEnumerable<int> enumerableThing = ...;
foreach (var x in enumerableThing.Reverse())

It isn't very efficient as it has to basically go through the enumerator forwards putting everything on a stack then pops everything back out in reverse order.

If you have a directly-indexable collection (e.g. IList) you should definitely use a for loop instead.

If you are on .NET 2.0 and cannot use a for loop (i.e. you just have an IEnumerable) then you will just have to write your own Reverse function. This should work:

static IEnumerable<T> Reverse<T>(IEnumerable<T> input)
{
    return new Stack<T>(input);
}

This relies on some behaviour which is perhaps not that obvious. When you pass in an IEnumerable to the stack constructor it will iterate through it and push the items onto the stack. When you then iterate through the stack it pops things back out in reverse order.

This and the .NET 3.5 Reverse() extension method will obviously blow up if you feed it an IEnumerable which never stops returning items.

Related questions

How do you get the index of the current iteration of a foreach loop?

Is there some rare language construct I haven't encountered (like the few I've learned recently, some on Stack Overflow) in C# to get a value representing the current iteration of a foreach loop? For instance, I currently do something like …

Read More
Update all objects in a collection using LINQ

Is there a way to do the following using LINQ? foreach (var c in collection) { c.PropertyToSet = value; } To clarify, I want to iterate through each object in a collection and then update a property on each object. My use …

Read More
How do I copy items from list to list without foreach?

How do I transfer the items contained in one List to another in C# without using foreach?

Read More