Question

I’m using .Aggregate on a List<String> to concatenate each item. I need to find out when the iteration has hit the last item. I was considering testing listOfStrings.Last() == currentString but the last item in the list could equal any other if the value is the same.

Answer 1

Why do you need to know this?

I’m just guessing, but it sounds like you’re doing something similar to generating a string such as “x, y, z” and you don’t want a comma after the last item.

If this is the case, you can use string.Join(separator, items) instead. Unless you want something which fits the form of “x, y & z”?

If you really need to use Aggregate, you can close over an external variable, like:

int count = 0;

var aggregated = items.Aggregate(new StringBuilder(), (acc, item) =>
{
    if (++count == items.Count)
    {
        // it's the last item
        acc.Append("last");
    }
    else
    {
        acc.Append("not last, ");
    }
    return acc;
}).ToString();