How to upsert a derived class in C#

Question

I have a base class called Entry:

[BsonDiscriminator(RootClass = true)]
[BsonKnownTypes(typeof(AccountEntry))]
public class Entry
{
    public ObjectId Id { get; set; }
    public string Register { get; set; }
}

With a derived class called AccountEntry as example

public class AccountEntry : Entry
{
    public Account Account { get; set; }
}

When I try to do an Update, a compile time error shows that says, AccountEntry cannot be converted to Entry

var filter = Builders<AccountEntry>.Filter.Where(x => x.Id = id);            
await context.Entries.UpdateOneAsync(filter, new AccountEntry()
{                                                
    Account = debitAccount,                
}, new UpdateOptions() { IsUpsert = true });

Inserting the derived class does not cause the same problem.

Answer 1

Your filter needs to be of type Entry, not AccountEntry.

var filter = Builders<Entry>.Filter.Where(x => x.Id == id);

This is because filter is not covariant (I think that's the right one). So, the compiler can't just use a FilterDefinitionBuilder in place of a FilterDefinitionBuilder.

We have a ticket open to better handle derived types: https://jira.mongodb.org/browse/CSHARP-1194.