public abstract class Entity : IEntity
{
 [BsonId]
 [BsonIgnoreIfDefault]
 [BsonRepresentation(BsonType.ObjectId)]
 public string Id { get; set; }
 [BsonIgnore]
 public abstract string CollectionName { get; }
}
[BsonDiscriminator(Required = true, RootClass = true)]
[BsonKnownTypes(typeof(LifeAccount), typeof(SimAccount))]
public class Account2 : Entity
{
 public string Name { get; set; }
 public string UserId { get; set; }
 public DateTime CreatedAt { get; set; }
 public DateTime? LastLoginAt { get; set; }
 public override string CollectionName { get; } = "Accounts";
}
public class LifeAccount : Account2
{
 public string ConsumerKey { get; set; }
 public string ConsumerSecret { get; set; }
}

public class SimAccount : Account2
{
 public decimal TransactionFee { get; set; }
 public decimal NetAssets { get; set; }
}

private readonly IMongoCollection<Account2> _accountCollection;
...
var projectDef = Builders<LifeAccount>.Projection
 .Include(a => a.Name)
 .Include(a => a.ConsumerKey)
 .Include(a => a.ConsumerSecret);

var account = await _accountCollection.OfType<LifeAccount>().Find(a => a.Id == accountId)
 .Project<LifeAccount>(projectDef)
 .SingleOrDefaultAsync();