Neat way to write loop that has special logic for the first item in a collection

You could try:

collection.first(x=>
{
    //...
}).rest(x=>
{
    //...
}).run();

first / rest would look like:

FirstPart<T> first<T>(this IEnumerable<T> c, Action<T> a)
{
    return new FirstPart<T>(c, a);
}

FirstRest rest<T>(this FirstPart<T> fp, Action<T> a)
{
    return new FirstRest(fp.Collection, fp.Action, a);
}

You would need to define classed FirstPart and FirstRest. FirstRest would need a run method like so (Collection, FirstAction, and RestAction are properties):

void run()
{
    bool first = true;
    foreach (var x in Collection)
    {
        if (first) {
            FirstAction(x);
            first = false;
        }
        else {
             RestAction(x);
        }
    }
}

How about:

using (var erator = enumerable.GetEnumerator())
{
    if (erator.MoveNext())
    {
        ProcessFirst(erator.Current);
        //ProcessOther(erator.Current); // Include if appropriate.

        while (erator.MoveNext())
            ProcessOther(erator.Current);
    }
}

You could turn that into an extension if you want:

public static void Do<T>(this IEnumerable<T> source, 
                         Action<T> firstItemAction,
                         Action<T> otherItemAction)
{
   // null-checks omitted

    using (var erator = source.GetEnumerator())
    {
        if (!erator.MoveNext())
            return;

        firstItemAction(erator.Current);

        while (erator.MoveNext())
           otherItemAction(erator.Current);            
    }
}

I'd be tempted to use a bit of linq

using System.Linq;

var theCollectionImWorkingOn = ...

var firstItem = theCollectionImWorkingOn.First();
firstItem.DoSomeWork();

foreach(var item in theCollectionImWorkingOn.Skip(1))
{
    item.DoSomeOtherWork();
}