Question

If I want to read and write mongo data with a POCO

public class Thingy
{
     public string Foo {get;set;}
}
...
coll.Insert(new Thing(Foo = "hello"));

When I read back I get a failure saying that _id is an unexpected attribute (which it is). So then I added a field called _id to the class. Now the insert doesnt work saying that the _id field cannot be null. A tried BsonIgnoreIfNull attribute, that didnt work.

Answer 1

When you insert an object, if it doesn’t have an _id field then the driver adds one and sets it to a 12-byte MongoDB ObjectId value.

You just need to add an Id property to your POCO, which will be deserialised from _id:

public class Thingy
{
     public ObjectId Id { get; set; }
}

Or, if you’d like to delegate another property to map onto _id then you can decorate it with the BsonIdAttribute, like this:

[BsonId]
public ObjectId MyKey { get; set; }   

The _id field doesn’t have to be an MongoDB ObjectId, you can set it to any value of any data type (except an array), it just needs to be unique within the collection.