Question

What would be the most elegant way to process the first IEnumerable item differently than others, without having to test on each iteration?

With a test on each iteration, it would look like this:

// "first item done" flag
bool firstDone = false;

// items is an IEnumerable<something>
foreach (var item in items)
{
    if (!firstDone)
    {
        // do this only once
        ProcessDifferently(item);
        firstDone = true;
        continue;
    }

    ProcessNormally(item);
}

If I do this:

ProcessDifferently(items.First());
ProcessNormally(items.Skip(1)); // this calls `items.GetEnumerator` again

it will invoke GetEnumerator twice, which I would like to avoid (for Linq-to-Sql cases, for example).

How would you do it, if you need to do several times around your code?

Answer 1

If I needed to do it in several places, I’d extract a method:

public void Process<T>(IEnumerable<T> source,
                       Action<T> firstAction,
                       Action<T> remainderAction)
{
    // TODO: Argument validation
    using (var iterator = source.GetEnumerator())
    {
        if (iterator.MoveNext())
        {
            firstAction(iterator.Current);
        }
        while (iterator.MoveNext())
        {
            remainderAction(iterator.Current);
        }
    }
}

Called as:

Process(items, ProcessDifferently, ProcessNormally);

There are other options too, but it would really depend on the situation.