Show an DisplayAlert at App.cs (Xamarin)
DisplayAlert is a method of the page class. However your app has a 'MainPage' property. So as long as the main page is set (should always be so after it's set during startup) you can use
Application.Current.MainPage.DisplayAlert
or from within App.cs
MainPage.DisplayAlert
So you can do this, is working for me
Device.BeginInvokeOnMainThread(async () =>
{
await Application.Current.MainPage.DisplayAlert("No Internet Connection", "Please connect to Internet", "OK");
});