diff --git a/csharp/src/Google.Protobuf.Test/Reflection/CustomOptionsTest.cs b/csharp/src/Google.Protobuf.Test/Reflection/CustomOptionsTest.cs
index ff4e9a7ffd..7aa2077778 100644
--- a/csharp/src/Google.Protobuf.Test/Reflection/CustomOptionsTest.cs
+++ b/csharp/src/Google.Protobuf.Test/Reflection/CustomOptionsTest.cs
@@ -41,6 +41,8 @@ using static UnitTest.Issues.TestProtos.ComplexOptionType2.Types;
 using static UnitTest.Issues.TestProtos.DummyMessageContainingEnum.Types;
 using static Google.Protobuf.Test.Reflection.CustomOptionNumber;
 
+#pragma warning disable CS0618
+
 namespace Google.Protobuf.Test.Reflection
 {
     // Internal enum to allow us to use "using static" for convenience.
diff --git a/csharp/src/Google.Protobuf/Reflection/EnumDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/EnumDescriptor.cs
index 36a1d0a5ec..33be96198d 100644
--- a/csharp/src/Google.Protobuf/Reflection/EnumDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/EnumDescriptor.cs
@@ -128,17 +128,16 @@ namespace Google.Protobuf.Reflection
         /// <summary>
         /// The (possibly empty) set of custom options for this enum.
         /// </summary>
-        //[Obsolete("CustomOptions are obsolete. Use GetOption")]
+        [Obsolete("CustomOptions are obsolete. Use GetOption")]
         public CustomOptions CustomOptions => new CustomOptions(Proto.Options._extensions?.ValuesByNumber);
 
-        /* // uncomment this in the full proto2 support PR
         /// <summary>
         /// Gets a single value enum option for this descriptor
         /// </summary>
         public T GetOption<T>(Extension<EnumOptions, T> extension)
         {
             var value = Proto.Options.GetExtension(extension);
-            return value is IDeepCloneable<T> clonable ? clonable.Clone() : value;
+            return value is IDeepCloneable<T> ? (value as IDeepCloneable<T>).Clone() : value;
         }
 
         /// <summary>
@@ -148,6 +147,5 @@ namespace Google.Protobuf.Reflection
         {
             return Proto.Options.GetExtension(extension).Clone();
         }
-        */
     }
 }
\ No newline at end of file
diff --git a/csharp/src/Google.Protobuf/Reflection/EnumValueDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/EnumValueDescriptor.cs
index 99c989130c..3936aa4e26 100644
--- a/csharp/src/Google.Protobuf/Reflection/EnumValueDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/EnumValueDescriptor.cs
@@ -73,17 +73,16 @@ namespace Google.Protobuf.Reflection
         /// <summary>
         /// The (possibly empty) set of custom options for this enum value.
         /// </summary>
-        //[Obsolete("CustomOptions are obsolete. Use GetOption")]
+        [Obsolete("CustomOptions are obsolete. Use GetOption")]
         public CustomOptions CustomOptions => new CustomOptions(Proto.Options._extensions?.ValuesByNumber);
 
-        /* // uncomment this in the full proto2 support PR
         /// <summary>
         /// Gets a single value enum option for this descriptor
         /// </summary>
         public T GetOption<T>(Extension<EnumValueOptions, T> extension)
         {
             var value = Proto.Options.GetExtension(extension);
-            return value is IDeepCloneable<T> clonable ? clonable.Clone() : value;
+            return value is IDeepCloneable<T> ? (value as IDeepCloneable<T>).Clone() : value;
         }
 
         /// <summary>
@@ -93,7 +92,6 @@ namespace Google.Protobuf.Reflection
         {
             return Proto.Options.GetExtension(extension).Clone();
         }
-        */
     }
 }
  
\ No newline at end of file
diff --git a/csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs
index 5a2f7f3d8e..c06328f99c 100644
--- a/csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs
@@ -277,17 +277,16 @@ namespace Google.Protobuf.Reflection
         /// <summary>
         /// The (possibly empty) set of custom options for this field.
         /// </summary>
-        //[Obsolete("CustomOptions are obsolete. Use GetOption")]
+        [Obsolete("CustomOptions are obsolete. Use GetOption")]
         public CustomOptions CustomOptions => new CustomOptions(Proto.Options._extensions?.ValuesByNumber);
 
-        /* // uncomment this in the full proto2 support PR
         /// <summary>
         /// Gets a single value enum option for this descriptor
         /// </summary>
         public T GetOption<T>(Extension<FieldOptions, T> extension)
         {
             var value = Proto.Options.GetExtension(extension);
-            return value is IDeepCloneable<T> clonable ? clonable.Clone() : value;
+            return value is IDeepCloneable<T> ? (value as IDeepCloneable<T>).Clone() : value;
         }
 
         /// <summary>
@@ -297,7 +296,6 @@ namespace Google.Protobuf.Reflection
         {
             return Proto.Options.GetExtension(extension).Clone();
         }
-        */
 
