diff --git a/Emby.Server.Implementations/Channels/ChannelManager.cs b/Emby.Server.Implementations/Channels/ChannelManager.cs
index 09aee602a..f65eaec1c 100644
--- a/Emby.Server.Implementations/Channels/ChannelManager.cs
+++ b/Emby.Server.Implementations/Channels/ChannelManager.cs
@@ -1075,14 +1075,6 @@ namespace Emby.Server.Implementations.Channels
                 forceUpdate = true;
             }
 
-            // was used for status
-            // if (!string.Equals(item.ExternalEtag ?? string.Empty, info.Etag ?? string.Empty, StringComparison.Ordinal))
-            // {
-            //    item.ExternalEtag = info.Etag;
-            //    forceUpdate = true;
-            //    _logger.LogDebug("Forcing update due to ExternalEtag {0}", item.Name);
-            // }
-
             if (!internalChannelId.Equals(item.ChannelId))
             {
                 forceUpdate = true;
diff --git a/Jellyfin.Api/Attributes/AcceptsFileAttribute.cs b/Jellyfin.Api/Attributes/AcceptsFileAttribute.cs
index 49b6689cd..58552d847 100644
--- a/Jellyfin.Api/Attributes/AcceptsFileAttribute.cs
+++ b/Jellyfin.Api/Attributes/AcceptsFileAttribute.cs
@@ -1,4 +1,6 @@
-using System;
+#pragma warning disable CA1813 // Avoid unsealed attributes
+
+using System;
 
 namespace Jellyfin.Api.Attributes
 {
diff --git a/Jellyfin.Api/Attributes/AcceptsImageFileAttribute.cs b/Jellyfin.Api/Attributes/AcceptsImageFileAttribute.cs
index 001f27409..244a29da4 100644
--- a/Jellyfin.Api/Attributes/AcceptsImageFileAttribute.cs
+++ b/Jellyfin.Api/Attributes/AcceptsImageFileAttribute.cs
@@ -3,7 +3,7 @@
     /// <summary>
     /// Produces file attribute of "image/*".
     /// </summary>
-    public class AcceptsImageFileAttribute : AcceptsFileAttribute
+    public sealed class AcceptsImageFileAttribute : AcceptsFileAttribute
     {
         private const string ContentType = "image/*";
 
diff --git a/Jellyfin.Api/Attributes/HttpSubscribeAttribute.cs b/Jellyfin.Api/Attributes/HttpSubscribeAttribute.cs
index 2fdd1e489..af8727552 100644
--- a/Jellyfin.Api/Attributes/HttpSubscribeAttribute.cs
+++ b/Jellyfin.Api/Attributes/HttpSubscribeAttribute.cs
@@ -7,7 +7,7 @@ namespace Jellyfin.Api.Attributes
     /// <summary>
     /// Identifies an action that supports the HTTP GET method.
     /// </summary>
-    public class HttpSubscribeAttribute : HttpMethodAttribute
+    public sealed class HttpSubscribeAttribute : HttpMethodAttribute
     {
         private static readonly IEnumerable<string> _supportedMethods = new[] { "SUBSCRIBE" };
 
diff --git a/Jellyfin.Api/Attributes/HttpUnsubscribeAttribute.cs b/Jellyfin.Api/Attributes/HttpUnsubscribeAttribute.cs
index d6d7e4563..1c0b70e71 100644
--- a/Jellyfin.Api/Attributes/HttpUnsubscribeAttribute.cs
+++ b/Jellyfin.Api/Attributes/HttpUnsubscribeAttribute.cs
@@ -7,7 +7,7 @@ namespace Jellyfin.Api.Attributes
     /// <summary>
     /// Identifies an action that supports the HTTP GET method.
     /// </summary>
-    public class HttpUnsubscribeAttribute : HttpMethodAttribute
+    public sealed class HttpUnsubscribeAttribute : HttpMethodAttribute
     {
         private static readonly IEnumerable<string> _supportedMethods = new[] { "UNSUBSCRIBE" };
 
