public class B
{
    public ObjectId Id;
}

public class C : B
{
}

var server = MongoServer.Create("mongodb://localhost/?safe=true");
var database = server.GetDatabase("test");
var collection = database.GetCollection<B>("test");

collection.Drop();
collection.Insert(new B());
collection.Insert(new C());
collection.Insert(new C());

Console.WriteLine(collection.AsQueryable<B>().OfType<C>().Count()); // should write 2

System.InvalidCastException: Unable to cast object of type 'MongoDB.Driver.Linq.Projector`2[ConsoleApplication1.B,ConsoleApplication1.C]' to type 'MongoDB.Drive
r.MongoCursor'.
   at MongoDB.Driver.Linq.SelectQuery.<TranslateCount>b__2(IEnumerable source) in C:\work\rstam\mongo-csharp-driver\Driver\Linq\Translators\SelectQuery.cs:line 416
   at MongoDB.Driver.Linq.SelectQuery.Execute() in C:\work\rstam\mongo-csharp-driver\Driver\Linq\Translators\SelectQuery.cs:line 210
   at MongoDB.Driver.Linq.MongoQueryProvider.Execute(Expression expression) in C:\work\rstam\mongo-csharp-driver\Driver\Linq\MongoQueryProvider.cs:line 155
   at MongoDB.Driver.Linq.MongoQueryProvider.Execute[TResult](Expression expression) in C:\work\rstam\mongo-csharp-driver\Driver\Linq\MongoQueryProvider.cs:line 131
   at System.Linq.Queryable.Count[TSource](IQueryable`1 source)
   at ConsoleApplication1.Program.Main(String[] _args) in C:\Users\Robert Stam\documents\visual studio 2010\Projects\ConsoleApplication1\ConsoleApplication1\Program.cs:line 47