how to manage _id field when using POCO with mongodb c# driver

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.