diff --git a/Jellyfin.Api/Attributes/ParameterObsoleteAttribute.cs b/Jellyfin.Api/Attributes/ParameterObsoleteAttribute.cs
index 56c9772b6..514e7ce97 100644
--- a/Jellyfin.Api/Attributes/ParameterObsoleteAttribute.cs
+++ b/Jellyfin.Api/Attributes/ParameterObsoleteAttribute.cs
@@ -6,7 +6,7 @@ namespace Jellyfin.Api.Attributes
     /// Attribute to mark a parameter as obsolete.
     /// </summary>
     [AttributeUsage(AttributeTargets.Parameter)]
-    public class ParameterObsoleteAttribute : Attribute
+    public sealed class ParameterObsoleteAttribute : Attribute
     {
     }
 }
diff --git a/Jellyfin.Api/Attributes/ProducesAudioFileAttribute.cs b/Jellyfin.Api/Attributes/ProducesAudioFileAttribute.cs
index 3adb700eb..9fc25f192 100644
--- a/Jellyfin.Api/Attributes/ProducesAudioFileAttribute.cs
+++ b/Jellyfin.Api/Attributes/ProducesAudioFileAttribute.cs
@@ -3,7 +3,7 @@
     /// <summary>
     /// Produces file attribute of "image/*".
     /// </summary>
-    public class ProducesAudioFileAttribute : ProducesFileAttribute
+    public sealed class ProducesAudioFileAttribute : ProducesFileAttribute
     {
         private const string ContentType = "audio/*";
 
diff --git a/Jellyfin.Api/Attributes/ProducesFileAttribute.cs b/Jellyfin.Api/Attributes/ProducesFileAttribute.cs
index 62a576ede..2bf77d729 100644
--- a/Jellyfin.Api/Attributes/ProducesFileAttribute.cs
+++ b/Jellyfin.Api/Attributes/ProducesFileAttribute.cs
@@ -1,4 +1,6 @@
-using System;
+#pragma warning disable CA1813 // Avoid unsealed attributes
+
+using System;
 
 namespace Jellyfin.Api.Attributes
 {
diff --git a/Jellyfin.Api/Attributes/ProducesImageFileAttribute.cs b/Jellyfin.Api/Attributes/ProducesImageFileAttribute.cs
index e15813676..1e5b542e2 100644
--- a/Jellyfin.Api/Attributes/ProducesImageFileAttribute.cs
+++ b/Jellyfin.Api/Attributes/ProducesImageFileAttribute.cs
@@ -3,7 +3,7 @@
     /// <summary>
     /// Produces file attribute of "image/*".
     /// </summary>
-    public class ProducesImageFileAttribute : ProducesFileAttribute
+    public sealed class ProducesImageFileAttribute : ProducesFileAttribute
     {
         private const string ContentType = "image/*";
 
diff --git a/Jellyfin.Api/Attributes/ProducesPlaylistFileAttribute.cs b/Jellyfin.Api/Attributes/ProducesPlaylistFileAttribute.cs
index 5d928ab91..5b15cb1a5 100644
--- a/Jellyfin.Api/Attributes/ProducesPlaylistFileAttribute.cs
+++ b/Jellyfin.Api/Attributes/ProducesPlaylistFileAttribute.cs
@@ -3,7 +3,7 @@
     /// <summary>
     /// Produces file attribute of "image/*".
     /// </summary>
-    public class ProducesPlaylistFileAttribute : ProducesFileAttribute
+    public sealed class ProducesPlaylistFileAttribute : ProducesFileAttribute
     {
         private const string ContentType = "application/x-mpegURL";
 
