Xamarin Forms Shell Navigation Failure with System.NullReferenceException

Question

This is a sample application attempting the Xamarin Forms navigation capability. We have an event handler with this simple logic:

async void Button_Clicked(System.Object sender, System.EventArgs e)
{
    await Shell.Current.GoToAsync(nameof(SchoolList));
}

We also have the route registered in App.xaml.cs as follows:

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

            MainPage = new MainPage();
            Routing.RegisterRoute(nameof(SchoolList), typeof(SchoolList));
        }

However, when the corresponding button is clicked, we get the error below:

System.NullReferenceException: Object reference not set to an instance of an object.
  at cross_app1.MainPage.Button_Clicked (System.Object sender, System.EventArgs e) [0x0000f] in /Users/klaus.nji/Projects/cross-app1/cross-app1/MainPage.xaml.cs:18
  at System.Runtime.CompilerServices.AsyncMethodBuilderCore+<>c.<ThrowAsync>b__7_0 (System.Object state) [0x00000] in /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/AsyncMethodBuilder.cs:1021
  at Android.App.SyncContext+<>c__DisplayClass2_0.<Post>b__0 () [0x00000] in /Users/builder/azdo/_work/1/s/xamarin-android/src/Mono.Android/Android.App/SyncContext.cs:36
  at Java.Lang.Thread+RunnableImplementor.Run () [0x00008] in /Users/builder/azdo/_work/1/s/xamarin-android/src/Mono.Android/Java.Lang/Thread.cs:36
  at Java.Lang.IRunnableInvoker.n_Run (System.IntPtr jnienv, System.IntPtr native__this) [0x00008] in /Users/builder/azdo/_work/1/s/xamarin-android/src/Mono.Android/obj/Release/monoandroid10/android-30/mcw/Java.Lang.IRunnable.cs:84
  at at (wrapper dynamic-method) Android.Runtime.DynamicMethodNameCounter.39(intptr,intptr)

The system is Mac Catalina v10.15.7, IDE is Visual Studio 2019 for Mac and I have the Andriod SDK installed and able to see other aspects of the app.

It looks like you don’t have a Shell, so Shell.Current is null. What Shell example did you follow? MainPage = new MainPage(); means your app is pointing to a page of type MainPage. It should be pointing to a Shell. I would expect to see MainPage = new AppShell();. There may be other details missing, so be sure to follow a working example. — ToolmakerSteve
– ToolmakerSteve, Commented Apr 11, 2022 at 5:24

ToolmakerSteve · Accepted Answer · 2022-04-11 05:40:03Z

2

It looks like you don’t have a Shell, so Shell.Current is null. What Shell example did you follow?

MainPage = new MainPage(); means your app is pointing to a page of type MainPage. To use Shell (Route) navigation, it needs to be pointing to a Shell.

I would expect to see MainPage = new AppShell();.

There may be other details missing, so be sure to follow a working example, such as Xaminals.

The new AppShell line can be seen in https://github.com/xamarin/xamarin-forms-samples/blob/main/UserInterface/Xaminals/Xaminals/App.xaml.cs.

Note: Its possible to navigate in Xamarin Forms without having Shell, nor defining routes. (Personally, I dislike the Shell, so I don’t use it.) You can define MainPage as a NavigationPage (see its doc). Or you can simply set App.MainPage to different pages, to move between them, without a navigation stack.

edited Apr 11, 2022 at 5:40

answered Apr 11, 2022 at 5:25

ToolmakerSteve

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1 Comment

this.myself Over a year ago

Personally I prefer working with Prism library for navigation. Working on both types for a while already and Prism is more flexible and handy to me than Shell.

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