如何使用应用程序ResourceDictionary进行RuntimeType映射

Question

I'm trying to add a mapping from one (ViewModel) RuntimeType to another (View) RuntimeType in the Application's ResourceDictionary. This is so that my Controller class can look-up a ViewModel object class and bind it to a new instance of the appropriate View class. The application is implemented as a collection of plug-ins, which mean that the mapping is not known at compile time.

In my sandbox application (which I use for prototyping) the mapping is added to the main Window's resource dictionary as follows:

<Window.Resources>
    <!-- This template associates the ConfirmDialog type
         with the ConfirmDialogViewModel type. -->
    <x:Type TypeName="v:ConfirmDialog" x:Key="{x:Type vm:ConfirmDialogViewModel}" />
</Window.Resources>

This compiles and runs perfectly, with the DependencyProperty that uses the mapping to display the ConfirmDialog Window finding the correct class and instantiating it when the attached ViewModel changes.

However, when I attempt to put the same mapping into my Application's Resource dictionary an exception is thrown:

A first chance exception of type 'System.Xaml.XamlObjectWriterException' occurred in System.Xaml.dll

System.Windows.Markup.XamlParseException occurred
HResult=-2146233087 Message='Missing key value on 'RuntimeType' object.' Line number '20' and line position '14'.
Source=PresentationFramework LineNumber=20 LinePosition=14
StackTrace: at System.Windows.Markup.WpfXamlLoader.Load(XamlReader xamlReader, IXamlObjectWriterFactory writerFactory, Boolean skipJournaledProperties, Object rootObject, XamlObjectWriterSettings settings, Uri baseUri)
InnerException: System.Xaml.XamlObjectWriterException HResult=-2146233088 Message='Missing key value on 'RuntimeType' object.' Line number '20' and line position '14'. Source=System.Xaml

The resource is included as follows in the App.xaml:

<Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <!-- This resource file contains the DataTemplates. -->
            <ResourceDictionary Source="Resources/DataTemplates.Resources.xaml" />
            <!-- This resource file contains the Styles. -->
            <ResourceDictionary Source="Resources/Styles.Resources.xaml" />

            <!-- This section is used for mapping Views to ViewModels. -->
            <ResourceDictionary>
                <!-- This template associates the ConfirmDialog type 
                     with the ConfirmDialogViewModel type. -->
                <x:Type TypeName="v:ConfirmDialog" x:Key="{x:Type vm:ConfirmDialogViewModel}" />
            </ResourceDictionary>
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</Application.Resources>

Any suggestions regarding why the Window and Application ResourceDictionary objects behave differently, how I can find more information from the exception, or things I might try to resolve it would be appreciated.

Answer 1

My solution is to add a new ResourceDictionary XAML file to the project and include that as a MergedDictionary.

Mappings.Resources.xaml (namespaces changed):

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:v="clr-namespace:MyWpfViewNamespace"
    xmlns:vm="clr-namespace:MyViewModelNamespace;assembly=MyViewModelAssembly"
    >

    <!-- This template associates the ConfirmDialog type 
        with the ConfirmDialogViewModel type. -->
    <x:Type TypeName="v:ConfirmDialog" x:Key="{x:Type vm:ConfirmDialogViewModel}" />

</ResourceDictionary>

App.xaml (extract):

<Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <!-- This resource file contains the DataTemplates. -->
            <ResourceDictionary Source="Resources/DataTemplates.Resources.xaml" />
            <!-- This resource file contains the Styles. -->
            <ResourceDictionary Source="Resources/Styles.Resources.xaml" />
            <!-- This resource file contains the RuntimeType mappings. -->
            <ResourceDictionary Source="Resources/Mappings.Resources.xaml" />
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</Application.Resources>

And (in case anyone is interested) the instantiation is done in the PropertyChanged event handler of a DependencyProperty that binds an object that exposes the interface to a top-level ViewModel base class.

In the Mappings.Resources.xaml mapped types:

• ConfirmDialogViewModel implements the ITopLevelViewModel interface.
• ConfirmDialog extends the BaseWpfWindow View base class.

ShowModalViewProperty.cs (extract):

public static void ShowModalViewPropertyChanged(
    DependencyObject d,
    DependencyPropertyChangedEventArgs e)
{
    var window = d as Window;
    ITopLevelViewModel newValue = e.NewValue as ITopLevelViewModel;
    if ((null != window) && (null != newValue))
    {
        // Search the local and Application resources for a mapping
        // between the ViewModel type of the new property and a
        // View type to use to display it.
        Type typeOfViewModel = newValue.GetType();
        Type typeOfView = (Type)window.TryFindResource(typeOfViewModel);
        if (null == typeOfView)
        {
            typeOfView = (Type)Application.Current.TryFindResource(typeOfViewModel);
        }

        // If a concrete type of the correct class is available...
        if ((null != typeOfView)&&
            (!typeOfView.IsAbstract)&&
            (typeOfView.IsSubclassOf(typeof(BaseWpfWindow))))
        {
            // Create a new window and show it as a (modal) dialog.
            BaseWpfWindow dialogWindow = (BaseWpfWindow)
                Activator.CreateInstance(typeOfView);
            if (null != dialogWindow)
            {
                dialogWindow.Owner = window;
                dialogWindow.DataContext = newValue;

                // ModalResult is a Property of ITopLevelViewModel, used to return
                // the Window.DialogResult back to the ViewModel object.
                newValue.ModalResult = dialogWindow.ShowDialog();
            }
        }
    }
}

I still don't know why the seperate MergedDictionary method works.