Implementing own ViewModelLocator

Question

I wanted to implement ViewModelLocator by my own. So I implemented the simplest app in the world. I did everything as in this tutorial. But I'm still getting an exception:

XamlParseException occured

Exception thrown: 'System.Windows.Markup.XamlParseException' in PresentationFramework.dll

Additional information: 'Provide value on 'System.Windows.StaticResourceExtension' threw an exception.' Line number '8' and line position '9'.

This is this line:

DataContext="{Binding MainWindowViewModel, Source={StaticResource ViewModelLocator}}">

Here is the code:

App.xaml

<Application x:Class="ViewModelLocatorDemo.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:viewModelLocatorDemo="clr-namespace:ViewModelLocatorDemo">
    <Application.Resources>
        <viewModelLocatorDemo:ViewModelLocator x:Key="ViewModelLocator"/>
    </Application.Resources>
</Application>

App.xaml.cs

namespace ViewModelLocatorDemo
{
    using System.Windows;
    using ViewModelLocatorDemo.Views;

    public partial class App : Application
    {
        protected override void OnStartup(StartupEventArgs e)
        {
            base.OnStartup(e);

            MainWindow mainWindow = new MainWindow();
            mainWindow.Show();
        }
    }
}

ViewModelLocator.cs

namespace ViewModelLocatorDemo
{
    using ViewModels;

    public class ViewModelLocator
    {
        public MainWindowViewModel MainWindowViewModel
        {
            get { return new MainWindowViewModel(); }
        }
    }
}

MainWindow.xaml

<Window x:Class="ViewModelLocatorDemo.Views.MainWindow"
        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"
        mc:Ignorable="d"
        Title="MainWindow" Height="300" Width="300"
        DataContext="{Binding MainWindowViewModel, Source={StaticResource ViewModelLocator}}">
    <Grid>
        <Frame x:Name="MainFrame" Margin="50" BorderThickness="2" BorderBrush="Black" />
    </Grid>
</Window>

MainWindowViewModel.cs

namespace ViewModelLocatorDemo.ViewModels
{
    public class MainWindowViewModel
    {
        public string MainText { get; set; }

        public MainWindowViewModel()
        {
            MainText = "The first page";
        }
    }
}

In this answer I found:

Make sure that the resources are defined before the usage (in Xaml parsing order). The easiest way is to place it into App.xaml

So I have it in App.xaml. If somebody would me explain what's going on here? Why am I getting this error?

score 5 · Accepted Answer · edited May 23 '17 at 12:07

You are running into this bug WPF - App.xaml file does not get parsed if my app does not set a StartupUri?

From that page:

There's a VS code generation bug where the code necessary to connect to the rest of the program sometimes is not inserted when contains only one entry and does not have a StartupUri attribute.

From that page, there are 3 solutions (summarizing here for completeness):

Add x:Name="App"
Add more resources in App.xaml like <viewModelLocatorDemo:ViewModelLocator x:Key="ViewModelLocator"/> and <viewModelLocatorDemo:ViewModelLocator x:Key="ViewModelLocator2"/>
Rather than overriding OnStartup, try using an event instead, Startup="Application_Startup"

This was definitely not obvious, and was difficult to troubleshoot and even find an answer for in my own search. Hopefully this answer will help others find the other answer.

Implementing own ViewModelLocator

1 Answers1