Is there a Linq method to add a single item to an IEnumerable?

Question

Basically I am trying to do something like this:

image.Layers

which returns an IEnumerable for all layers except the Parent layer,

Answer 1

One way would be to create a singleton-sequence out of the item (such as an array), and then Concat it onto the original:

image.Layers.Concat(new[] { image.ParentLayer } )

If you're doing this really often, consider writing an Append (or similar) extension-method, such as the one listed here, which would let you do:

image.Layers.Append(image.ParentLayer)

.NET Core Update (per the "best" answer below):

Append and Prepend have now been added to the .NET Standard framework, so you no longer need to write your own. Simply do this:

image.Layers.Append(image.ParentLayer)

Answer 2

I once made a nice little function for this:

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

Now this is possible:

image.Layers.Append(CoreUtil.AsEnumerable(image.ParentLayer, image.AnotherLayer))

Answer 3

/// <summary>Concatenates elements to a sequence.</summary>
/// <typeparam name="T">The type of the elements of the input sequences.</typeparam>
/// <param name="target">The sequence to concatenate.</param>
/// <param name="items">The items to concatenate to the sequence.</param>
public static IEnumerable<T> ConcatItems<T>(this IEnumerable<T> target, params T[] items)
{
    if (items == null)
        items = new [] { default(T) };
    return target.Concat(items);
}

This solution is based on realbart's answer. I adjusted it to allow the use of a single null value as a parameter:

var newCollection = collection.ConcatItems(null)

Answer 4

If you like the syntax of .With, write it as an extension method. IEnumerable won't notice another one.

Answer 5

I use the following extension methods to avoid creating a useless Array:

public static IEnumerable<T> ConcatSingle<T>(this IEnumerable<T> enumerable, T value) {
   return enumerable.Concat(value.Yield());
}

public static IEnumerable<T> Yield<T>(this T item) {
    yield return item;
}