C# - Custom serializer not giving value to _id

Dor_Ben-Senior · 2023-09-06T11:52:33.850Z

Hi all,
I’m using the C# driver and needed to write a custom serializer for a collection that contains different document types (all children of the same base type)
In the deserialize method I figure out the document type and deserialize to the relevant class
In the serialize method I wanted to use the driver’s default behavior since I didn’t need to do anything special, so I created an instance of a BsonClassMapSerializer and used its serialize method
The problem is that when I do this it doesn’t generate a value for the _id property
So the question is do I need to do it myself because I’m using a custom serializer, or am I just doing something wrong here with how I’m serializing?
I thought that using the ClassMapSerializer will take care of this for me

Some code example for better understanding:

public class BaseClass
{
    [BsonId]
    [BsonRepresentation(BsonType.ObjectId)]
    public string Id { get; set; }
	public string Title { get; set; }
}

public class Child1 : BaseClass
{
	public string Description { get; set; }
	public int Value { get; set; }
}

public class Child2 : BaseClass
{
	public string Creator { get; set; }
	public int Amount { get; set; }
	public int NumberOfUsers { get; set; } 
}

public class MyCustomSerializer : IBsonSerializer<BaseClass>, IBsonDocumentSerializer
{
	public Type ValueType => typeof(BaseClass);

	private readonly IBsonDocumentSerializer _baseSerializer = new BsonClassMapSerializer<BaseClass>(BsonClassMap.LookupClassMap(typeof(BaseClass)));

	public BaseClass Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
	{
		// Deserialize to the relevant child class
	}

	public void Serialize(BsonSerializationContext context, BsonSerializationArgs args, BaseClass value)
	{
        // This doesn't give a value to _id field so the document is inserted with _id: null
		_baseSerializer.Serialize(context, args, value);
	}

	public void Serialize(BsonSerializationContext context, BsonSerializationArgs args, object value)
	{
		Serialize(context, args, (BaseInteractionInstance)value);
	}

	public bool TryGetMemberSerializationInfo(string memberName, out BsonSerializationInfo serializationInfo)
	{
		return _baseSerializer.TryGetMemberSerializationInfo(memberName, out serializationInfo);
	}

	object IBsonSerializer.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
	{
		return Deserialize(context, args);
	}
}

Dor_Ben-Senior · 2023-09-06T12:57:18.654Z

For now I fixed it by adding this in the Serialize method:

if (string.IsNullOrEmpty(value.Id))
            {
                value.Id = ObjectId.GenerateNewId().ToString();
            }

but leaving this thread open because I do want to know if I’m using the base serializer wrong or if this is the correct way to do it