diff --git a/Jellyfin.Api/Attributes/ProducesVideoFileAttribute.cs b/Jellyfin.Api/Attributes/ProducesVideoFileAttribute.cs
index d8b2856dc..6857d45ec 100644
--- a/Jellyfin.Api/Attributes/ProducesVideoFileAttribute.cs
+++ b/Jellyfin.Api/Attributes/ProducesVideoFileAttribute.cs
@@ -3,7 +3,7 @@
     /// <summary>
     /// Produces file attribute of "video/*".
     /// </summary>
-    public class ProducesVideoFileAttribute : ProducesFileAttribute
+    public sealed class ProducesVideoFileAttribute : ProducesFileAttribute
     {
         private const string ContentType = "video/*";
 
diff --git a/MediaBrowser.Controller/Entities/Audio/Audio.cs b/MediaBrowser.Controller/Entities/Audio/Audio.cs
index 536668e50..30111a9af 100644
--- a/MediaBrowser.Controller/Entities/Audio/Audio.cs
+++ b/MediaBrowser.Controller/Entities/Audio/Audio.cs
@@ -146,11 +146,7 @@ namespace MediaBrowser.Controller.Entities.Audio
             return info;
         }
 
-        protected override List<Tuple<BaseItem, MediaSourceType>> GetAllItemsForMediaSources()
-        {
-            var list = new List<Tuple<BaseItem, MediaSourceType>>();
-            list.Add(new Tuple<BaseItem, MediaSourceType>(this, MediaSourceType.Default));
-            return list;
-        }
+        protected override IEnumerable<(BaseItem, MediaSourceType)> GetAllItemsForMediaSources()
+            => new[] { ((BaseItem)this, MediaSourceType.Default) };
     }
 }
diff --git a/MediaBrowser.Controller/Entities/BaseItem.cs b/MediaBrowser.Controller/Entities/BaseItem.cs
index a76ca2305..b1ac2fe8e 100644
--- a/MediaBrowser.Controller/Entities/BaseItem.cs
+++ b/MediaBrowser.Controller/Entities/BaseItem.cs
@@ -5,7 +5,6 @@
 using System;
 using System.Collections.Generic;
 using System.Globalization;
-using System.IO;
 using System.Linq;
 using System.Text;
 using System.Text.Json.Serialization;
@@ -333,13 +332,6 @@ namespace MediaBrowser.Controller.Entities
         [JsonIgnore]
         public string ExternalSeriesId { get; set; }
 
-        /// <summary>
-        /// Gets or sets the etag.
-        /// </summary>
-        /// <value>The etag.</value>
-        [JsonIgnore]
-        public string ExternalEtag { get; set; }
-
         [JsonIgnore]
         public virtual bool IsHidden => false;
 
@@ -1161,9 +1153,9 @@ namespace MediaBrowser.Controller.Entities
             .ToList();
         }
 
-        protected virtual List<Tuple<BaseItem, MediaSourceType>> GetAllItemsForMediaSources()
+        protected virtual IEnumerable<(BaseItem, MediaSourceType)> GetAllItemsForMediaSources()
         {
-            return new List<Tuple<BaseItem, MediaSourceType>>();
+            return Enumerable.Empty<(BaseItem, MediaSourceType)>();
         }
 
         private MediaSourceInfo GetVersionInfo(bool enablePathSubstitution, BaseItem item, MediaSourceType type)
diff --git a/MediaBrowser.Controller/Entities/Video.cs b/MediaBrowser.Controller/Entities/Video.cs
index 7dd95b85c..de42c67d3 100644
--- a/MediaBrowser.Controller/Entities/Video.cs
+++ b/MediaBrowser.Controller/Entities/Video.cs
@@ -509,35 +509,35 @@ namespace MediaBrowser.Controller.Entities
             }).FirstOrDefault();
         }
 
-        protected override List<Tuple<BaseItem, MediaSourceType>> GetAllItemsForMediaSources()
+        protected override IEnumerable<(BaseItem, MediaSourceType)> GetAllItemsForMediaSources()
         {
-            var list = new List<Tuple<BaseItem, MediaSourceType>>();
+            var list = new List<(BaseItem, MediaSourceType)>
+            {
+                (this, MediaSourceType.Default)
+            };
 
