How can I create a singleton IEnumerable?

Question

Does C# offer some nice method to cast a single entity of type T to IEnumerable<T>?

The only way I can think of is something like:

T entity = new T();
IEnumerable<T> = new List { entity }.AsEnumerable();

And I guess there should be a better way.

Uis

possible duplicate: #4779942 – Ammamaria

possible duplicate of Passing a single item as IEnumerable<T> – Acclamation

Answer 1

Your call to AsEnumerable() is unnecessary. AsEnumerable is usually used in cases where the target object implements IQueryable<T> but you want to force it to use LINQ-to-Objects (when doing client-side filtering on a LINQ-compatible ORM, for example). Since List<T> implements IEnumerable<T> but not IQueryable<T>, there's no need for it.

Anyway, you could also create a single-element array with your item;

IEnumerable<T> enumerable = new[] { t };

Or Enumerable.Repeat

IEnumerable<T> enumerable = Enumerable.Repeat(t, 1);

Kilburn

Notes: the array constructor syntax is significantly more efficient (but such micro-optimization is often not relevant); the repeat method on the other hand is guaranteed to be immutable. – Insuperable

I like that Enumerable.Repeat allows you to hide the backing type. Also, I >assume< it's probably implemented with a loop/yield, and so can be the basis for a deferred execution implementation. – Sheet

@Sprague: If by "hide the backing type", you're referring to the ability to call Repeat without explicitly specifying the generic arguments, then the implicit array syntax (new[] { t }) does the same thing. – Kilburn

@AdamRobinson No, perhaps a confusing choice of words on my part. I was referring to the implementation of IEnumerable (array v. list v. value-yielding function) – Sheet

Answer 2

I use

Enumerable.Repeat(entity, 1);

Besought

Answer 3

var entity = new T();
var singleton = Enumerable.Repeat(entity, 1);

(Although I'd probably just do var singleton = new[] { entity }; in most situations, especially if it was only for private use.)

Foxtail

Strangely VB.NET is less verbose than C#! Creating the array is just { entity }. – Porush

@Martinho: In C# you can do T[] x = { y } or var x = new[] { y }, but not var x = { y }. – Foxtail

Answer 4

With the new collection expression that introduced in C# 12:

var singleton = [entity];

Just like the plain Dim singleton = { entity } in VB.NET: How can I create a singleton IEnumerable?

Aborn