Xamarin.Android中的数据库

Question

Hello I have a ListView that shows users from an SQL-table. I made a drop down menu that appears when I click on the items of the list. My goal is to delete the item I clicked on, but im kinda stuck because I get some errors.

Here is my what I am trying to do:

I created a List<User> users = new List<Users>();

and I my OnCreate method I have this:

 lView = FindViewById<ListView>(Resource.Id.usersListView);
            string dpPath = Path.Combine(System.Environment.GetFolderPath(System.Environment.SpecialFolder.Personal), "user.db3");
            var db = new SQLiteConnection(dpPath);

            users = db.Table<User>().ToList();

            adapter = new UsersAdapter(this, users);
            lView.Adapter = adapter;

            lView.ItemClick += ShowOptions;  

Here is my also my code for the delete option from the popup menu

 string dpPath = Path.Combine(System.Environment.GetFolderPath(System.Environment.SpecialFolder.Personal), "user.db3"); //Call Database  
 var db = new SQLiteConnection(dpPath);

 TableQuery<User> data = db.Table<User>(); //Call Table  

  ** User userToDelete = data.Where(x => x.id == users[e.Position].id).FirstOrDefault(); //Linq Query 

  if(userToDelete != null)
  {
       db.Query<User>("DELETE FROM User Where username=?", userToDelete.username);
       adapter.NotifyDataSetChanged();

  }  

When I debug the app I get an Error on the line with **. It says System.NotSupportedException: Member access failed to compile expression
I would be really thankful if someone tells me what I'm missing or doing wrong.

UPDATE :

User

class User
{
    [PrimaryKey, AutoIncrement, Column("_Id")]
    public int id { get; set; } // AutoIncrement and set primarykey  

    [MaxLength(25)]
    public string username { get; set; }

    [MaxLength(15)]
    public string password { get; set; }

    public int age { get; set; }
    public string email { get; set; }

    public override string ToString()
    {
        return string.Format("Username: {0}, Pass: {1}, Age: {2}, email: {3}", username, password, age, email);
    }
}  

Adapter

class UsersAdapter : BaseAdapter<User>
{
    List<User> users;
    private Context context;

    public UsersAdapter(Context cont, List<User> list) : base()
    {
        users = list;
        context = cont;
    }

    public override long GetItemId(int position)
    {
        return position;
    }
    public override int Count
    {
        get { return users.Count; }
    }

    public override Java.Lang.Object GetItem(int position)
    {
        return position;
    }

    public override User this[int position]
    {
        get { return users[position]; }
    }

    public override View GetView(int position, View convertView, ViewGroup parent)
    {
        View view = convertView;

        if(view == null)
        {
            view = LayoutInflater.From(context).Inflate(Resource.Layout.UserView, null, false);
        }

        TextView user = view.FindViewById<TextView>(Resource.Id.usernameView);
        TextView pass = view.FindViewById<TextView>(Resource.Id.passView);
        TextView age = view.FindViewById<TextView>(Resource.Id.ageView);
        TextView email = view.FindViewById<TextView>(Resource.Id.emailView);
        TextView id = view.FindViewById<TextView>(Resource.Id.idView);

        id.Text = string.Format("id #{0}", users[position].id.ToString());
        user.Text = users[position].username;
        pass.Text = string.Format("Password: {0}", users[position].password);
        age.Text = string.Format("Age: {0}", users[position].age);
        email.Text = string.Format("Email: {0}", users[position].email);

        return view;
    }

}  

Click

private void ShowOptions(object sender, AdapterView.ItemClickEventArgs e)
{
     PopupMenu menu = new PopupMenu(this, lView.GetChildAt(e.Position));

            menu.MenuInflater.Inflate(Resource.Menu.popupmenu, menu.Menu);

            menu.MenuItemClick += (s, args) => 
            {
                //different functionality according to the pressed popup menu option
                switch (args.Item.ItemId)
                {
                    //delete user from sql table
                    case Resource.Id.deletePop:
                        Toast.MakeText(this, string.Format("You clicked Delete"), ToastLength.Short).Show();

                        string dpPath = Path.Combine(System.Environment.GetFolderPath(System.Environment.SpecialFolder.Personal), "user.db3"); //Call Database  
                        var db = new SQLiteConnection(dpPath);

                        TableQuery<User> data = db.Table<User>(); //Call Table  
                        //var data1 = data.Where(x => x.id == users[e.Position].id).FirstOrDefault(); //Linq Query 

                        User userToDelete = data.Where(x => x.id == users[e.Position].id).First(); //Linq Query 

                        if(userToDelete != null)
                        {
                            db.Query<User>("DELETE FROM User Where username=?", userToDelete.username);
                            adapter.NotifyDataSetChanged();

                        }
                        break;                        
                }
            };


            menu.Show();    }

Answer 1

I figured it out I had to change TableQuery<User> data = db.Table<User>(); to var data = db.Table<User>().ToList(); . Now it works fine. It seems that the TableQuery<User> was not compatible with List<User> and that's why I got the error.