CSHARP-4648: Standardize handling of decimal vs Decimal128.

Author: rstam

src/MongoDB.Bson/Serialization/Serializers/ObjectSerializer.cs

@@ -303,10 +303,6 @@ public override void Serialize(BsonSerializationContext context, BsonSerializati
                                 bsonWriter.WriteDateTime(bsonDateTime.MillisecondsSinceEpoch);
                                 return;
 
-                            case TypeCode.Decimal:
-                                bsonWriter.WriteDecimal128((decimal)value);
-                                return;
-
                             case TypeCode.Double:
                                 bsonWriter.WriteDouble((double)value);
                                 return;
@@ -571,6 +567,11 @@ private static class DefaultFrameworkAllowedTypes
 
             private static bool AllowedTypesImplementation(Type type)
             {
+                if (typeof(BsonValue).IsAssignableFrom(type))
+                {
+                    return true;
+                }
+
                 return type.IsConstructedGenericType ? IsAllowedGenericType(type) : IsAllowedType(type);
 
                 static bool IsAllowedType(Type type) =>

tests/MongoDB.Bson.Tests/Serialization/Serializers/ObjectSerializerTests.cs

@@ -102,6 +102,23 @@ public void TestBoolean()
             Assert.True(bson.SequenceEqual(rehydrated.ToBson()));
         }
 
+        [Fact]
+        public void TestBsonDecimal128()
+        {
+            var value = (BsonDecimal128)1.5M;
+            var c = new C { Obj = value };
+
+            var json = c.ToJson();
+            json.Should().Be("{ \"Obj\" : { \"_t\" : \"MongoDB.Bson.BsonDecimal128, MongoDB.Bson\", \"_v\" : NumberDecimal(\"1.5\") } }");
+
+            var bson = c.ToBson();
+            var rehydrated = BsonSerializer.Deserialize<C>(bson);
+            rehydrated.Obj.Should().BeOfType<BsonDecimal128>();
+            rehydrated.Obj.Should().Be(value);
+
+            rehydrated.ToBson().Should().Equal(bson);
+        }
+
         [Fact]
         public void TestDateTime()
         {
@@ -118,27 +135,35 @@ public void TestDateTime()
         [Fact]
         public void TestDecimal()
         {
-            var c = new C { Obj = 1.5M };
+            var value = 1.5M;
+            var c = new C { Obj = value };
+
             var json = c.ToJson();
-            var expected = "{ 'Obj' : NumberDecimal('1.5') }".Replace("'", "\"");
-            Assert.Equal(expected, json);
+            json.Should().Be("{ \"Obj\" : { \"_t\" : \"System.Decimal\", \"_v\" : \"1.5\" } }");
 
             var bson = c.ToBson();
             var rehydrated = BsonSerializer.Deserialize<C>(bson);
-            Assert.True(bson.SequenceEqual(rehydrated.ToBson()));
+            rehydrated.Obj.Should().BeOfType<decimal>();
+            rehydrated.Obj.Should().Be(value);
+
+            rehydrated.ToBson().Should().Equal(bson);
         }
 
         [Fact]
         public void TestDecimal128()
         {
-            var c = new C { Obj = (Decimal128)1.5M };
+            var value = (Decimal128)1.5M;
+            var c = new C { Obj = value };
+
             var json = c.ToJson();
-            var expected = "{ 'Obj' : NumberDecimal('1.5') }".Replace("'", "\"");
-            Assert.Equal(expected, json);
+            json.Should().Be("{ \"Obj\" : NumberDecimal(\"1.5\") }");
 
             var bson = c.ToBson();
             var rehydrated = BsonSerializer.Deserialize<C>(bson);
-            Assert.True(bson.SequenceEqual(rehydrated.ToBson()));
+            rehydrated.Obj.Should().BeOfType<Decimal128>();
+            rehydrated.Obj.Should().Be(value);
+
+            rehydrated.ToBson().Should().Equal(bson);
         }
 
         [Fact]