Deserializing an object without a default constructor out of MongoDb + C#

Question

Consider the two classes:

public class Entity
{
    public ObjectId Id { get; set; }
    public E2 e = new E2(ConfigClass.SomeStaticMethod());
}

public class E2
{
    [BsonIgnore]
    public int counter = 5;
    public DateTime last_update { get; set; }

    public E2(int c)
    {
        counter = c;
    }
}

I'll store and then retrieve an object of type Entity to / from a MongoDb like this (assuming collection is empty):

var collection = database.GetCollection<Entity>("temp");
collection.Save<Entity>(new Entity());
var list = collection.FindAs<Entity>(new QueryDocument());
var ent = list.Single();

Regardless of what ConfigClass.SomeStaticMethod() returns, the counter field will be zero (the default value for integers). But if I add a default constructor to the E2 class then counter will be 5.

This means that MongoDb's C# driver has got a problem with calling non-default constructors (which is totally understandable). I know there's a BsonDefaultValue attribute defined within BSON library but it can only accept constant expressions.

What I'm trying to do is to load the default value of a field out of config files while the rest of object is retrieved from MongoDb!? And of course with the least effort.

[UPDATE]

I've also tested this with the same results:

public class Entity
{
    public ObjectId Id { get; set; }
    public E2 e = new E2();
    public Entity()
    {
        e.counter = ConfigClass.SomeStaticMethod();
    }
}

public class E2
{
    [BsonIgnore]
    public int counter = 5;
    public DateTime last_update { get; set; }
}

Running this code results in counter to be zero again!

Answer 1

I managed to accomplish it like this:

public class Entity
{
    public ObjectId Id { get; set; }
    public E2 e = new E2();
}

public class E2 : ISupportInitialize
{
    [BsonIgnore]
    public int counter = 5;
    public DateTime last_update { get; set; }

    public void BeginInit()
    {
    }

    public void EndInit()
    {
        counter = ConfigClass.SomeStaticMethod();
    }
}

The ISupportInitialize interface comes with two methods of BeginInit and EndInit which are called before and after the process of deserialization. They are the best places to set default values.