EnumerableExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Umbraco.Core.Models.PublishedContent;
using Umbraco.Web;

public static class EnumerableExtensions {
  public static bool HasAny<T> (this IEnumerable<T> items) => items != null && items.Any();
  
  public static bool IsNullOrEmpty<T>(this IEnumerable<T> items)
  {
    return items == null && !items.Any();
  }

  public static T FirstOrDefaultOfType<T> (this IEnumerable<IPublishedContent> publishedContents) => publishedContents.OfType<T> ().FirstOrDefault();

  public static string JoinIfNotNull <TResult> (this IEnumerable <IPublishedContent> publishedContents, Func <IPublishedContent, TResult> func, string separator = StringConstants.Separators.Space) => !publishedContents.HasValue() ?
    string.Empty :
    string.Join(separator, publishedContents.Select(func));

  public static IEnumerable <(T item, int index)> WithIndex<T> (this IEnumerable <T> items) => items.HasAny() ?
    items
    .Select((item, index) => (item, index))
    .ToList() :
    Enumerable.Empty < (T item, int index) > ();

  public static IEnumerable<T> OrderBySequence <T, TId> (this IEnumerable<T> source,
    IEnumerable <TId> order,
    Func <T, TId> idSelector) {
    var lookup = source.ToDictionary(idSelector, t => t);

    foreach(var id in order) {
      yield
      return lookup[id];
    }
  }
}