Passing a single item as IEnumerable<T>

Well, if the method expects an IEnumerable you've got to pass something that is a list, even if it contains one element only.

passing

new[] { item }

as the argument should be enough I think

In C# 3.0 you can utilize the System.Linq.Enumerable class:

// using System.Linq

Enumerable.Repeat(item, 1);

This will create a new IEnumerable that only contains your item.

Your helper method is the cleanest way to do it, IMO. If you pass in a list or an array, then an unscrupulous piece of code could cast it and change the contents, leading to odd behaviour in some situations. You could use a read-only collection, but that's likely to involve even more wrapping. I think your solution is as neat as it gets.

In C# 3 (I know you said 2), you can write a generic extension method which might make the syntax a little more acceptable:

static class IEnumerableExtensions
{
    public static IEnumerable<T> ToEnumerable<T>(this T item)
    {
        yield return item;
    }
}

client code is then item.ToEnumerable().

This helper method works for item or many.

public static IEnumerable<T> ToEnumerable<T>(params T[] items)
{
    return items;
}    

I'm kind of surprised that no one suggested a new overload of the method with an argument of type T to simplify the client API.

public void DoSomething<T>(IEnumerable<T> list)
{
    // Do Something
}

public void DoSomething<T>(T item)
{
    DoSomething(new T[] { item });
}

Now your client code can just do this:

MyItem item = new MyItem();
Obj.DoSomething(item);

or with a list:

List<MyItem> itemList = new List<MyItem>();
Obj.DoSomething(itemList);

I agree with @EarthEngine's comments to the original post, which is that 'AsSingleton' is a better name. See this wikipedia entry. Then it follows from the definition of singleton that if a null value is passed as an argument that 'AsSingleton' should return an IEnumerable with a single null value instead of an empty IEnumerable which would settle the if (item == null) yield break; debate. I think the best solution is to have two methods: 'AsSingleton' and 'AsSingletonOrEmpty'; where, in the event that a null is passed as an argument, 'AsSingleton' will return a single null value and 'AsSingletonOrEmpty' will return an empty IEnumerable. Like this:

public static IEnumerable<T> AsSingletonOrEmpty<T>(this T source)
{
    if (source == null)
    {
        yield break;
    }
    else
    {
        yield return source;
    }
}

public static IEnumerable<T> AsSingleton<T>(this T source)
{
    yield return source;
}

Then, these would, more or less, be analogous to the 'First' and 'FirstOrDefault' extension methods on IEnumerable which just feels right.

Either (as has previously been said)

MyMethodThatExpectsAnIEnumerable(new[] { myObject });

or

MyMethodThatExpectsAnIEnumerable(Enumerable.Repeat(myObject, 1));

As a side note, the last version can also be nice if you want an empty list of an anonymous object, e.g.

var x = MyMethodThatExpectsAnIEnumerable(Enumerable.Repeat(new { a = 0, b = "x" }, 0));

As I have just found, and seen that user LukeH suggested too, a nice simple way of doing this is as follows:

public static void PerformAction(params YourType[] items)
{
    // Forward call to IEnumerable overload
    PerformAction(items.AsEnumerable());
}

public static void PerformAction(IEnumerable<YourType> items)
{
    foreach (YourType item in items)
    {
        // Do stuff
    }
}

This pattern will allow you to call the same functionality in a multitude of ways: a single item; multiple items (comma-separated); an array; a list; an enumeration, etc.

I'm not 100% sure on the efficiency of using the AsEnumerable method though, but it does work a treat.

Update: The AsEnumerable function looks pretty efficient! (reference)

Although it's overkill for one method, I believe some people may find the Interactive Extensions useful.

The Interactive Extensions (Ix) from Microsoft includes the following method.

public static IEnumerable<TResult> Return<TResult>(TResult value)
{
    yield return value;
}

Which can be utilized like so:

var result = EnumerableEx.Return(0);

Ix adds new functionality not found in the original Linq extension methods, and is a direct result of creating the Reactive Extensions (Rx).

Think, Linq Extension Methods + Ix = Rx for IEnumerable.

You can find both Rx and Ix on CodePlex.

