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

Question

Often I have to code a loop that needs a special case for the first item, the code never seems as clear as it should ideally be.

Short of a redesign of the C# language, what is the best way to code these loops?

// this is more code to read then I would like for such a common concept
// and it is to easy to forget to update "firstItem"
foreach (x in yyy)
{
  if (firstItem)
  {
     firstItem = false;
     // other code when first item
  }
  // normal processing code
}

// this code is even harder to understand
if (yyy.Length > 0)
{
   //Process first item;
   for (int i = 1; i < yyy.Length; i++)
   {  
      // process the other items.
   }
}

Answer 1

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);            
    }
}

Answer 2

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);
        }
    }
}