         /// <summary>
         /// Look up and cross-link all field types etc.
diff --git a/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs
index 9b20e3d22a..13266f725d 100644
--- a/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs
@@ -504,17 +504,16 @@ namespace Google.Protobuf.Reflection
         /// <summary>
         /// The (possibly empty) set of custom options for this file.
         /// </summary>
-        //[Obsolete("CustomOptions are obsolete. Use GetOption")]
+        [Obsolete("CustomOptions are obsolete. Use GetOption")]
         public CustomOptions CustomOptions => new CustomOptions(Proto.Options._extensions?.ValuesByNumber);
 
-        /* // uncomment this in the full proto2 support PR
         /// <summary>
         /// Gets a single value enum option for this descriptor
         /// </summary>
         public T GetOption<T>(Extension<FileOptions, T> extension)
         {
             var value = Proto.Options.GetExtension(extension);
-            return value is IDeepCloneable<T> clonable ? clonable.Clone() : value;
+            return value is IDeepCloneable<T> ? (value as IDeepCloneable<T>).Clone() : value;
         }
 
         /// <summary>
@@ -524,7 +523,6 @@ namespace Google.Protobuf.Reflection
         {
             return Proto.Options.GetExtension(extension).Clone();
         }
-        */
 
         /// <summary>
         /// Performs initialization for the given generic type argument.
diff --git a/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs
index 15bfbfadd0..4ca9e36989 100644
--- a/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs
@@ -243,17 +243,16 @@ namespace Google.Protobuf.Reflection
         /// <summary>
         /// The (possibly empty) set of custom options for this message.
         /// </summary>
-        //[Obsolete("CustomOptions are obsolete. Use GetOption")]
+        [Obsolete("CustomOptions are obsolete. Use GetOption")]
         public CustomOptions CustomOptions => new CustomOptions(Proto.Options._extensions?.ValuesByNumber);
 
-        /* // uncomment this in the full proto2 support PR
         /// <summary>
         /// Gets a single value enum option for this descriptor
         /// </summary>
         public T GetOption<T>(Extension<MessageOptions, T> extension)
         {
             var value = Proto.Options.GetExtension(extension);
-            return value is IDeepCloneable<T> clonable ? clonable.Clone() : value;
+            return value is IDeepCloneable<T> ? (value as IDeepCloneable<T>).Clone() : value;
         }
 
         /// <summary>
@@ -263,7 +262,6 @@ namespace Google.Protobuf.Reflection
         {
             return Proto.Options.GetExtension(extension).Clone();
         }
-        */
 
         /// <summary>
         /// Looks up and cross-links all fields and nested types.
diff --git a/csharp/src/Google.Protobuf/Reflection/MethodDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/MethodDescriptor.cs
index 1b37d9b172..c138576b32 100644
--- a/csharp/src/Google.Protobuf/Reflection/MethodDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/MethodDescriptor.cs
@@ -73,17 +73,16 @@ namespace Google.Protobuf.Reflection
         /// <summary>
         /// The (possibly empty) set of custom options for this method.
         /// </summary>
-        //[Obsolete("CustomOptions are obsolete. Use GetOption")]
+        [Obsolete("CustomOptions are obsolete. Use GetOption")]
         public CustomOptions CustomOptions => new CustomOptions(Proto.Options._extensions?.ValuesByNumber);
 
-        /* // uncomment this in the full proto2 support PR
         /// <summary>
         /// Gets a single value enum option for this descriptor
         /// </summary>
         public T GetOption<T>(Extension<MethodOptions, T> extension)
         {
             var value = Proto.Options.GetExtension(extension);
-            return value is IDeepCloneable<T> clonable ? clonable.Clone() : value;
+            return value is IDeepCloneable<T> ? (value as IDeepCloneable<T>).Clone() : value;
         }
 
         /// <summary>
@@ -93,7 +92,6 @@ namespace Google.Protobuf.Reflection
         {
             return Proto.Options.GetExtension(extension).Clone();
         }
-        */
 
         internal MethodDescriptor(MethodDescriptorProto proto, FileDescriptor file,
                                   ServiceDescriptor parent, int index)
diff --git a/csharp/src/Google.Protobuf/Reflection/ServiceDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/ServiceDescriptor.cs
index 54f4df7298..215aff5d44 100644
--- a/csharp/src/Google.Protobuf/Reflection/ServiceDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/ServiceDescriptor.cs
@@ -94,17 +94,16 @@ namespace Google.Protobuf.Reflection
         /// <summary>
         /// The (possibly empty) set of custom options for this service.
         /// </summary>
-        //[Obsolete("CustomOptions are obsolete. Use GetOption")]
+        [Obsolete("CustomOptions are obsolete. Use GetOption")]
         public CustomOptions CustomOptions => new CustomOptions(Proto.Options._extensions?.ValuesByNumber);
 
-        /* // uncomment this in the full proto2 support PR
         /// <summary>
         /// Gets a single value enum option for this descriptor
         /// </summary>
         public T GetOption<T>(Extension<ServiceOptions, T> extension)
         {
             var value = Proto.Options.GetExtension(extension);
-            return value is IDeepCloneable<T> clonable ? clonable.Clone() : value;
+            return value is IDeepCloneable<T> ? (value as IDeepCloneable<T>).Clone() : value;
         }
 
         /// <summary>
@@ -114,7 +113,6 @@ namespace Google.Protobuf.Reflection
         {
             return Proto.Options.GetExtension(extension).Clone();
         }
-        */
 
         internal void CrossLink()
         {