This is 30% faster than yield or Enumerable.Repeat when used in foreach due to this C# compiler optimization, and of the same performance in other cases.

public struct SingleSequence<T> : IEnumerable<T> {
    public struct SingleEnumerator : IEnumerator<T> {
        private readonly SingleSequence<T> _parent;
        private bool _couldMove;
        public SingleEnumerator(ref SingleSequence<T> parent) {
            _parent = parent;
            _couldMove = true;
        }
        public T Current => _parent._value;
        object IEnumerator.Current => Current;
        public void Dispose() { }

        public bool MoveNext() {
            if (!_couldMove) return false;
            _couldMove = false;
            return true;
        }
        public void Reset() {
            _couldMove = true;
        }
    }
    private readonly T _value;
    public SingleSequence(T value) {
        _value = value;
    }
    public IEnumerator<T> GetEnumerator() {
        return new SingleEnumerator(ref this);
    }
    IEnumerator IEnumerable.GetEnumerator() {
        return new SingleEnumerator(ref this);
    }
}

in this test:

    // Fastest among seqs, but still 30x times slower than direct sum
    // 49 mops vs 37 mops for yield, or c.30% faster
    [Test]
    public void SingleSequenceStructForEach() {
        var sw = new Stopwatch();
        sw.Start();
        long sum = 0;
        for (var i = 0; i < 100000000; i++) {
            foreach (var single in new SingleSequence<int>(i)) {
                sum += single;
            }
        }
        sw.Stop();
        Console.WriteLine($"Elapsed {sw.ElapsedMilliseconds}");
        Console.WriteLine($"Mops {100000.0 / sw.ElapsedMilliseconds * 1.0}");
    }

This may not be any better but it's kind of cool:

Enumerable.Range(0, 1).Select(i => item);

I recently asked the same thing on another post

Is there a way to call a C# method requiring an IEnumerable<T> with a single value? ...with benchmarking.

I wanted people stopping by here to see the brief benchmark comparison shown at that newer post for 4 of the approaches presented in these answers.

It seems that simply writing new[] { x } in the arguments to the method is the shortest and fastest solution.

Sometimes I do this, when I'm feeling impish:

"_".Select(_ => 3.14)  // or whatever; any type is fine

This is the same thing with less shift key presses, heh:

from _ in "_" select 3.14

For a utility function I find this to be the least verbose, or at least more self-documenting than an array, although it'll let multiple values slide; as a plus it can be defined as a local function:

static IEnumerable<T> Enumerate (params T[] v) => v;
// usage:
IEnumerable<double> example = Enumerate(1.234);

Here are all of the other ways I was able to think of (runnable here):

using System;
using System.Collections.Generic;
using System.Linq;

public class Program {
    
    public static IEnumerable<T> ToEnumerable1 <T> (T v) {
        yield return v;
    }
    
    public static T[] ToEnumerable2 <T> (params T[] vs) => vs;
    
    public static void Main () {
        static IEnumerable<T> ToEnumerable3 <T> (params T[] v) => v;
        p( new string[] { "three" } );
        p( new List<string> { "three" } );
        p( ToEnumerable1("three") ); // our utility function (yield return)
        p( ToEnumerable2("three") ); // our utility function (params)
        p( ToEnumerable3("three") ); // our local utility function (params)
        p( Enumerable.Empty<string>().Append("three") );
        p( Enumerable.Empty<string>().DefaultIfEmpty("three") );
        p( Enumerable.Empty<string>().Prepend("three") );
        p( Enumerable.Range(3, 1) ); // only for int
        p( Enumerable.Range(0, 1).Select(_ => "three") );
        p( Enumerable.Repeat("three", 1) );
        p( "_".Select(_ => "three") ); // doesn't have to be "_"; just any one character
        p( "_".Select(_ => 3.3333) );
        p( from _ in "_" select 3.0f );
        p( "a" ); // only for char
        // these weren't available for me to test (might not even be valid):
        //   new Microsoft.Extensions.Primitives.StringValues("three")
        
    }

    static void p <T> (IEnumerable<T> e) =>
        Console.WriteLine(string.Join(' ', e.Select((v, k) => $"[{k}]={v,-8}:{v.GetType()}").DefaultIfEmpty("<empty>")));

}

The easiest way I'd say would be new T[]{item};; there's no syntax to do this. The closest equivalent that I can think of is the params keyword, but of course that requires you to have access to the method definition and is only usable with arrays.

