Styjun

I've created my MVVM Light app with Windows Template Studio and now i'm trying to use databindings in my XAML. But this does not seem possible to me with the generated code because it does not declare the Viewmodel in the XAML. I changed the code to do this but then i stumbeled across this error:

XAML ViewModelLocator type cannot be constructed. In order to be constructed in XAML, a type cannot be abstract, interface, nested, generic or a struct, and must have a public default constructor

I've tried making the constructor public but that does not solve the issue. Any idea's on how i can get databindings to work?

This is all the code:

App.XAML

<Application
x:Class="PatientApp.UWP.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:vm="using:PatientApp.UWP.ViewModels">

<Application.Resources>
 <ResourceDictionary>
 <ResourceDictionary.MergedDictionaries>
 <XamlControlsResources xmlns="using:Microsoft.UI.Xaml.Controls"/>
 <ResourceDictionary Source="/Styles/_Colors.xaml"/>
 <ResourceDictionary Source="/Styles/_FontSizes.xaml"/>
 <ResourceDictionary Source="/Styles/_Thickness.xaml"/>
 <ResourceDictionary Source="/Styles/TextBlock.xaml"/>
 <ResourceDictionary Source="/Styles/Page.xaml"/>
 </ResourceDictionary.MergedDictionaries>
 <vm:ViewModelLocator xmlns:vm="using:PatientApp.UWP.ViewModels" x:Key="Locator" />
 </ResourceDictionary>
</Application.Resources>

ViewModelLocator:

 [Windows.UI.Xaml.Data.Bindable]
public class ViewModelLocator

 private static ViewModelLocator _current;

 public static ViewModelLocator Current => _current ?? (_current = new ViewModelLocator());

 private ViewModelLocator()
 
 SimpleIoc.Default.Register(() => new NavigationServiceEx());
 SimpleIoc.Default.Register<ShellViewModel>();
 Register<PatientsViewModel, AllPatientsPage>();
 Register<DiseasesViewModel, AllDiseasesPage>();
 

 public DiseasesViewModel DiseasesViewModel => SimpleIoc.Default.GetInstance<DiseasesViewModel>();

 public PatientsViewModel PatientsViewModel => SimpleIoc.Default.GetInstance<PatientsViewModel>();

 public ShellViewModel ShellViewModel => SimpleIoc.Default.GetInstance<ShellViewModel>();

 public NavigationServiceEx NavigationService => SimpleIoc.Default.GetInstance<NavigationServiceEx>();

 public void Register<VM, V>()
 where VM : class
 
 SimpleIoc.Default.Register<VM>();

 NavigationService.Configure(typeof(VM).FullName, typeof(V));
 

PatientPage:

<Page
x:Class="PatientApp.UWP.Views.AllPatientsPage"
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"
Style="StaticResource PageStyle"
mc:Ignorable="d"
xmlns:datamodel="using:PatientApp.Model"
DataContext="Binding PatientViewModelInstance, Source=StaticResource Locator">
<Grid
 Background="ThemeResource SystemControlPageBackgroundChromeLowBrush">
 <Grid.RowDefinitions>
 <RowDefinition Height="Auto" />
 <RowDefinition Height="*" />
 </Grid.RowDefinitions>

 <CommandBar DefaultLabelPosition="Right"
 Grid.Row="0"
 OverflowButtonVisibility="Collapsed">

 <AppBarButton Icon="Sort" Label="Sort" />
 <AppBarButton Icon="Edit" Label="Edit" />
 <AppBarButton Icon="Add" Label="Add" />
 <AppBarButton Icon="Zoom" Label="Search" />
 </CommandBar>
 <GridView x:Name="Patients"
 Grid.Row="1"
 ItemsSource="Binding Patients">
 <GridView.ItemsPanel>
 <ItemsPanelTemplate>
 <ItemsStackPanel Margin="14,0,0,0" Orientation="Vertical" />
 </ItemsPanelTemplate>
 </GridView.ItemsPanel>
 <GridView.ItemTemplate>
 <DataTemplate x:DataType="datamodel:Patient">
 <TextBlock Text="x:Bind Name"
 FontWeight="Medium"
 TextWrapping="NoWrap"
 HorizontalAlignment="Left" />
 </DataTemplate>
 </GridView.ItemTemplate>
 </GridView>
</Grid>

XAML ViewModelLocator type cannot be constructed. In order to be constructed in XAML, a type cannot be abstract, interface, nested, generic or a struct, and must have a public default constructor

The reason is ViewModelLocator construct method is private in your scenario. So you could not instantiate it in the xaml. you could change it to public. Even that you can't use it. Because the ViewModelLocator instance in xaml is created by a constructor rather than a singleton method. In other words <vm:ViewModelLocator xmlns:vm="using:PatientApp.UWP.ViewModels" x:Key="Locator" /> not equal to ViewModelLocator.Current. Unfortunately, ActivationService used ViewModelLocator.Current to get NavigationService. So the app throw exception when start.

public static NavigationServiceEx NavigationService => ViewModelLocator.Current.NavigationService;

It is hard to approach with MVVMLight. In general, the page DataContext was defended in code behind. And with x:bind to binding data source.

private SettingsViewModel ViewModel

 get return ViewModelLocator.Current.SettingsViewModel;