Small fixes

Author: papafe

src/MongoDB.Driver/Core/WireProtocol/Messages/Encoders/BinaryEncoders/CommandMessageBinaryEncoder.cs

@@ -28,13 +28,13 @@ namespace MongoDB.Driver.Core.WireProtocol.Messages.Encoders.BinaryEncoders
     internal sealed class CommandMessageBinaryEncoder : MessageBinaryEncoderBase, IMessageEncoder
     {
         private const int EncryptedMaxBatchSize = 2 * 1024 * 1024; // 2 MiB
-        private readonly ICommandMessageSectionFormatter<Type0CommandMessageSection> _type0SectionFormatter;
+        private static readonly ICommandMessageSectionFormatter<Type0CommandMessageSection> __type0SectionFormatter = new Type0SectionFormatter(BsonSerializer.DefaultSerializationDomain);
+        //QUESTION Looking at the spec and our implementation, it seems that type 0 sections always serialize/deserialize RawBsonDocument, so they should use the default domain. Am I missing something?
 
         // constructors
         public CommandMessageBinaryEncoder(Stream stream, MessageEncoderSettings encoderSettings)
             : base(stream, encoderSettings)
         {
-            _type0SectionFormatter = new Type0SectionFormatter(SerializationDomain);
         }
 
         // public methods
@@ -250,7 +250,7 @@ private void WriteSection(BsonBinaryWriter writer, CommandMessageSection section
             switch (section)
             {
                 case Type0CommandMessageSection type0Section:
-                    _type0SectionFormatter.FormatSection(type0Section, writer);
+                    __type0SectionFormatter.FormatSection(type0Section, writer);
                     break;
                 case Type1CommandMessageSection type1Section:
                     var type1SectionFormatter = new Type1SectionFormatter(GetSectionMaxSize(), SerializationDomain);

src/MongoDB.Driver/Core/WireProtocol/Messages/Encoders/BinaryEncoders/MessageBinaryEncoderBase.cs

@@ -83,7 +83,8 @@ protected int? MaxWireDocumentSize
         }
 
         protected IBsonSerializationDomain SerializationDomain
-            => _encoderSettings?.GetOrDefault<IBsonSerializationDomain>(MessageEncoderSettingsName.SerializationDomain, null);
+            => _encoderSettings?.GetOrDefault<IBsonSerializationDomain>(MessageEncoderSettingsName.SerializationDomain, null)  ?? BsonSerializer.DefaultSerializationDomain;
+        //QUESTION Is this correct? If we don't have a domain in the encoder settings, just use the default one?
 
         // methods
         public BsonBinaryReader CreateBinaryReader()

src/MongoDB.Driver/Core/WireProtocol/Messages/Encoders/JsonEncoders/CommandMessageJsonEncoder.cs

@@ -33,7 +33,8 @@ internal sealed class CommandMessageJsonEncoder : MessageJsonEncoderBase, IMessa
         public CommandMessageJsonEncoder(TextReader textReader, TextWriter textWriter, MessageEncoderSettings encoderSettings)
             : base(textReader, textWriter, encoderSettings)
         {
-            _type0SectionFormatter = new Type0SectionFormatter(SerializationDomain);
+            //QUESTION Looking at the spec and our implementation, it seems that type 0 sections always serialize/deserialize RawBsonDocument, so they should use the default domain. Am I missing something?
+            _type0SectionFormatter = new Type0SectionFormatter(BsonSerializer.DefaultSerializationDomain);
             _type1SectionFormatter = new Type1SectionFormatter(SerializationDomain);
         }