public void signIn(string userName, string userPass)
        {
            User user = new User();
            user.getUser(userName , userPass);

            if (user.userName != null)
            {
                Response.Redirect("/Home/Menu");
            }
            else
            {
                Response.Redirect("/Index/Index?Fail=" + "fail");
            }
        }

public class User
{
    public ObjectId _id { get; set; }
    public string userName { get; set; }
    public string userPass { get; set; }

    public User getUser(string name , string pass)
    {
        var client = new MongoClient("mongodb://localhost:27017");
        var db = client.GetDatabase("testdb");
        var coll = db.GetCollection<User>("user");

        List<User> list = coll.Find(x => x.userName == name && x.userPass == pass).ToList<User>();


        User uObj = new User();
        uObj = list.FirstOrDefault();

        return uObj;
    }
}