public class DummyClass
{
    [BsonId]
    public int Id { set; get; }

    [BsonElement("first")]
    public string First { set { _name = value; } }

    [BsonConstructor]
    public DummyClass()
    {
        Id = 2;
        First = "1";
    }
}

_dbClient = new MongoClient();
_database = _dbClient.GetDatabase("testDB");
_collection = _database.GetCollection<BsonDocument>("Collection");

var doc = BsonDocument.Create(dummy);
_collection.InsertOneAsync(doc);

System.ArgumentException : .NET type DummyClass cannot be mapped to BsonType.Document.
Parameter name: value

{
    {"Id", "2"},
    {"First", "1"},
}

BsonClassMap.RegisterClassMap<DummyClass>();

_collection = _database.GetCollection<DummyClass>("Collection");
await _collection.InsertOneAsync(doc);

_collection.InsertOneAsync(doc.ToBsonDocument())