Xamarin.Forms: Application.Current.MainPage.DisplayAlert() throws System.NullReferenceException

I’m using Application.Current.MainPage.DisplayAlert from a separate class and I’m getting this error:

System.NullReferenceException: ‘Object reference not set to an instance of an object.’

App.xaml.cs

public partial class App : Application
{
    public App()
    {
        InitializeComponent();

        MainPage = new NavigationPage(new MainPage());
    }
}

MainPage.cs

public partial class MainPage : ContentPage
{
    private static readonly DisplayMessage DisplayMessage = new();

    public MainPage()
    {
        InitializeComponent();

        DisplayMessage.Display();
    }
}

DisplayMessage.cs

public class DisplayMessage
{
    public async void Display()
    {
        await Application.Current.MainPage.DisplayAlert("Hello World", "Welcome", "OK");
    }
}

I found this article proposing this solution but it doesn’t help

public static Page RootPage;

public App()
{
    InitializeComponent();
    
    MainPage = new MainPage();
    
    App.RootPage = this.MainPage;
}
    
App.RootPage.DisplayAlert(title, message, ok);

Does someone have a direction to find out ? Thanks a lot.

>Solution :

The problem is that you’re trying to access the MainPage instance before it has finished construction.

Instead of doing this:

public partial class MainPage : ContentPage
{
    private static readonly DisplayMessage DisplayMessage = new();

    public MainPage()
    {
        InitializeComponent();

        DisplayMessage.Display();
    }
}

You can do the following, which should work:

public partial class MainPage : ContentPage
{
    private static readonly DisplayMessage DisplayMessage = new();

    public MainPage()
    {
        InitializeComponent();
    }

    public void ShowMessage()
    {
        DisplayMessage.Display();
    }
}

Then, after construction finished, the ShowMessage() method can be called, which in turn will display the message.

public partial class App : Application
{
    public App()
    {
        InitializeComponent();

        MainPage = new MainPage();

        MainPage.ShowMessage();
    }
}

Now, the overall design of your code is debatable, but making these changes should lead to a usable solution.

Update:

You may also want to add a null-check to the following piece of code:

public class DisplayMessage
{
    public async void Display()
    {
        //note the "?"
        await Application.Current.MainPage?.DisplayAlert("Hello World", "Welcome", "OK");
    }
}

Note the ? which serves to ensure that DisplayAlert() only gets called when MainPage is set to an instance.