Enumerable.Range(1,1).Select(_ => {
    //Do some stuff... side effects...
    return item;
});

The above code is useful when using like

var existingOrNewObject = MyData.Where(myCondition)
       .Concat(Enumerable.Range(1,1).Select(_ => {
           //Create my object...
           return item;
       })).Take(1).First();

In the above code snippet there is no empty/null check, and it is guaranteed to have only one object returned without afraid of exceptions. Furthermore, because it is lazy, the closure will not be executed until it is proved there is no existing data fits the criteria.

To be filed under "Not necessarily a good solution, but still...a solution" or "Stupid LINQ tricks", you could combine Enumerable.Empty<>() with Enumerable.Append<>()...

IEnumerable<string> singleElementEnumerable = Enumerable.Empty<string>().Append("Hello, World!");

...or Enumerable.Prepend<>()...

IEnumerable<string> singleElementEnumerable = Enumerable.Empty<string>().Prepend("Hello, World!");

The latter two methods are available since .NET Framework 4.7.1 and .NET Core 1.0.

This is a workable solution if one were really intent on using existing methods instead of writing their own, though I'm undecided if this is more or less clear than the Enumerable.Repeat<>() solution. This is definitely longer code (partly due to type parameter inference not being possible for Empty<>()) and creates twice as many enumerator objects, however.

Rounding out this "Did you know these methods exist?" answer, Array.Empty<>() could be substituted for Enumerable.Empty<>(), but it's hard to argue that makes the situation...better.

I'm a bit late to the party but I'll share my way anyway. My problem was that I wanted to bind the ItemSource or a WPF TreeView to a single object. The hierarchy looks like this:

Project > Plot(s) > Room(s)

There was always going to be only one Project but I still wanted to Show the project in the Tree, without having to pass a Collection with only that one object in it like some suggested. Since you can only pass IEnumerable objects as ItemSource I decided to make my class IEnumerable:

public class ProjectClass : IEnumerable<ProjectClass>
{
    private readonly SingleItemEnumerator<AufmassProjekt> enumerator;

    ... 

    public IEnumerator<ProjectClass > GetEnumerator() => this.enumerator;

    IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();
}

And create my own Enumerator accordingly:

public class SingleItemEnumerator : IEnumerator
{
    private bool hasMovedOnce;

    public SingleItemEnumerator(object current)
    {
        this.Current = current;
    }

    public bool MoveNext()
    {
        if (this.hasMovedOnce) return false;
        this.hasMovedOnce = true;
        return true;
    }

    public void Reset()
    { }

    public object Current { get; }
}

public class SingleItemEnumerator<T> : IEnumerator<T>
{
    private bool hasMovedOnce;

    public SingleItemEnumerator(T current)
    {
        this.Current = current;
    }

    public void Dispose() => (this.Current as IDisposable).Dispose();

    public bool MoveNext()
    {
        if (this.hasMovedOnce) return false;
        this.hasMovedOnce = true;
        return true;
    }

    public void Reset()
    { }

    public T Current { get; }

    object IEnumerator.Current => this.Current;
}

This is probably not the "cleanest" solution but it worked for me.

EDIT
To uphold the single responsibility principle as @Groo pointed out I created a new wrapper class:

public class SingleItemWrapper : IEnumerable
{
    private readonly SingleItemEnumerator enumerator;

    public SingleItemWrapper(object item)
    {
        this.enumerator = new SingleItemEnumerator(item);
    }

    public object Item => this.enumerator.Current;

    public IEnumerator GetEnumerator() => this.enumerator;
}

public class SingleItemWrapper<T> : IEnumerable<T>
{
    private readonly SingleItemEnumerator<T> enumerator;

    public SingleItemWrapper(T item)
    {
        this.enumerator = new SingleItemEnumerator<T>(item);
    }

    public T Item => this.enumerator.Current;

    public IEnumerator<T> GetEnumerator() => this.enumerator;

    IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();
}

Which I used like this

TreeView.ItemSource = new SingleItemWrapper(itemToWrap);

EDIT 2
I corrected a mistake with the MoveNext() method.

I prefer

public static IEnumerable<T> Collect<T>(this T item, params T[] otherItems)
{
    yield return item;
    foreach (var otherItem in otherItems)
    {
        yield return otherItem;
    }
}

This lets you call item.Collect() if you want the singleton, but it also lets you call item.Collect(item2, item3) if you want