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:

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

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

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

    <StackPanel
        HorizontalAlignment="Left"
        VerticalAlignment="top"
        DockPanel.Dock="Top"
        Orientation="Horizontal">
        <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

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 :-)

First make sure that you have the following XML namespace defined in your root element:

xmlns:d="http://schemas.microsoft.com/expression/blend/2008"

Then you can add the following attribute to an element in your xaml:

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