-            list.Add(new Tuple<BaseItem, MediaSourceType>(this, MediaSourceType.Default));
-            list.AddRange(GetLinkedAlternateVersions().Select(i => new Tuple<BaseItem, MediaSourceType>(i, MediaSourceType.Grouping)));
+            list.AddRange(GetLinkedAlternateVersions().Select(i => ((BaseItem)i, MediaSourceType.Grouping)));
 
             if (!string.IsNullOrEmpty(PrimaryVersionId))
             {
-                var primary = LibraryManager.GetItemById(PrimaryVersionId) as Video;
-                if (primary != null)
+                if (LibraryManager.GetItemById(PrimaryVersionId) is Video primary)
                 {
                     var existingIds = list.Select(i => i.Item1.Id).ToList();
-                    list.Add(new Tuple<BaseItem, MediaSourceType>(primary, MediaSourceType.Grouping));
-                    list.AddRange(primary.GetLinkedAlternateVersions().Where(i => !existingIds.Contains(i.Id)).Select(i => new Tuple<BaseItem, MediaSourceType>(i, MediaSourceType.Grouping)));
+                    list.Add((primary, MediaSourceType.Grouping));
+                    list.AddRange(primary.GetLinkedAlternateVersions().Where(i => !existingIds.Contains(i.Id)).Select(i => ((BaseItem)i, MediaSourceType.Grouping)));
                 }
             }
 
             var localAlternates = list
                 .SelectMany(i =>
                 {
-                    var video = i.Item1 as Video;
-                    return video == null ? new List<Guid>() : video.GetLocalAlternateVersionIds();
+                    return i.Item1 is Video video ? video.GetLocalAlternateVersionIds() : Enumerable.Empty<Guid>();
                 })
                 .Select(LibraryManager.GetItemById)
                 .Where(i => i != null)
                 .ToList();
 
-            list.AddRange(localAlternates.Select(i => new Tuple<BaseItem, MediaSourceType>(i, MediaSourceType.Default)));
+            list.AddRange(localAlternates.Select(i => (i, MediaSourceType.Default)));
 
             return list;
         }
diff --git a/MediaBrowser.Model/Net/MimeTypes.cs b/MediaBrowser.Model/Net/MimeTypes.cs
index 748170a0e..043cee2a2 100644
--- a/MediaBrowser.Model/Net/MimeTypes.cs
+++ b/MediaBrowser.Model/Net/MimeTypes.cs
@@ -191,7 +191,7 @@ namespace MediaBrowser.Model.Net
             // Catch-all for all video types that don't require specific mime types
             if (_videoFileExtensions.Contains(ext))
             {
-                return "video/" + ext.Substring(1);
+                return string.Concat("video/", ext.AsSpan(1));
             }
 
             // Type text
diff --git a/jellyfin.ruleset b/jellyfin.ruleset
index 469f61021..7adc35087 100644
--- a/jellyfin.ruleset
+++ b/jellyfin.ruleset
@@ -50,8 +50,12 @@
     <Rule Id="CA1725" Action="Error" />
     <!-- error on CA1725: Call async methods when in an async method -->
     <Rule Id="CA1727" Action="Error" />
+    <!-- error on CA1813: Avoid unsealed attributes -->
+    <Rule Id="CA1813" Action="Error" />
     <!-- error on CA1843: Do not use 'WaitAll' with a single task -->
     <Rule Id="CA1843" Action="Error" />
+    <!-- error on CA1845: Use span-based 'string.Concat' -->
+    <Rule Id="CA1845" Action="Error" />
     <!-- error on CA2016: Forward the CancellationToken parameter to methods that take one
         or pass in 'CancellationToken.None' explicitly to indicate intentionally not propagating the token -->
     <Rule Id="CA2016" Action="Error" />