Concatenate enumerable and a number in Linq

Question

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What is the best wayto add single element to an IEnumerable collection?

Let's say I want to concantenate Enumerable.Repeat(100, 100) and the number 3, what is the nicest way to do this?

Of course I can do Enumerable.Repeat(100, 100).Concat(Enumerable.Repeat(3,1)), but it doesn't look very expressive...

"but it doesn't look very expressive"... k; what would your *ideal* syntax look like? — Marc Gravell, Oct 24 '12 at 08:45
possible [duplicate](http://stackoverflow.com/q/4189563/944681) — Michal Klouda, Oct 24 '12 at 08:47
@MarcGravell: I think it would be like `numbers.Concat(3)`. An extension method does it. Anyway, I don't like doing things *too linq*. — Cheng Chen, Oct 24 '12 at 08:51
@LouisRhys: It works, but what's wrong with `numbers.Concat(new[] { 3 })`? Creating a method to avoid the `new[] { }` syntax, is it worthy? — Cheng Chen, Oct 24 '12 at 09:20
@DannyChen IMO new[] { } obscures the meaning for people reading the code.. — Louis Rhys, Oct 24 '12 at 09:27

score 3 · Answer 1 · edited Oct 24 '12 at 08:59

3

If you really want to make it neat then the best thing to do would be to create an extension method for something like ConcatSingle and then call it.

edited Oct 24 '12 at 08:59

CodesInChaos

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answered Oct 24 '12 at 08:49

wizzardmr42

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+1 for using a different name from `Concat`. Reusing `Concat` can be ambiguous. – CodesInChaos Oct 24 '12 at 09:00

Tim Schmelter · Answer 2 · 2012-10-24T09:05:37.100

2

You could use an intermediate array which contains only that element:

Enumerable.Range(100, 100).Concat(new []{ 3 });

You could also create an extension method to allow that without creating an additional array:

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

Now this is possible:

Enumerable.Range(100, 100).Concat(3.ToEnumerable());

edited Oct 24 '12 at 09:05

answered Oct 24 '12 at 08:47

Tim Schmelter

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I don't like the name of your extension method. Too close to `AsEnumerable` which does something completely different. – CodesInChaos Oct 24 '12 at 08:59
@CodesInChaos: Renamed to `ToEnumerable` if that's better. – Tim Schmelter Oct 24 '12 at 09:05
That's better, but I'm still not a fan. You don't convert the item to a sequence, you create a sequence that *contains* the item. So I still think that the name doesn't really fit its semantics. – CodesInChaos Oct 24 '12 at 09:08
@CodesInChaos: What do you think would be a better name? That's why i've named it `AsIEnumerable` in the first place, it doesn't really convert it to anything. It's convenient if you need to add static objects to existing sequences or queries. – Tim Schmelter Oct 24 '12 at 09:11
Perhaps `AsSequenceElement` or `Enumerable.FromElement` (no extension method). Personally I just go with `new[]{element}`. For the OP's use `ConcatSingle` is nicest. – CodesInChaos Oct 24 '12 at 09:16

score 0 · Answer 3 · answered Oct 24 '12 at 08:50

You can create an extension method for converting a single value to an IEnumerable

public static class LinqEx
{
    public static IEnumerable<T> ToIEnumerable<T>(this T singleItem)
    {
        yield return singleItem;
    }
}

and then use

Enumerable.Repeat(100, 100).Concat(3.ToIEnumerable())

Cheng Chen · Accepted Answer · 2012-10-24T09:28:06.330

0

public static IEnumerable<T> Concat(this IEnumerable<T> source, T item)
{
    //the code here is not very expressive to you :)
}

EDIT It's suggested not to use the method name Concat, using ConcatSingle or ConcatOne instead, see the discussions in below comments.

edited Oct 24 '12 at 09:28

answered Oct 24 '12 at 08:53

Cheng Chen

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The problem with this extension method is that it can break existing code. Consider the case where the item implements `IEnumerable`. – CodesInChaos Oct 24 '12 at 09:02
@CodesInChaos: I don't think so. You can easily tell if `T item` is a *single* object or a collection in this method. – Cheng Chen Oct 24 '12 at 09:17
`class X : IEnumerable{...}` => boom. While that's not the best design, introducing breaking changes with an extension method is simply unacceptable IMO. There is a reason the standard collections have both `Add` and `AddRange` instead of overloading `Add`. – CodesInChaos Oct 24 '12 at 09:20
@CodesInChaos: Ok..I admit what you said makes sense...because I can hardly imagine the usage scenario of `class X : IEnumerable`.. – Cheng Chen Oct 24 '12 at 09:26
You can use `class TreeNode:IEnumerable` to express a tree. I nowadays prefer having a `Children` property, but I think I used it in some of my earlier projects. – CodesInChaos Oct 24 '12 at 09:29
@CodesInChaos: Good example. I remember I used such a structure to store a tree data in the past, and like you I'd prefer a child property now. – Cheng Chen Oct 24 '12 at 09:30
@CodesInChaos I'm sure renaming the method to `ConcatSingle` or similar will help? – Louis Rhys Oct 24 '12 at 09:47
@LouisRhys Yes, naming it `ConcatSingle` is what I recommend, just like wizzardmr42's answer suggests. It's a useful method with a clear name and avoids overloading ambiguities. – CodesInChaos Oct 24 '12 at 09:57

Concatenate enumerable and a number in Linq

4 Answers4