Serializing Immutable Value types with Mongo C# Driver

Question

I have many immutable value type classes, for example EmailAddress, which ensure any non null instance is valid. I would like to control the serialization of these types of objects to be just the standard string representation ("[email protected]") when persisted using MongoDB C# Driver.

I have tried implementing the IBsonSerilizer however it will only allow for objects or arrays at the root level. I was able to implement proper Json Serilization with Json.NET, is there a different approach I should be taking?

Answer 1

I assume you mean an EmailAddress class something like this:

[BsonSerializer(typeof(EmailAddressSerializer))]
public class EmailAddress
{
    private string _value;

    public EmailAddress(string value)
    {
        _value = value;
    }

    public string Value
    {
        get { return _value; }
    }
}

I've used an attribute to link the EmailAddress class to a custom serializer, which could be implemented like this:

public class EmailAddressSerializer : BsonBaseSerializer
{
    public override object Deserialize(BsonReader bsonReader, Type nominalType, Type actualType, IBsonSerializationOptions options)
    {
        if (bsonReader.GetCurrentBsonType() == BsonType.Null)
        {
            bsonReader.ReadNull();
            return null;
        }
        else
        {
            var value = bsonReader.ReadString();
            return new EmailAddress(value);
        }
    }

    public override void Serialize(BsonWriter bsonWriter, Type nominalType, object value, IBsonSerializationOptions options)
    {
        if (value == null)
        {
            bsonWriter.WriteNull();
        }
        else
        {
            var emailAddress = (EmailAddress)value;
            bsonWriter.WriteString(emailAddress.Value);
        }
    }
}

You can't serialize an EmailAddress as the root document (because it's not a document...). But you could use an EmailAddress embedded in some other document. For example:

public class Person
{
    public int Id { get; set; }
    public EmailAddress EmailAddress { get; set; }
}

Which you could test using code like the following:

var person = new Person { Id = 1, EmailAddress = new EmailAddress("[email protected]") };
var json = person.ToJson();
var rehyrdated = BsonSerializer.Deserialize<Person>(json);

The resulting JSON/BSON document is:

{ "_id" : 1, "EmailAddress" : "[email protected]" }