how do I check if an entity is the first element of a foreach loop

I like the Linq way, but without the Skip(1), this way you can also use it for the last item in a list and your code remains clean imho :)

foreach(var item in items)
{
    if (items.First()==item)
        item.firstStuff();

    else if (items.Last() == item)
        item.lastStuff();

    item.otherStuff();
}

There are several ways that you could do that.

1. Use a for loop instead
2. Set a Boolean flag
3. Use Linq to get the list.First() and then foreach over list.Skip(1)

Something like this:

bool first = true;

foreach(var item in items)
{
    if (first)
    {
        item.firstStuff();
        first = false;
    }
    item.otherStuff();
}

Here's a performant solution:

using (var erator = enumerable.GetEnumerator())
{
    if (erator.MoveNext())
    {
        DoActionOnFirst(erator.Current);

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

EDIT: And here's a LINQ one:

if (enumerable.Any())
    {
        DoActionOnFirst(enumerable.First());

        foreach (var item in enumerable.Skip(1))
            DoActionOnOther(item);
    }

EDIT: If the actions on the items have signatures assignable to Func<TItem, TResult>, you can do:

enumerable.Select((item, index) => index == 0 ? GetResultFromFirstItem(item) : GetResultFromOtherItem(item));

In my opinion this is the simplest way

foreach (var item in list)
{
    if((list.IndexOf(item) == 0)
    {
        // first
    }
    // others
}