diff --git a/csharp/PhoneNumbers/MetadataFilter.cs b/csharp/PhoneNumbers/MetadataFilter.cs
index ac9f62cb..5c6219ff 100644
--- a/csharp/PhoneNumbers/MetadataFilter.cs
+++ b/csharp/PhoneNumbers/MetadataFilter.cs
@@ -29,7 +29,7 @@ namespace PhoneNumbers
     /// changes without notice. Any changes are not guaranteed to be reflected in the versioning scheme
     /// of the public API, nor in release notes.
     /// </summary>
-    public class MetadataFilter
+    public sealed class MetadataFilter
     {
         // The following 3 sets comprise all the PhoneMetadata fields as defined at phonemetadata.proto
         // which may be excluded from customized serializations of the binary metadata. Fields that are
diff --git a/csharp/PhoneNumbers/PhoneNumberOfflineGeocoder.cs b/csharp/PhoneNumbers/PhoneNumberOfflineGeocoder.cs
index 82b6ef13..9ddeaebd 100644
--- a/csharp/PhoneNumbers/PhoneNumberOfflineGeocoder.cs
+++ b/csharp/PhoneNumbers/PhoneNumberOfflineGeocoder.cs
@@ -92,7 +92,7 @@ private static string GetCountryName(string country, string language)
     /// files generated at build time by <c>PhoneNumbers.MetadataBuilder</c>, via
     /// <see cref="PrefixFileReader"/>.
     /// </remarks>
-    public class PhoneNumberOfflineGeocoder
+    public sealed class PhoneNumberOfflineGeocoder
     {
         private static PhoneNumberOfflineGeocoder instance;
         private const string MAPPING_DATA_DIRECTORY = "geocoding.";
diff --git a/csharp/PhoneNumbers/PhoneNumberToCarrierMapper.cs b/csharp/PhoneNumbers/PhoneNumberToCarrierMapper.cs
index 745c8f38..ac6fddef 100644
--- a/csharp/PhoneNumbers/PhoneNumberToCarrierMapper.cs
+++ b/csharp/PhoneNumbers/PhoneNumberToCarrierMapper.cs
@@ -27,7 +27,7 @@ namespace PhoneNumbers
     /// number portability the number might not belong to the returned carrier anymore.
     /// </para>
     /// </summary>
-    public class PhoneNumberToCarrierMapper
+    public sealed class PhoneNumberToCarrierMapper
     {
         // Corresponds to resources/carrier/ embedded with LinkBase="carrier".
         private const string MAPPING_DATA_DIRECTORY = "carrier.";
diff --git a/csharp/PhoneNumbers/PhoneNumberToTimeZonesMapper.cs b/csharp/PhoneNumbers/PhoneNumberToTimeZonesMapper.cs
index e8f10065..ecd52eb4 100644
--- a/csharp/PhoneNumbers/PhoneNumberToTimeZonesMapper.cs
+++ b/csharp/PhoneNumbers/PhoneNumberToTimeZonesMapper.cs
@@ -12,7 +12,7 @@
 
 namespace PhoneNumbers
 {
-    public class PhoneNumberToTimeZonesMapper
+    public sealed class PhoneNumberToTimeZonesMapper
     {
         private static readonly string[] UNKNOWN_TIMEZONE = { "Etc/Unknown" };
 
diff --git a/csharp/PhoneNumbers/PhoneNumberUtil.cs b/csharp/PhoneNumbers/PhoneNumberUtil.cs
index 5cdfb7fb..e2f8e9ed 100644
--- a/csharp/PhoneNumbers/PhoneNumberUtil.cs
+++ b/csharp/PhoneNumbers/PhoneNumberUtil.cs
@@ -53,7 +53,7 @@ namespace PhoneNumbers
     /// @author Lara Rennie
     /// -->
     /// </summary>
-    public partial class PhoneNumberUtil
+    public sealed partial class PhoneNumberUtil
     {
         // The minimum and maximum length of the national significant number.
         private const int MIN_LENGTH_FOR_NSN = 2;
diff --git a/csharp/PhoneNumbers/ShortNumberInfo.cs b/csharp/PhoneNumbers/ShortNumberInfo.cs
index 14756efd..8fc02c8e 100644
--- a/csharp/PhoneNumbers/ShortNumberInfo.cs
+++ b/csharp/PhoneNumbers/ShortNumberInfo.cs
@@ -32,7 +32,7 @@ namespace PhoneNumbers
     /// @author Shaopeng Jia
     /// @author David Yonge-Mallo
     /// </summary>
-    public class ShortNumberInfo
+    public sealed class ShortNumberInfo
     {
         private static readonly ShortNumberInfo Instance = new();