Question

I can set the relationship between View Model and view through following DataContext syntax:

 <UserControl.DataContext>
    <view_model:MainMenuModel />
</UserControl.DataContext>

And I can also set the relationship between View Model and view through following DataTemplate syntax:

    <DataTemplate
        DataType="{x:Type viewModel:UserViewModel}">
        <view:UserView />
    </DataTemplate>

What is the difference between the two? Is the second XAML not setting the data context of a view?

Answer 1

Your second XAML defines a template that can be used to display an object of type viewModel:UserViewModel. It doesn’t set data for anything but, if a ContentPresenter is asked to display an object of that type, it will use your template.

Your first XAML is setting the DataContext property of your control. It defines that any bindings you perform within that scope will use the DataContext as the root of the binding (unless explicitly overriden). For a simple example of DataContext at work, compare these two (both assume that “s” is the System namespace):

<StackPanel>  
  <TextBlock Text="{Binding Day, Source={x:Static s:DateTime.Now}}" />
  <TextBlock Text="{Binding Month, Source={x:Static s:DateTime.Now}}" />
  <TextBlock Text="{Binding Year, Source={x:Static s:DateTime.Now}}" />
</StackPanel>

<StackPanel DataContext="{Binding Source={x:Static s:DateTime.Now}}">  
  <TextBlock Text="{Binding Day}" />
  <TextBlock Text="{Binding Month}" />
  <TextBlock Text="{Binding Year}" />
</StackPanel>

Both StackPanels will render identically, but the second is more easily reused. (E.g.: you only have to change the binding in one place if you wanted to display a different date.)