Question

I have a strongly typed View class that all my UserControls derive from. It looks more or less like this:

public class View<TContext> : UserControl 
{

    /// <summary>
        /// Gets or sets a value indicating whether to auto create the data context type.
        /// </summary>
    public static DependencyProperty AutoCreateDataContextProperty = DependencyProperty.Register("AutoCreateDataContext", typeof(bool), typeof(View<TContext>), new PropertyMetadata(false));
    /// <summary>
    /// Gets or sets a value indicating whether to auto create the data context type.
    /// </summary>
    /// <value>
    ///     <c>true</c> if [auto resolve data context]; otherwise, <c>false</c>.
    /// </value>
    public bool AutoCreateDataContext
    {
        get { return (bool)GetValue(AutoCreateDataContextProperty); }
        set { SetValue(AutoCreateDataContextProperty, value); }
    }

    /// <summary>
    /// Gets or sets the view model.
    /// </summary>
    /// <value>
    /// The view model.
    /// </value>
    public new TContext DataContext
    {
        get
        {
            if (AutoCreateDataContext && !DesignerProperties.GetIsInDesignMode(new ContentControl()))
            {
                base.DataContext = ServiceProvider.Current.GetService<TContext>();
            }
            return (TContext)base.DataContext;
        }
        set { base.DataContext = value; }
    }
}

The bit about AutoCreateDataContext is new…and is the source of my question. Adding this to the View<TContext> base class hasn’t caused any problems itself…but once I set the value to true in one of my derived Views:

<s:View x:TypeArguments="local:PersonSearchViewModel"
    x:Class="PersonSearchView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" ...
             Height="600" Width="800" Background="White" AutoCreateDataContext="True">

InitializeComponent for this view throws the following exception:

System.NullReferenceException occurred
  Message=Object reference not set to an instance of an object.
  Source=PresentationFramework
  StackTrace:
       at System.Windows.Markup.WpfXamlLoader.TransformNodes(XamlReader xamlReader, XamlObjectWriter xamlWriter, Boolean onlyLoadOneNode, Boolean skipJournaledProperties, Boolean shouldPassLineNumberInfo, IXamlLineInfo xamlLineInfo, IXamlLineInfoConsumer xamlLineInfoConsumer, XamlContextStack`1 stack, IStyleConnector styleConnector)
       at System.Windows.Markup.WpfXamlLoader.Load(XamlReader xamlReader, IXamlObjectWriterFactory writerFactory, Boolean skipJournaledProperties, Object rootObject, XamlObjectWriterSettings settings, Uri baseUri)
       at System.Windows.Markup.WpfXamlLoader.LoadBaml(XamlReader xamlReader, Boolean skipJournaledProperties, Object rootObject, XamlAccessLevel accessLevel, Uri baseUri)
       at System.Windows.Markup.XamlReader.LoadBaml(Stream stream, ParserContext parserContext, Object parent, Boolean closeStream)
       at System.Windows.Application.LoadComponent(Object component, Uri resourceLocator)
       at ....

As soon as I remove the AutoCreateDataContext=True from the markup, it works fine again. There is no inner exception or further exception detail. How can I debug/resolve this?

Answer 1

I did some guessing and then disassembling and found it to be a bug in how WPF handles DependencyProperties declared on generic DependencyObjects (like my View<T>).

Made an abstract non-generic base class (called View, which View<T> now inherits from) and declared my DependencyProperties there instead. Problem solved.

I guess I’ve gotten used to just how poor Microsoft quality is…so I’ve actually started recognizing trends in bugs like this.