Question

I have been working fine with MVVM Light on a silverlight 4 Windows Phone 7 project in Visual Studio 2010 and Blend 4 for sometime.
Then suddenly I start getting the “Cannot create an instance of “ViewModelLocator”” error in both VS 2010 and Blend. Can’t work out why it should appear now. Any ideas on how I can possibly track down if something somewhere has changed that I have not spotted.
The only change I made, and then put back, was to add a “d:” infront of the data context expression in one of the user controls. I had the idea of binding in design, but programmatically binding in a delayed manner at run time.

I see others have posted this problem, but the answers refer to a bug in Blend, which has apparently been fixed. Also this is occuring in VS2010 too.

App.xaml looks like

<Application x:Class="BillSplitter2.App"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:phone="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone"
         xmlns:shell="clr-namespace:Microsoft.Phone.Shell;assembly=Microsoft.Phone"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
         mc:Ignorable="d"
         xmlns:vm="clr-namespace:BillSplitter2.ViewModel"
         xmlns:converters="clr-namespace:HardMediumSoft.WP7.Tools.Converters;assembly=HardMediumSoft.WP7.Tools">


<Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="Resources/ResourceDictionary1.xaml"/>
        </ResourceDictionary.MergedDictionaries>
        <!--Global View Model Locator-->
        <vm:ViewModelLocator x:Key="Locator" d:IsDataSource="True"/>
        <!--Localization String Library-->
        <local:LocalizedStrings xmlns:local="clr-namespace:BillSplitter2.Utilities"
                                x:Key="LocalizedStrings" />
        <!--Converters -->
        <converters:FloatConverter x:Key="FloatConverter" />
        <converters:BoolToVisibilityConverter x:Key="BoolToVisibilityConverter" />
        <converters:StringToBrush x:Key="StringToBrushConverter" />
    </ResourceDictionary>
</Application.Resources>


<Application.ApplicationLifetimeObjects>
    <!--Required object that handles lifetime events for the application-->
    <shell:PhoneApplicationService Launching="Application_Launching"
                                   Closing="Application_Closing"
                                   Activated="Application_Activated"
                                   Deactivated="Application_Deactivated" />
</Application.ApplicationLifetimeObjects>

Answer 1

Thanks to the recommendation above, I started to investigate the constructor of my ViewModel. Although I had no errors, I did find that belnd had issues with event listeners and handlers.

I was using the

if (IsInDesignMode)
{
   //populate values here for blend
}
else
{
   //runtime initiation
}

to populate some values for design time. I would initiate values in the model by setting their properties. This was all fine until I started adding more complex event handling routines based on property changes.

To rectify this and recover my ‘blendability’ I did two things!

1. Set private fields rather than properties in the IsDesignMode section. This avoid PropertyChanged events firing.
2. Added IsInDesignMode detection in event handlers that remained a problem and skipped any cascded updating.

Hope this helps!