How do you increment a field in mongodb using c# - mongodb

Question

Thought this would be pretty straight forward, but my value is remaining the same (0).
What I'd like to do is increment my UnreadMessages field when the user receives a message they haven't read and then decrement it when they have. So I thought code like this would work:
var userHelper = new MongoHelper<User>();
//increment
userHelper.Collection.Update(Query.EQ("Id", userId.ToId()), Update.Inc("UnreadMessages", 1));
//decrement
userHelper.Collection.Update(Query.EQ("Id", userId.ToId()), Update.Inc("UnreadMessages", -1));
After running these no errors are thrown but the value doesn't change either. And no I'm not running one after the other as the code above could be interpreted :)
Update
Here's my helper class:
public class MongoHelper<T> : Sandbox.Services.IMongoHelper<T> where T : class
{
public MongoCollection<T> Collection { get; private set; }
public MongoHelper()
{
var con = new MongoConnectionStringBuilder(ConfigurationManager.ConnectionStrings["MongoDB"].ConnectionString);
var server = MongoServer.Create(con);
var db = server.GetDatabase(con.DatabaseName);
Collection = db.GetCollection<T>(typeof(T).Name.ToLower());
}
}
and thanks to Travis' answer I was able to pull this off:
MongoHelper<UserDocument> userHelper = new MongoHelper<UserDocument>();
var user = userHelper.Collection.FindAndModify(Query.EQ("Username", "a"), SortBy.Null, Update.Inc("MessageCount", 1), true).GetModifiedDocumentAs<UserDocument>();

Answer 1

Not sure what your helper does. Here is a working snippet I use:
var query = Query.And(Query.EQ("_id", keyName));
var sortBy = SortBy.Null;
var update = Update.Inc("KeyValue", adjustmentAmount);
var result = collection.FindAndModify(query, sortBy, update, true);
So, "query" finds the document, update does the increment, and FindAndModify puts them together and actually hits the database.