Linq Select and Aggregate within a single iteration

Question

Is there a way to do this with linq without enumerating the fooCollection twice?

var fooCollection = // get foo
var selectedIds = new List<int>();
var aggregateContent = String.Empty;

foreach (var f in foo)
{
    selectedIds.Add(foo.Id);
    aggregateContent += foo.Content
}

var results = new FooResults
{
    Content = aggregateContent,
    SelectedIds = selectedIds
};

return results;

Answer 1

Yes, you can use the Enumerable.Aggregate method:

var result = fooCollection.Aggregate(new FooResult(),
                                    (r,f) => 
                                    { 
                                        r.SelectedIds.Add(f.Id);
                                        r.Content += f.Content;
                                        return r;
                                    });

This has the benefit of being side-effect free. I dislike side effects in my LINQ. =)

Answer 2

There is one possiblity, but I consider it a hack:

var aggregateContent = String.Empty;
var selectedIds = foo.Select(x => { aggregateContent += x.Content;
                                    return x.Id; })
                     .ToList();

I would go with the loop you already have. It is much cleaner as any LINQ solution you could come up with.

Answer 3

You could do this:

foo.ForEach(x => { selectedIds.Add(x.Id); aggregateContent += x.Content; });

I would recommend not concatenating the Content into a string, but to use a StringBuilder instead.

EDIT

If you don't have a LINQ extension library that implements ForEach for IEnumerable, here is a method you can use:

public static void ForEach<T>(this IEnumerable<T> enumeration, Action<T> action) 
{ 
    foreach(T item in enumeration) 
    { 
        action(item); 
    } 
}