WPF implicit DataTemplate declared in App.xaml won't take effect - wpf

Question

In the MainWindow.xaml, I set:
<Window.DataContext>
<vm:MainViewModel/>
</Window.DataContext>
In the App.xaml file, I added the following:
<Application.Resources>
<DataTemplate DataType="vm:MainViewModel">
<v:MainView/>
</DataTemplate>
</Application.Resources>
I was hoping the MainWindow will automatically load and show the MainView with its DataContext property set to the windows's one (which was set to MainViewModel at design-time as above), but it won't work - the MainWindow doesn't use the DataTemplate set in App.xaml.
Any better ideas for this scenario?

Answer 1

You should make a minor changes -
First, in your window. Try this:
<Window>
<!-- setup window... -->
<ContentPresenter>
<ContentPresenter.Content>
<vm:MainViewModel/>
</ContentPresenter.Content>
</ContentPresenter>
</Window>
This creates a single content item within your Window. DataTemplates work by mapping content to a new View - in this case, since the Content here is the MainViewModel, it will automatically create and instantiate a new MainView for you. Setting the DataContext will not trigger DataTemplates, since you're never making the ViewModel "content" of an object.
You can shorten this by just setting the Window's Content directly, if you prefer:
<Window>
<Window.Content>
<vm:MainViewModel/>
</Window.Content>
</Window>
Or, even, binding the Content to the DataContext (though this only makes sense if you need the DataContext set for some other purpose):
<Window Content="{Binding}">
<Window.DataContext>
<vm:MainViewModel/>
</Window.DataContext>
</Window>

Answer 2

I think you need
<DataTemplate DataType="{x:Type vm:MainViewModel}">
EDIT:
I really don't think I'm wrong, the code
<Window.DataContext>
<WpfApplication1:ViewModel />
</Window.DataContext>
<Window.Resources>
<DataTemplate DataType="{x:Type WpfApplication1:ViewModel}">
<TextBlock>Custom template</TextBlock>
</DataTemplate>
</Window.Resources>
<ContentPresenter Content="{Binding}" />
shows “Custom template”. If I remove the x:Type, what's shown instead is “WpfApplication1.ViewModel”, which is the result of calling ToString() on the view model object. This is used in the absence of a DataTemplate.