WPF data context for design time and run time

Question

I'm learning WPF, MVVM Light and the ViewModelLocator pattern and running into difficulties with my main window's data context.

public class ViewModelLocator
   {
      public ViewModelLocator()
      {
         var mainModel = new MainModel();
         Main = new MainViewModel(mainModel);
      }

      public MainViewModel Main { get; private set; }

      public static ViewModelLocator Instance
      {
         get { return Application.Current.Resources["Locator"] as ViewModelLocator; }
      }
   }

and in my app.xaml:

<Application.Resources>
    <viewModels:ViewModelLocator x:Key="Locator" />
</Application.Resources>

When I set the data context in my main window using:

DataContext="Binding Main, Source={StaticResource Locator}"

it compiles but all of MainViewModel's properies I bind to elsewhere in the xaml show up red with tooltip "cannot resolve symbol". I thought I could get around this by also specifying a designer-only data context:

<Window x:Class="WPFDemo.Windows.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:converters="clr-namespace:WPFDemo.Converters"
    xmlns:local="clr-namespace:WPFDemo"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:models="clr-namespace:WPFDemo.Models"
    xmlns:viewModels="clr-namespace:WPFDemo.ViewModels"
    Title="MainWindow" Height="350" Width="525"
    DataContext="Binding Main, Source={StaticResource Locator}"
    mc:Ignorable="d"
    d:DataContext="{d:DesignInstance, Type=viewModels:MainViewModel,
    IsDesignTimeCreatable=True}">

but the compiler doesn't like that last line ("The character ',' is unexpected at this position", referrring to the first comma). Note I'm not using ExpressionBlend, but I thought I heard in a course that this line would enable VisualStudio designer as well:

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

How do I use a ViewModelLocator while still enabling Visual Studio to recognize bound properties at design time?

Answer 1

Using the default data context should also work in design time:

DataContext="{Binding Main, Source={StaticResource Locator}}"

If not, try to compile the proyect and check out again. You can manage the properties values that you want to show in design time by using the IsInDesignMode property that the MvvmLight Toolkit provides. By default the MainViewModel's constructor looks like this:

    /// <summary>
    /// Initializes a new instance of the MainViewModel class.
    /// </summary>
    public MainViewModel()
    {
        if (IsInDesignMode)
        {
            // Code runs in Blend --> create design time data.
        }
        else
        {
            // Code runs "for real"
        }
    }

Hope this helps...

Answer 2

The last line will work nicely if you remove the comma after d:DesignInstance:

d:DataContext="{d:DesignInstance Type=viewModels:MainViewModel,
IsDesignTimeCreatable=True}">