public class Base
        {
            public string A { get; set; }
        }

        public class T1 : Base
        {
            public string B { get; set; }
        }

        public class T2 : Base
        {
            public string C { get; set; }
        }

        [Test]
        public void CastTest()
        {
            var server = MongoServer.Create("...");
            var db = server.GetDatabase("test");
            var collection = db.GetCollection<Base>("castTest");

            var t1 = new T1 { A = "T1.A", B = "T1.B" };
            var t2 = new T2 { A = "T2.A" };
            collection.Insert(t1);
            collection.Insert(t2);

            var query = from t in collection.AsQueryable() where t is T1 && ((T1)t).B == "T1.B" select t;
            var results = query.ToList();
            Assert.That(results.Count, Is.EqualTo(1));
            Assert.That(results[0], Is.InstanceOf(typeof(T1)));
            Assert.That(results[0].A, Is.EqualTo("T1.A"));
        }