Question

I have an IEnumerable of a custom type. (That I’ve gotten from a SelectMany)

I also have an item (myItem) in that IEnumerable that I desire the previous and next item from the IEnumerable.

Currently, I’m doing the desired like this:

var previousItem = myIEnumerable.Reverse().SkipWhile( 
    i => i.UniqueObjectID != myItem.UniqueObjectID).Skip(1).FirstOrDefault();

I can get the next item by simply ommitting the .Reverse.

or, I could:

int index = myIEnumerable.ToList().FindIndex( 
    i => i.UniqueObjectID == myItem.UniqueObjectID)

and then use .ElementAt(index +/- 1) to get the previous or next item.

1. Which is better between the two options?
2. Is there an even better option available?

“Better” includes a combination of performance (memory and speed) and readability; with readability being my primary concern.

Answer 1

First off

“Better” includes a combination of performance (memory and speed)

In general you can’t have both, the rule of thumb is, if you optimise for speed, it’ll cost memory, if you optimise for memory, it’ll cost you speed.

There is a better option, that performs well on both memory and speed fronts, and can be used in a readable manner (I’m not delighted with the function name, however, FindItemReturningPreviousItemFoundItemAndNextItem is a bit of a mouthful).

So, it looks like it’s time for a custom find extension method, something like . . .

public static IEnumerable<T> FindSandwichedItem<T>(this IEnumerable<T> items, Predicate<T> matchFilling)
{
    if (items == null)
        throw new ArgumentNullException("items");
    if (matchFilling == null)
        throw new ArgumentNullException("matchFilling");

    return FindSandwichedItemImpl(items, matchFilling);
}

private static IEnumerable<T> FindSandwichedItemImpl<T>(IEnumerable<T> items, Predicate<T> matchFilling)
{
    using(var iter = items.GetEnumerator())
    {
        T previous = default(T);
        while(iter.MoveNext())
        {
            if(matchFilling(iter.Current))
            {
                yield return previous;
                yield return iter.Current;
                if (iter.MoveNext())
                    yield return iter.Current;
                else
                    yield return default(T);
                yield break;
            }
            previous = iter.Current;
        }
    }
    // If we get here nothing has been found so return three default values
    yield return default(T); // Previous
    yield return default(T); // Current
    yield return default(T); // Next
}

You can cache the result of this to a list if you need to refer to the items more than once, but it returns the found item, preceded by the previous item, followed by the following item. e.g.

var sandwichedItems = myIEnumerable.FindSandwichedItem(item => item.objectId == "MyObjectId").ToList();
var previousItem = sandwichedItems[0];
var myItem = sandwichedItems[1];
var nextItem = sandwichedItems[2];

The defaults to return if it’s the first or last item may need to change depending on your requirements.

Hope this helps.