My ObservableCollection<T> in a view model file cannot be seen by other files

Question

I have a view model class in a folder called ViewModel. The file is called PgViewMode.cs and looks like this:

using CommunityToolkit.Mvvm.ComponentModel;
using LockAndKeyMaui.Models;
using CommunityToolkit.Mvvm.Input;
using System.Collections.ObjectModel;
using SQLite;
using System.ComponentModel.DataAnnotations;
using CommunityToolkit.Maui.Storage;
using Mopups.Services;

namespace LockAndKeyMaui.ViewModel
{
    public partial class PgViewModel : ObservableValidator
    {
        public ObservableCollection<Groups> Grps { get; set; } = new ObservableCollection<Groups>();

        public PgViewModel()
        {
            Grps = new ObservableCollection<Groups>();
        }
     }
}

In one of my view files, I have the XAML looking like this:

<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:sf="clr-namespace:Syncfusion.Maui.DataGrid;assembly=Syncfusion.Maui.DataGrid"
             xmlns:viewmodel="clr-namespace:LockAndKeyMaui.ViewModel"
             x:DataType="viewmodel:PgViewModel"
             x:Class="LockAndKeyMaui.View.PasswdInfo"
             BackgroundColor="DarkGreen">

And in the code-behind of this view file the constructor looks like this:

public PasswdInfo()
{
    laks = new LakFuncs();
    InitializeComponent();
    BindingContext = new PgViewModel();
}

Now at one point in the code-behind of the view, I have:

Grps.Clear();
foreach (var grp in grps)
{
    Grps.Add(new Groups() { GrpName = grp.GrpName });
}

grplist.ItemsSource = Grps;
grplist.IsVisible = (Grps.Count > 0);

And this is where I get the error message: "The name 'Grps' does not exist in the current context." Grps is the definition of the ObservableCollection<Groups> in the view model file. I realize it is a different file name, but shouldn't it be recognized in some way?

"And this is where I get the errors" is not so helpful in understanding the problem. Please post the error message. — Shameel
– Shameel, Commented May 20, 2024 at 3:27
You should read How to Ask and then give us a minimal reproducible example. — Enigmativity
– Enigmativity, Commented May 20, 2024 at 4:28
Grpsis not a property of the view but of the viewmodel. The Bindingcontext is implicitly used from within XAML, but not from the code behind. So you should replace (e.g.) Grps.Clear(); with BindingContext.Grps.Clear(); — Johan Donne
– Johan Donne, Commented May 20, 2024 at 5:02
@JohanDonne The second part of your comment is wrong. It's not possible to directly access Grps from the BindingContext like that. To do this, a cast is required, e.g. : (BindingContext as PgViewModel).Grps.Clear();. However, I would discourage to do so and use a reference to the ViewModel instead. — Julian
– Julian, Commented May 20, 2024 at 7:52
@Julian My bad, forgot about the cast, Thanks for the correction. — Johan Donne
– Johan Donne, Commented May 20, 2024 at 13:04

Julian · Accepted Answer · 2024-05-20 09:43:42Z

1

You need to store a reference to your PgViewModel in the code-behind of your PasswdInfo page:

public partial class PasswdInfo : ContentPage
{
    private PgViewModel _vm;

    public PasswdInfo()
    {
          InitializeComponent();
          BindingContext = _vm = new PgViewModel();
    }

    //...
}

Then you can access any of the public properties directly:

_vm.Grps.Clear();
foreach (var grp in grps)
{
    _vm.Grps.Add(new Groups() { GrpName = grp.GrpName });
}

grplist.ItemsSource = _vm.Grps;
grplist.IsVisible = _vm.Grps.Count > 0;

This is necessary, because the ViewModel is not in any way directly related to the View. If you always update everything from within the code-behind, you might not even need to set the BindingContext, but that depends on whether you use any binding expressions in your XAML.

edited May 20, 2024 at 9:43

answered May 20, 2024 at 7:47

Julian

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