Object is not set in ViewModel class and control is directly move from data access class to View

Question

I want to fetch first record from AboutUs table and display as a content label.

I have created 4 classes in MVVM pattern.

First is Model class AboutUs.cs

[Table("tblAboutUs")]
    public class AboutUs
    {
        [PrimaryKey, AutoIncrement, NotNull]
        public int IDP { get; set; }
        public string Content { get; set; }
    }

Second is Data Access class

SQLiteAboutUs.cs

public class SQLiteAboutUs
    {
        private static readonly AsyncLock Mutex = new AsyncLock();
        private SQLiteAsyncConnection dbConn;

        public int StatusCode { get; set; }

        public SQLiteAboutUs(ISQLitePlatform sqlitePlatform, string dbPath)
        {

            if (dbConn == null)
            {
                var connectionFunc = new Func<SQLiteConnectionWithLock>(() =>
                    new SQLiteConnectionWithLock
                    (
                        sqlitePlatform,
                        new SQLiteConnectionString(dbPath, storeDateTimeAsTicks: false)
                    ));

                dbConn = new SQLiteAsyncConnection(connectionFunc);
                dbConn.CreateTableAsync<Model.AboutUs>();
            }
        }

        public SQLiteAboutUs()
        {
        }
        public async Task Save(Model.AboutUs content)
        {
            using (await Mutex.LockAsync().ConfigureAwait(false))
            {
                StatusCode = 0;


            await dbConn.InsertAsync(new Model.AboutUs { Content = content.Content });
            StatusCode = 1;
        }

        //For Get first Row from Table
        public async Task<Model.AboutUs> GetAllData()
        {
            return await dbConn.Table<Model.AboutUs>().Where(x => x.IDP == 1).FirstOrDefaultAsync();
        }
    }

Third class ViewModel Class

AboutUsViewModel.cs

public class AboutUsViewModel
    {
        readonly SQLiteAboutUs _db;
        public string AboutUsContent { get; set; }

        //public string AboutUs;  
        public AboutUsViewModel()
        {
            _db = new SQLiteAboutUs();
        }
        public async void FirstRecord()
        {
            Model.AboutUs obj = await _db.GetAllData();
            this.AboutUsContent = obj.Content;
        }
    }

Forth one is Code behind file of my xaml pages.

AboutUs.xaml.cs

public partial class AboutUs : ContentPage
    {
        readonly AboutUsViewModel aboutUsViewModel;
        public AboutUs()
        {
            InitializeComponent();

            aboutUsViewModel = new AboutUsViewModel();

            aboutUsViewModel.FirstRecord();

            lblContent.Text = aboutUsViewModel.AboutUsContent;
        }
    }

I have debug code but problem is In AboutUsViewModel.cs class in FirstRecord Method object can not be set that's why AboutUsContent string property is also not set.

I can't figure out why my debugger directly jump from GetAllData() method in SQLiteAboutUs.cs to label.text in code behind file of view?

What is exactly being null here? Are you sure dbConn is not null? It happened to me many times. can you also try to assign a variable like this? var result= await dbConn.Table().Where(x => x.IDP == 1).FirstOrDefaultAsync(); with a try catch maybe, just to make sure if FirstOrDefaultAsync makes the problem or return type makes the problem? — Emil, Jul 25 '16 at 10:30

score 1 · Answer 1 · edited May 23 '17 at 12:14

Welcome in the wonderfull world of asynchronicity. I encourage you to read carefully about how await is working: How and When to use `async` and `await`

It is not a blocking call. Thus you create the view AboutUs. It creates the ViewModel AboutUsViewModel. It calls

aboutUsViewModel.FirstRecord();

But does not wait for the call to be complete (dont't forget you marked your FirstRecord function as async...)

So it calls

Model.AboutUs obj = await _db.GetAllData();

And directly return to the caller because of the await operator. That's why it directly jump to

lblContent.Text = aboutUsViewModel.AboutUsContent;

What you would like is Something like

await aboutUsViewModel.FirstRecord();

To wait the call to be complete before going to the next line. But of course you can't do that, because you are in a constructor and you can't have an async constructor. And calling a database (or actually anything that could likely failed) in a constructor is a bad practice anyway.

I would advise you to let only InitializeComponent() in the constructor, and then use Something like the OnDataLoaded() event of your view to perform your async call with a await.

Hope it helps.

Thank you for encourage me. Not I see the actual problem. I can't find OnDataLoaded() method in xamarin.forms. So I am using OnAppearing() method. — RMR, Jul 25 '16 at 09:20
but problem is when GetAllData Method call it gives me error "Object reference not set to the instance of the object". — RMR, Jul 25 '16 at 09:22
OnAppearing is probably to soon, you should look for another event later, or just call something like a initialize function by hand. — Ouarzy, Jul 25 '16 at 11:53

Object is not set in ViewModel class and control is directly move from data access class to View

1 Answers1