Prism and Simple Injector - mvvm

Question

I'm trying a simple HelloWorld using simple inject and prism.
Git Source
When application starts, this error come up
Failed to assign to property
'Prism.Windows.Mvvm.ViewModelLocator.AutoWireViewModel'. [Line: 8
Position: 5]"
Exception thrown: 'System.MissingMethodException' in Prism.Windows.dll
Exception thrown: 'Windows.UI.Xaml.Markup.XamlParseException' in
HelloWorldPrism.exe WinRT information: Failed to assign to property
'Prism.Windows.Mvvm.ViewModelLocator.AutoWireViewModel'. [Line: 8
Position: 5] An exception of type
'Windows.UI.Xaml.Markup.XamlParseException' occurred in
HelloWorldPrism.exe but was not handled in user code WinRT
information: Failed to assign to property
'Prism.Windows.Mvvm.ViewModelLocator.AutoWireViewModel'. [Line: 8
Position: 5] Additional information: The text associated with this
error code could not be found. Failed to assign to property
'Prism.Windows.Mvvm.ViewModelLocator.AutoWireViewModel'. [Line: 8
Position: 5]
e.StackTrace " at Windows.UI.Xaml.Application.LoadComponent(Object
component, Uri resourceLocator, ComponentResourceLocation
componentResourceLocation)\r\n at
HelloWorldPrism.Views.MainView.InitializeComponent()\r\n at
HelloWorldPrism.Views.MainView..ctor()" string
<Page
x:Class="HelloWorldPrism.Views.MainView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:mvvm="using:Prism.Windows.Mvvm"
mvvm:ViewModelLocator.AutoWireViewModel="True"
mc:Ignorable="d"
>
public MainViewModel(INavigationService navigationService)
{
_navigationService = navigationService;
}
If I add a parameterless constructor it works normal.
public MainViewModel()
{
}
App.cs
protected override Task OnLaunchApplicationAsync(LaunchActivatedEventArgs args)
{
Window.Current.Activate();
return Task.FromResult(true);
}
protected override void CreateAndConfigureContainer()
{
Logger.Log("Creating and Configuring Container", Category.Debug, Priority.Low);
Container = CreateContainer();
}
protected override Container CreateContainer()
{
return new Container();
}
protected override UIElement CreateShell(Frame rootFrame)
{
var shell = Container.GetInstance<MainView>();
shell.SetFrame(rootFrame);
return shell;
}
protected override Type GetPageType(string pageToken)
{
var type = Type.GetType(string.Format(CultureInfo.InvariantCulture, GetType().AssemblyQualifiedName.Replace(GetType().FullName, GetType().Namespace + ".Views.{0}View"), pageToken));
if (type != null)
return type;
throw new ArgumentException(string.Format(CultureInfo.InvariantCulture, ResourceLoader.GetForCurrentView("/Prism.Windows/Resources/").GetString("DefaultPageTypeLookupErrorMessage"), pageToken, GetType().Namespace + ".Views"), nameof(pageToken));
}
protected override Task OnInitializeAsync(IActivatedEventArgs args)
{
Container.RegisterSingleton(SessionStateService);
Container.RegisterSingleton(DeviceGestureService);
Container.RegisterSingleton(NavigationService);
Container.RegisterSingleton(EventAggregator);
return Task.CompletedTask;
}
protected override void ConfigureViewModelLocator()
{
ServiceLocator.SetLocatorProvider(() => new
SimpleInjectorServiceLocatorAdapter(Container));
}

Answer 1

When application starts, this error come up
In the MainView.xaml, you defined the AutoWireViewModel property to true. Once this property is set to True, the ViewModelLocator will try to instantiate the corresponding ViewModel based on a particular convention. Since the names of your View and ViewModel met the convention, when you set this property to true, Prism will help you instantiate the corresponding ViewModel.
Inside Prism.mvvm namespace, the ViewModelLocationProvider class locates the view model for the view that has the AutoWireViewModelChangedattached property set to true. And the error is thrown by the following code line of ViewModelLocationProvider class:
/// <summary>
/// The default view model factory which provides the ViewModel type as a parameter.
/// </summary>
static Func<Type, object> _defaultViewModelFactory = type => Activator.CreateInstance(type);
System.MissingMethodException: 'No parameterless constructor defined for this object.'
So it is caused by the Activator.CreateInstance(Type) method requires a public constructor, please see MissingMethodException.
If I add a parameterless constructor it works normal.
It seems like this is the correct solution. If you just don't want a parameterless constructor for the ViewModel, you may try instantiate it and set the DataContext to the View by yourself. If you doubt this is an issue for Prism library maybe you can open a thread here.
Update:
According to #rubStackOverflow, it was missing ViewModelLocationProvider.SetDefaultViewModelFactory((viewMo‌​delType) => Container.GetInstance(viewModelType)); on OnInitializeAsync method.