I added a click event in the submit button to display a confirmation message:
$("#btnSubmit").click(function () {
alert("An event is clicked");
});
That broke all client-side validations of ASP.Net Core MVC, however, the server side validations is still working. You cannot submit an event with invalid data, but it won't display the errors messages.
I tried to add the javascript code in the submit event in form instead of the click event of the event, that is not working too. Even I added another invisible submit button, after the confirmation message is displayed, then the code will trigger the click event of that invisible submit button. That is still not working.
I randomly placed the script tags. I found the script file which contains the click event needs to place after the validation scripts:
<script src="~/lib/jquery-validation/dist/jquery.validate.js"></script><script src="~/lib/jquery-validation-unobtrusive/jquery.validate.unobtrusive.js"></script>
<script src="~/js/site.js" asp-append-version="true"></script>

I did the ASP.Net Core MVC web app in Azure. I found all DateTime values are displayed in US format. Even I set in Startup.cs set DefaultRequestCulture to be en-au. It does not work. Finally, I used to use DisplayFormat attribute in The object.

Code

I started to use ASP.Net Core in our web development. The first challenge I faced is about the nested View Model.
I have not used much in nested view model in the previous of MVC,
I got the View Models like this:

Code

If I wish to do the binding in the razor html

You have to add the bind in the controller

Code

mc_code_odd">
[Bind("FirstName,LastName")]
public Customer{
   public string FirstName {get;set;}
   public string LastName {get;set;}
}

That is impossible to detect screen size in the Xamarin Form. You have to do in the native Android code.l suggest to do in MainActivity and then put in the static variable in the Forms PCL project like this:
Android.Util.DisplayMetrics displayMetrics = new Android.Util.DisplayMetrics();
this.WindowManager.DefaultDisplay.GetMetrics(displayMetrics);
App.Height=(int)(displayMetrics.HeightPixels/displayMetrics.Density);
Also, the unit of HeightRequest is device-independent units. But I found if you use HeightPixel divide by Density of the screen, then it will convert into the unit for HeightRequest or WidthRequest

If you want to use RequestLocationUpdates method in LocationManager, you need to add a class which implements ILocationListener to receive the callback from LocationManager. Most of the examples on the internet used the Activity as ILocationListener. That is easy if you choose the implementation in this way. But I wish to build a self-contained class which does not inherit any UI class.
locationManager.RequestLocationUpdates(LocationProviders.GPS, 10, 10, this);
Firstly, I just implemented all methods from the interface. Then I got an invalid casting exception. It is because LocationManager required the class is a sub-class of Java.lang.Object. In the native Android SDK, all classes are sub-class of Java.lang.Object, but we are using Xamarin, that is .Net, not Java. Thus we need to inherit our class to be a Java Object. After I make my class to be a sub-class of Java.lang.The object, I got an exception about the invalid handle. That is because ILocationListener has a property
public IntPtr Handle

I built a property to get from Application Context
Application.Context.Handle

Actually, Handle Property is in the Java.lang.Object. I do not need to build my own, I need to remove the section. Then it works.