Fix warnings, improve performance (#1665)

* Fix warnings, improve performance

`QueryResult.Items` is now a `IReadOnlyList` so we don't need to
allocate a new `Array` when we have a `List` (and `Items` shouldn't need to
be mutable anyway)

* Update Providers .csproj to latest C#

* Remove extra newline from DtoService.cs

* Remove extra newline from UserLibraryService.cs

MediaBrowser.Controller/Entities/IHasTrailers.cs

@@ -1,6 +1,5 @@
 using System;
 using System.Collections.Generic;
-using System.Linq;
 using MediaBrowser.Model.Entities;
 
 namespace MediaBrowser.Controller.Entities
@@ -11,29 +10,82 @@ namespace MediaBrowser.Controller.Entities
         /// Gets or sets the remote trailers.
         /// </summary>
         /// <value>The remote trailers.</value>
-        MediaUrl[] RemoteTrailers { get; set; }
+        IReadOnlyList<MediaUrl> RemoteTrailers { get; set; }
 
         /// <summary>
         /// Gets or sets the local trailer ids.
         /// </summary>
         /// <value>The local trailer ids.</value>
-        Guid[] LocalTrailerIds { get; set; }
-        Guid[] RemoteTrailerIds { get; set; }
+        IReadOnlyList<Guid> LocalTrailerIds { get; set; }
+
+        /// <summary>
+        /// Gets or sets the remote trailer ids.
+        /// </summary>
+        /// <value>The remote trailer ids.</value>
+        IReadOnlyList<Guid> RemoteTrailerIds { get; set; }
+
         Guid Id { get; set; }
     }
 
+    /// <summary>
+    /// Class providing extension methods for working with <see cref="IHasTrailers" />.
+    /// </summary>
     public static class HasTrailerExtensions
     {
+        /// <summary>
+        /// Gets the trailer count.
+        /// </summary>
+        /// <returns><see cref="IReadOnlyList{Guid}" />.</returns>
+        public static int GetTrailerCount(this IHasTrailers item)
+            => item.LocalTrailerIds.Count + item.RemoteTrailerIds.Count;
+
         /// <summary>
         /// Gets the trailer ids.
         /// </summary>
-        /// <returns>List&lt;Guid&gt;.</returns>
-        public static List<Guid> GetTrailerIds(this IHasTrailers item)
+        /// <returns><see cref="IReadOnlyList{Guid}" />.</returns>
+        public static IReadOnlyList<Guid> GetTrailerIds(this IHasTrailers item)
         {
-            var list = item.LocalTrailerIds.ToList();
-            list.AddRange(item.RemoteTrailerIds);
-            return list;
+            var localIds = item.LocalTrailerIds;
+            var remoteIds = item.RemoteTrailerIds;
+
+            var all = new Guid[localIds.Count + remoteIds.Count];
+            var index = 0;
+            foreach (var id in localIds)
+            {
+                all[index++] = id;
+            }
+
+            foreach (var id in remoteIds)
+            {
+                all[index++] = id;
+            }
+
+            return all;
         }
 
+        /// <summary>
+        /// Gets the trailers.
+        /// </summary>
+        /// <returns><see cref="IReadOnlyList{BaseItem}" />.</returns>
+        public static IReadOnlyList<BaseItem> GetTrailers(this IHasTrailers item)
+        {
+            var localIds = item.LocalTrailerIds;
+            var remoteIds = item.RemoteTrailerIds;
+            var libraryManager = BaseItem.LibraryManager;
+
+            var all = new BaseItem[localIds.Count + remoteIds.Count];
+            var index = 0;
+            foreach (var id in localIds)
+            {
+                all[index++] = libraryManager.GetItemById(id);
+            }
+
+            foreach (var id in remoteIds)
+            {
+                all[index++] = libraryManager.GetItemById(id);
+            }
+
+            return all;
+        }
     }
 }