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,
In cases like this I would just use a for loop like this:
for(int i = 0; i < yyy.Count; i++){ if(i == 0){ //special logic here } }
Using a for loop also would allow you to do something special in other cases like on the last item, on even items in the sequence, ..etc.