using System;

using System.Linq;

using System.Collections.Generic;

public static class Program {

    public static IEnumerable<T> Delimit1a <T> (this IEnumerable<T> source, T delimiter) =>

        source.SelectMany((item) => Enumerable.Empty<T>().Append(delimiter).Append(item)).Skip(1);

​

    public static IEnumerable<T> Delimit1b <T> (this IEnumerable<T> source, T delimiter) =>

        source.SelectMany((item) => new T[] { delimiter, item }).Skip(1);

    public static IEnumerable<T> Delimit2a <T> (this IEnumerable<T> source, T delimiter) =>

        source.Aggregate(Enumerable.Empty<T>(), (delimited, item) => delimited.Append(delimiter).Append(item)).Skip(1);

    public static IEnumerable<T> Delimit2b <T> (this IEnumerable<T> source, T delimiter) =>

        source.Aggregate(null as IEnumerable<T>, (delimited, item) => (delimited?.Append(delimiter) ?? Enumerable.Empty<T>()).Append(item)) ?? Enumerable.Empty<T>();

    public static IEnumerable<T> Delimit3a <T> (this IEnumerable<T> source, T delimiter) {

        foreach (T item in source.Take(1)) // protects agains empty source

            yield return item;

        foreach (T item in source.Skip(1)) {

            yield return delimiter;

            yield return item;

        }

    }

    public static IEnumerable<T> Delimit3b <T> (this IEnumerable<T> source, T delimiter) {

        static IEnumerable<U> Helper<U> (IEnumerable<U> source, U delimiter) {

            foreach (U item in source) {

                yield return delimiter;

                yield return item;

            }

        }

        return Helper(source, delimiter).Skip(1);

    }

    public static void Main () {

        foreach (int count in new int[] { 11, 2, 1, 0 }) {

            p( Enumerable.Range(10, count).Delimit1a(-1) );

            p( Enumerable.Range(10, count).Delimit1b(-1) );

            p( Enumerable.Range(10, count).Delimit2a(-1) );

            p( Enumerable.Range(10, count).Delimit2b(-1) );

            p( Enumerable.Range(10, count).Delimit3a(-1) );

            p( Enumerable.Range(10, count).Delimit3b(-1) );

        }

    }

    static void p <T> (IEnumerable<T> e) =>

        Console.WriteLine($"[ {string.Join(", ", e)} ]");

}