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

Many implementations have been given already. Mine looks a bit different (but performs just as well)

Also, I find it practicle to also have control over the ORDER. thus often, I also have a ConcatTo method, putting the new element op front.

public static class Utility
{
    /// 

    /// Adds the specified element at the end of the IEnummerable.
    /// 
    /// The type of elements the IEnumerable contans.
    /// The target.
    /// The item to be concatenated.
    /// An IEnumerable, enumerating first the items in the existing enumerable
    public static IEnumerable ConcatItem(this IEnumerable target, T item)
    {
        if (null == target) throw new ArgumentException(nameof(target));
        foreach (T t in target) yield return t;
        yield return item;
    }

    /// 

    /// Inserts the specified element at the start of the IEnumerable.
    /// 
    /// The type of elements the IEnumerable contans.
    /// The IEnummerable.
    /// The item to be concatenated.
    /// An IEnumerable, enumerating first the target elements, and then the new element.
    public static IEnumerable ConcatTo(this IEnumerable target, T item)
    {
        if (null == target) throw new ArgumentException(nameof(target));
        yield return item;
        foreach (T t in target) yield return t;
    }
}

Or alternatively, use an implicitly created array. (using the params keyword) so you can call the method to add one or more items at a time:

public static class Utility
{
    /// 

    /// Adds the specified element at the end of the IEnummerable.
    /// 
    /// The type of elements the IEnumerable contans.
    /// The target.
    /// The items to be concatenated.
    /// An IEnumerable, enumerating first the items in the existing enumerable
    public static IEnumerable ConcatItems(this IEnumerable target, params T[] items) =>
        (target ?? throw new ArgumentException(nameof(target))).Concat(items);

    /// 

    /// Inserts the specified element at the start of the IEnumerable.
    /// 
    /// The type of elements the IEnumerable contans.
    /// The IEnummerable.
    /// The items to be concatenated.
    /// An IEnumerable, enumerating first the target elements, and then the new elements.
    public static IEnumerable ConcatTo(this IEnumerable target, params T[] items) =>
        items.Concat(target ?? throw new ArgumentException(nameof(target)));