CSHARP-750: Fixed array of value type serialization performance issue

Author: Dmitry Naumov

MongoDB.Bson/IO/BsonBinaryWriter.cs

@@ -30,6 +30,7 @@ public class BsonBinaryWriter : BsonWriter
         private bool _disposeBuffer;
         private BsonBinaryWriterSettings _binaryWriterSettings; // same value as in base class just declared as derived class
         private BsonBinaryWriterContext _context;
+        private readonly UTF8Encoding _encoding = new UTF8Encoding(false, true);
 
         // constructors
         /// <summary>
@@ -724,7 +725,7 @@ private void WriteNameHelper()
                 name = Name;
             }
 
-            _buffer.WriteCString(new UTF8Encoding(false, true), name); // always use strict encoding for names
+            _buffer.WriteCString(_encoding, name); // always use strict encoding for names
         }
     }
 }

MongoDB.Bson/Serialization/Serializers/ArraySerializer.cs

@@ -65,10 +65,14 @@ public override object Deserialize(
                     bsonReader.ReadStartArray();
                     var discriminatorConvention = BsonSerializer.LookupDiscriminatorConvention(typeof(T));
                     var list = new List<T>();
+
+                    var itemNominalType = typeof(T);
+                    var itemNominalTypeSerializer = BsonSerializer.LookupSerializer(typeof(T));
+
                     while (bsonReader.ReadBsonType() != BsonType.EndOfDocument)
                     {
-                        var elementType = discriminatorConvention.GetActualType(bsonReader, typeof(T));
-                        var serializer = BsonSerializer.LookupSerializer(elementType);
+                        var elementType = itemNominalType.IsValueType ? itemNominalType : discriminatorConvention.GetActualType(bsonReader, typeof(T));
+                        var serializer = itemNominalType.IsValueType ? itemNominalTypeSerializer : BsonSerializer.LookupSerializer(elementType);
                         var element = (T)serializer.Deserialize(bsonReader, typeof(T), elementType, itemSerializationOptions);
                         list.Add(element);
                     }
@@ -136,11 +140,17 @@ public override void Serialize(
                 var arraySerializationOptions = EnsureSerializationOptions<ArraySerializationOptions>(options);
                 var itemSerializationOptions = arraySerializationOptions.ItemSerializationOptions;
 
+                var itemNominalType = typeof(T);
+                var itemNominalTypeSerializer = BsonSerializer.LookupSerializer(itemNominalType);
+
                 bsonWriter.WriteStartArray();
-                for (int index = 0; index < array.Length; index++)
+
+                foreach (var item in array)
                 {
-                    BsonSerializer.Serialize(bsonWriter, typeof(T), array[index], itemSerializationOptions);
+                    var itemSerializer = (itemNominalType.IsValueType || item == null) ? itemNominalTypeSerializer : BsonSerializer.LookupSerializer(item.GetType());
+                    itemSerializer.Serialize(bsonWriter, itemNominalType, item, itemSerializationOptions);
                 }
+
                 bsonWriter.WriteEndArray();
             }
         }