How to force serialization of decimal values inside dictionaries as doubles in mongodb csharp driver? - mongodb-.net-driver

Question

I have the following dictionary:
var dict = new Dictionary<string, object> {
{ "decimal", 3.503m },
{ "int", 45 }
};
var serializedString = dict.ToJson();
By default that is serialized as:
{ "decimal" : { "_t" : "System.Decimal", "_v" : "3.503" }, "int" : 45 }
If I override DecimalSerializer as:
BsonSerializer.RegisterSerializer<decimal>(new DecimalSerializer().WithRepresentation(BsonType.Double));
That only influences on how "_v" value is serialized, e.g.:
{ "decimal" : { "_t" : "System.Decimal", "_v" : 3.503 }, "int" : 45 }
Expected result:
{ "decimal" : 3.503, "int" : 45 }
Please advise

Answer 1

The cause of the .Net types in the bson, is the lack of type in the dictionary. The Bson serializers are trying to get enough state to restore the original object of the items in the dictionary. From the context (the dictionary) they are of type "object", so the .Net type is inserted to know enough when deserializing.
The following solutions answer your question but lose the type information for deserializing.
Solution 1: Change the dictionary type to <string, decimal>
var dict = new Dictionary<string, decimal> {
{ "decimal", 3.503m },
{ "int", 45 }
};
var serializedString = dict.ToJson();
Results in: { "decimal" : "3.503", "int" : "45" }
With your override of the decimal serializer, you get the expected result.
{ "decimal" : 3.503, "int" : 45 }
Solution 2: Change the dictionary type to <string, double>
var dict = new Dictionary<string, double> {
{ "decimal", (double)3.503m },
{ "int", 45 }
};
var serializedString = dict.ToJson();
Results in the expected result: { "decimal" : 3.503, "int" : 45 }
Solution 3: Use custom serializer
public class MyDictionarySerializer : SerializerBase<Dictionary<string, object>>
{
public override void Serialize(MongoDB.Bson.Serialization.BsonSerializationContext context, MongoDB.Bson.Serialization.BsonSerializationArgs args, Dictionary<string, object> dictionary)
{
context.Writer.WriteStartArray();
foreach (var item in dictionary)
{
context.Writer.WriteStartDocument();
context.Writer.WriteString(item.Key);
// TODO your converstions from object to double
var value = (double)item.Value;
context.Writer.WriteDouble(value);
context.Writer.WriteEndDocument();
}
context.Writer.WriteEndArray();
}
public override Dictionary<string, object> Deserialize(MongoDB.Bson.Serialization.BsonDeserializationContext context, MongoDB.Bson.Serialization.BsonDeserializationArgs args)
{
context.Reader.ReadStartArray();
var result = new Dictionary<string, object>();
while (true)
{
try
{
//this catch block only need to identify the end of the Array
context.Reader.ReadStartDocument();
}
catch (Exception exp)
{
context.Reader.ReadEndArray();
break;
}
var key = context.Reader.ReadString();
double value = context.Reader.ReadDouble();
result.Add(key, value);
context.Reader.ReadEndDocument();
}
return result;
}
}

Answer 2

As another option, it's possible to override object serializer
public class DecimalsOverridingObjectSerializer : ObjectSerializer
{
public override void Serialize(BsonSerializationContext context, BsonSerializationArgs args, object value) {
if (value != null && value is decimal) {
base.Serialize(context, args, Convert.ToDouble(value));
} else {
base.Serialize(context, args, value);
}
}
}
BsonSerializer.RegisterSerializer(typeof(object), new DecimalsOverridingObjectSerializer());
that still will not work for Hashtables.
Possible workaround for Hashtables:
public class DecimalsOverridingDictionarySerializer<TDictionary>:
DictionaryInterfaceImplementerSerializer<TDictionary>
where TDictionary : class, IDictionary, new()
{
public DecimalsOverridingDictionarySerializer(DictionaryRepresentation dictionaryRepresentation)
: base(dictionaryRepresentation, new DecimalsOverridingObjectSerializer(), new DecimalsOverridingObjectSerializer())
{ }
}
BsonSerializer.RegisterSerializer(typeof(Hashtable), new DecimalsOverridingDictionarySerializer<Hashtable>(DictionaryRepresentation.Document));