ViewModel always returns null when trying to access it from code-behind

Question

I am still learning Xamarin.Forms, so please bear with me.

I am developing a control, that let's you take pictures with your camera, displays them in a FlowListView and then ultimately I want to take these pictures, turn them into E-Mail attachments and send them off.

While the Gallery part of the control is working and the E-Mail part was already written by somebody else, for some reason I can't access the control's ViewModel from my E-Mail Code-Behind.

I wrote the control in MVVM, the E-Mail part is written in xaml with code-behind.

Whenever I try to access the ImagePickerViewModel to get the contents of my AttachmentList, I get returned null.

I have already tried not initializing the ImagePickerViewModel in the E-Mail code-behind, it still returns null.

What would be the correct way to do this?

ImagePickerViewModel.cs

 [XamlCompilation(XamlCompilationOptions.Compile)]
    public class ImagePickerViewModel : BindableObject, INotifyPropertyChanged
    {
        // HERE WOULD BE A TON OF DEPENDENCIES

        public ImagePickerViewModel()

        {
            // Commands
            TakePictureCmd = new Command(TakePicture);

            //ObservableCollection
            TakenPicturesList = new ObservableCollection();
            AttachmentList = new List();   
        }

        public event PropertyChangedEventHandler PropertyChanged;

        public Model.ImagePicker NewPicture { get; set; }

        public BaseObject attachment { get; set; }
        public List AttachmentList { get; set; }

        public ObservableCollection TakenPicturesList { get; set; }

        public string selectedImage { get; set; }
        public string Path
        {
            get { return NewPicture.ImagePath; }
            set { NewPicture.ImagePath = value; OnPropertyChanged(nameof(Path)); }
        }

        public Command TakePictureCmd { get; set; }


        private async void TakePicture()
        {
            var path = await _cameraService.TakePictureAsync();
            TakenPicturesList.Add(new Model.ImagePicker { ImagePath = path });
            GetFilesForAttachment(path);
        }


        private void GetFilesForAttachement(string filePath)
        {
            attachment = Attachment.FromFile(filePath);
            if (attachment != null)
            {
                AttachmentList.Add(attachment);
            }
        }

        public void RaisedOnPropertyChanged(string _PropertyName)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(_PropertyName));
        }
    }

ImagePickerView.xaml

EmailCreatePage.xaml.cs

 [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class EmailCreatePage: ContentPage
    {
        // HERE WOULD BE A TON OF DEPENDENCIES

        public EmailCreateViewModel ViewModel { get; }
        public ImagePickerViewModel ViewModelImagePicker {get;}

        public EmailCreatePage()
        {
            InitializeComponent();
            BindingContext = ViewModel = new EmailCreateViewModel();
            ViewModelImagePicker = new ImagePickerViewModel();
        }
private void CreateAttachment()
        {
 
                var attachmentList = GetFileList();
                foreach (var file in attachmentList)
                {
                    var attachment = Attachment.FromFile(file.ToString());
                    if (attachment != null)
                    {

                        ViewModel.CubesMessage.ConnectedObjects.Add(attachment);
                    }
            }

public List GetFileList()
        {
// GET LIST FROM OTHER VIEWMODEL HERE
// VIEWMODEL IS ALWAYS NULL
          var attachmentList = ImagePickerViewModel.AttachmentList;
          return attachmentList;
}

// HERE WOULD BE THE CODE THAT SENDS THE EMAIL OFF AFTER A BUTTON CLICK

Jessie Zhang -MSFT · Accepted Answer

From the following code you posted , we find that you want to access variable AttachmentList from ImagePickerViewModel. But here, you just create a new instance of class ImagePickerViewModel, so the attachmentList will always be null.

   ViewModelImagePicker = new ImagePickerViewModel();

Besides, you also use code ImagePickerViewModel.AttachmentList; to access variable AttachmentList in ImagePickerViewModel, that's confusing. Here, you can remove code ViewModelImagePicker = new ImagePickerViewModel();.

public List GetFileList()
        {
// GET LIST FROM OTHER VIEWMODEL HERE
// VIEWMODEL IS ALWAYS NULL
          var attachmentList = ImagePickerViewModel.AttachmentList;
          return attachmentList;
}

In fact, if you want to get the value of variable AttachmentList in class ImagePickerViewModel from other page, you can define a global static varible AttachmentList.

Just as follows:

   public stactic List AttachmentList { get; set; }

Note: Make sure AttachmentList in class ImagePickerViewModel could been assigned values correctly.

ViewModel always returns null when trying to access it from code-behind

Answers (1)

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