FireClient不起作用并返回到Xamarin中有任何错误的类？

Question

this is login function but when compiler compile the line where "value" mean its return to Xamarin form without proceeding further Thank You

    async public  Task<User> GetLogin(User info)
    {            
        User login = new User();       
        var firebase = new FirebaseClient("https://xyz.firebaseio.com/");
        try
        {
            var value = await firebase.Child("User").OrderByValue().OnceAsync<User>();            
            foreach (var item in value)
            {
            if (info.Username == item.Object.Username && info.Password == item.Object.Password )
            {
                login.Id = item.Object.Id;
                login.Fullname = item.Object.Fullname;
                login.Username = item.Object.Username;
                login.Password = item.Object.Password;
                login.Role = item.Object.Role;
                MasterPage session = new MasterPage(login); 
                return login;
                }
                    else { }
                }
        }
        catch (Exception ex) { }
        return null;
    }