MongoDB C# Driver - how to store _id as ObjectId but map to string Id property?

Question

I'm having trouble getting my model to represent an entity's Id property as a string but have it auto-generated and represented internally by MongoDb as a native ObjectId.

class Account
{
    public string Id { get; set; }
    ...
}

class AccountStore
{
    static AccountStore()
    {
        BsonClassMap.RegisterClassMap<Account>(cm =>
        {
            cm.AutoMap();
            cm.SetIgnoreExtraElements(true);

            // map Id property here
        });
    }

    public void Save(Account account)
    {
        _accounts.Save(account);
    }
}

For the line // map Id property here in the above code, I've tried numerous different ways of configuring the Id mapping and none have worked. The ways I have tried, and the associated exceptions that are thrown when I call the Save method, are:

// Exception: No IdGenerator found.
cm.IdMemberMap
  .SetRepresentation(BsonType.ObjectId);

// Exception: No IdGenerator found.
cm.IdMemberMap
  .SetRepresentation(BsonType.String);

// Exception: Unable to cast object of type 'MongoDB.Bson.ObjectId' to type 'System.String'.
cm.IdMemberMap
  .SetRepresentation(BsonType.ObjectId)
  .SetIdGenerator(ObjectIdGenerator.Instance);

// Exception: Unable to cast object of type 'MongoDB.Bson.ObjectId' to type 'System.String'.
cm.IdMemberMap
  .SetRepresentation(BsonType.String)
  .SetIdGenerator(ObjectIdGenerator.Instance);

// Exception: Unable to cast object of type 'MongoDB.Bson.ObjectId' to type 'System.String'.
cm.IdMemberMap
  .SetIdGenerator(ObjectIdGenerator.Instance);

What am I doing wrong? I thought this was a standard use case for id handling?

Answer 1

This has changed, I'm using the latest 1.x driver (Nuget package <package id="mongocsharpdriver" version="2.0.0" targetFramework="net45" />) and instead of using SetRepresentation you set the serialiser.

public class RegistrationAttempt
{
    public string AttemptId { get; set; }
}

BsonClassMap.RegisterClassMap<RegistrationAttempt>(cm =>
{
    cm.AutoMap();
    cm.MapIdProperty(c => c.AttemptId)
        .SetIdGenerator(StringObjectIdGenerator.Instance)
        .SetSerializer(new StringSerializer(BsonType.ObjectId));
});

Answer 2

Found the answer:

cm.IdMemberMap
  .SetRepresentation(BsonType.ObjectId)
  .SetIdGenerator(StringObjectIdGenerator.Instance);

This allows me to save as a native ObjectId and still have the Id represented in C# as a string. As a small gotcha, the id must be parsed before being queried upon:

public Account GetAccountById(string id)
{
    return _accounts.FindOneById(ObjectId.Parse(id));
}

Edit May 2015
Apparently the driver has changed since I wrote this answer. The other answer above is correct for newer versions, but this answer can still be referred to if using a legacy version of the driver.