Adding single item to enumerable

Question

Is there a more elegant solution for adding an item to an IEnumerable than this

myNewIEnumerable = myIEnumerable.Concat ( Enumerable.Repeat (item, 1) );

?

Some context:

public void printStrings (IEnumerable<string> myStrings) {
   Console.WriteLine ("The beginning.");
   foreach (var s in myStrings) Console.WriteLine (s);
   Console.WriteLine ("The end.");
}

...

var result = someMethodDeliveringAnIEnumerableOfStrings ();
printStrings (result.Concat ( Enumerable.Repeat ("finished", 1) ) );

Probably not, IEnumerable is the wrong medium for this. Show more context for a better answer. — Henk Holterman, Jul 13 '12 at 20:11
Poor context. You could just (and better) use an extra WriteLine() here. — Henk Holterman, Jul 13 '12 at 20:25
@Henk, yup, the example is a little disappointing. I was expecting more injection of a data point into an existing query and less writing to the console. — Frédéric Hamidi, Jul 13 '12 at 20:31
Ok, I changed the example for you guys such that calling WriteLine outside printStrings does not work anymore. — JohnB, Jul 13 '12 at 20:47

score 5 · Answer 1 · answered Jul 13 '12 at 20:11

5

Sure, there is. Concat() takes any IEnumerable<T>, including arrays:

var myNewIEnumerable = myIEnumerable.Concat(new[] { item });

answered Jul 13 '12 at 20:11

Frédéric Hamidi

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Hm, but this creates an array!? I'm not sure that's what I want. – JohnB Jul 13 '12 at 20:13
@Henk, shouldn't the performance of `Concat()` be comparable to that of a custom iterator block that exhausts `myIEnumerable` then yields `item`? – Frédéric Hamidi Jul 13 '12 at 20:14
2

@JohnB, yes, this creates an array of one element, the simplest enumerable construct I could think of. – Frédéric Hamidi Jul 13 '12 at 20:14
Yeah, of course you are right, but I would really appreciate something like an AppendItem extension function. – JohnB Jul 13 '12 at 20:16
@FrédéricHamidi - you're probably right, consuming the result should only iterate the original once. – Henk Holterman Jul 13 '12 at 20:23

score 2 · Accepted Answer · edited May 23 '17 at 12:20

2

Maybe the easiest way to append a single element to an IEnumerable<T> is to use this AsIEnumerable extension (although Eric Lippert advises against creating extension for objects):

public static IEnumerable<T> AsIEnumerable<T>(this T obj)
{
    yield return obj;
}  

var appended = source.Concat(element.AsIEnumerable())

or use this Append<T> extension: https://stackoverflow.com/a/3645715/284240

edited May 23 '17 at 12:20

Community

1
1

answered Jul 13 '12 at 20:32

Tim Schmelter

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The AsEnumerable () is what I was looking for. Thank you very much! – JohnB Jul 13 '12 at 20:46
Just a remark: As I just noticed, it's important not to confuse AsIEnumerable with AsEnumerable ! It must be AsIEnumerable, of course. – JohnB Jul 21 '12 at 09:14
@JohnB: Well, that is the core of this answer (and your question) since a single object is never an `IEnumerable` of it's type and has no `AsEnumerable()` method. We could rename it to `AsEnumerable` but that could cause misunderstandings even more. – Tim Schmelter Jul 21 '12 at 14:32

score 1 · Answer 3 · answered Jul 13 '12 at 20:24

You can define your own SingleConcat extension methods like so

public static class CustomConcat
    {
        public static IEnumerable<T> SingleConcat<T>(this IEnumerable<T> first, T lastElement)
        {
            foreach (T t in first)
            {
                yield return t;
            }

            yield return lastElement;
        }
    }

    class Program
    {


        static void Main(string[] args)
        {
            List<int> mylist = new List<int> { 1 , 2 , 3};

            var newList = mylist.SingleConcat(4);

            foreach (var s in newList)
            {
                Console.WriteLine(s);
            }

            Console.Read();
        }
    }

This should print 1,2,3 followed by a 4

Adding single item to enumerable

3 Answers3