What is the advantage of setting DataContext in code instead of XAML?

Question

There seem to be two main ways to define DataContext in WPF:

• either in code like this:

App.xaml.cs (taken from the WPF MVVM Toolkit template):

public partial class App : Application
{
    private void OnStartup(object sender, StartupEventArgs e)
    {
        // Create the ViewModel and expose it using the View's DataContext
        MainView mainView = new MainView();
        MainViewModel mainViewModel = new MainViewModel();
        mainViewModel.LoadCustomers("c:\\testdata2\\Customers.xml");
        mainView.DataContext = mainViewModel;
        mainView.Show();
    }
}

• or in XAML like this:

Window1.xaml:

<Window.Resources>
    <ObjectDataProvider x:Key="FileNames" ObjectType="{x:Type local:OldLibrary}" MethodName="GetFileNames"/>
    <ObjectDataProvider x:Key="Directories" ObjectType="{x:Type local:OldLibrary}" MethodName="GetDirectories"/>
</Window.Resources>

<DockPanel>

    <StackPanel DockPanel.Dock="Top" HorizontalAlignment="Left" Orientation="Horizontal">
        <StackPanel.DataContext>
            <local:CustomerViewModel/>
        </StackPanel.DataContext>
        <TextBlock Text="{Binding Path=FirstName}"/>
        <TextBlock Text=" "/>
        <TextBlock Text="{Binding Path=LastName}"/>
    </StackPanel>

    <StackPanel DockPanel.Dock="Top" HorizontalAlignment="Left" Orientation="Horizontal" VerticalAlignment="top">
        <ListBox ItemsSource="{Binding Source={StaticResource FileNames}}"/>
    </StackPanel>

    <StackPanel DockPanel.Dock="Top" HorizontalAlignment="Left" Orientation="Horizontal" VerticalAlignment="top">
        <ComboBox ItemsSource="{Binding Source={StaticResource Directories}}" SelectedIndex="0"/>
    </StackPanel>

    <StackPanel DockPanel.Dock="Top" HorizontalAlignment="Left" Orientation="Horizontal" VerticalAlignment="top">
        <StackPanel.DataContext>
            <local:SystemInformationViewModel/>
        </StackPanel.DataContext>
        <TextBlock Text="{Binding Path=CurrentTime}"/>
    </StackPanel>

</DockPanel>

One advantage that defining the DataContext in XAML has is that your data shows up in Expression Blend design mode and Expression Blend allows you to do quite a lot within the GUI e.g. choose fields from your datasource, etc. as shown here.

I have read that binding ADO.NET objects cannot be bound in XAML (although I don't see why you could write a minimal wrapper for them to which you could bind from XAML).

Strange that the WPF Team in making the WPF MVVM templates define the DataContext in code which very quickly makes it impracticable to edit your Views in Expression Blend, since your data doesn't show up in design mode which is often a significant part of the layout.

So I'm thinking there must be some advantage down the road to setting the DataContext in code instead of XAML, anyone know what it is?

Answer 1

Having it in codebehind makes it easy to inject the datacontext using unity.

Answer 2

I don't like the idea of having Expression Blend try to instantiate my data objects.

I set the DataContext through code where I am able to use Dependency Injection to inject the proper objects, services, providers or what else I am using to find my code.

Answer 3

There could be a kind of solution to this, using the DataObjectProvider to mask the fact that the data is instantiated outside of XAML.

It will state what the type of the DataContext is, which should be enough for Blend to pick up the properties.

I have not tried this yet, so take it with a grain of salt, but it is certainly worth investigating.

Answer 4

See Rob's article about design time data in Blend: http://www.robfe.com/2009/08/design-time-data-in-expression-blend-3/

Answer 5

You can (maybe in 2009 you couldn't) get the best of both worlds by using the d:DataContext attribute. You don't need any of that ViewModelLocator craziness if you're not ready for that yet :-)

d:DataContext="{d:DesignInstance IsDesignTimeCreatable=True, Type=vm:CustomerInsightViewModel}"

In your xaml codebehind :

    public CustomerInsightUserControl()
    {
        InitializeComponent();

        if (!DesignerProperties.IsInDesignTool)
        {
            DataContext = new CustomerInsightViewModel();
        }
    }

Then in your ViewModel:

    public CustomerInsightViewModel()
    {
        if (IsInDesignMode)
        {
            // Create design time data
            Customer = new Customer() {
                FirstName=... 
            }
        }
        else {
            // Create datacontext and load customers
        }
    }

Don't miss the IsDesignTimeCreatable=True or else Blend won't instantiate your class