Using StaticResource as an argument for MarkupExtension causes NullReferenceException in WPF

Question

Even after simplifying the problem to minimal code, I still can't understand what can cause NullReferenceException.

Create new WPF Application "MarkupExtParam". Replace code:

MainWindow.xaml:

<Window x:Class="MarkupExtParam.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:my="clr-namespace:MarkupExtParam"
        Content="{Binding}">
    <Control.DataContext>
        <my:Foo/>
    </Control.DataContext>
    <Control.Resources>
        <my:Foo x:Key="foo"/>
        <DataTemplate DataType="{x:Type my:Foo}">
            <TextBlock Text="{my:Ext {StaticResource foo}}"/>
        </DataTemplate>
    </Control.Resources>
</Window>

MainWindow.xaml.cs:

using System;
using System.Windows.Markup;

namespace MarkupExtParam
{
    public partial class MainWindow
    {
        public MainWindow () { InitializeComponent(); }
    }

    [MarkupExtensionReturnType (typeof(string))]
    public class ExtExtension : MarkupExtension
    {
        [ConstructorArgument ("foo")]
        public Foo Foo { get; set; }
        public ExtExtension (Foo foo) { Foo = foo; }
        public override object ProvideValue (IServiceProvider provider)
        {
            return Foo.ToString();
        }
    }

    public class Foo { }
}

When I run the application, the following exception is thrown:

System.NullReferenceException was unhandled

Message=Object reference not set to an instance of an object.
Source=System.Xaml
StackTrace:
   at MS.Internal.Xaml.Runtime.ClrObjectRuntime.GetConverterInstance[TConverterBase](XamlValueConverter`1 converter)
   at MS.Internal.Xaml.Runtime.ClrObjectRuntime.CreateObjectWithTypeConverter(ServiceProviderContext serviceContext, XamlValueConverter`1 ts, Object value)
   at MS.Internal.Xaml.Runtime.ClrObjectRuntime.CreateFromValue(ServiceProviderContext serviceContext, XamlValueConverter`1 ts, Object value, XamlMember property)
   at System.Xaml.XamlObjectWriter.Logic_CreateFromValue(ObjectWriterContext ctx, XamlValueConverter`1 typeConverter, Object value, XamlMember property, String targetName, IAddLineInfo lineInfo)
   at System.Xaml.XamlObjectWriter.Logic_ConvertPositionalParamsToArgs(ObjectWriterContext ctx)
   at System.Xaml.XamlObjectWriter.WriteEndMember()
   at System.Xaml.XamlWriter.WriteNode(XamlReader reader)
   at System.Windows.FrameworkTemplate.LoadTemplateXaml(XamlReader templateReader, XamlObjectWriter currentWriter)
   at System.Windows.FrameworkTemplate.LoadTemplateXaml(XamlObjectWriter objectWriter)
   at System.Windows.FrameworkTemplate.LoadOptimizedTemplateContent(DependencyObject container, IComponentConnector componentConnector, IStyleConnector styleConnector, List`1 affectedChildren, UncommonField`1 templatedNonFeChildrenField)
   at System.Windows.FrameworkTemplate.LoadContent(DependencyObject container, List`1 affectedChildren)
   at System.Windows.StyleHelper.ApplyTemplateContent(UncommonField`1 dataField, DependencyObject container, FrameworkElementFactory templateRoot, Int32 lastChildIndex, HybridDictionary childIndexFromChildID, FrameworkTemplate frameworkTemplate)
   at System.Windows.FrameworkTemplate.ApplyTemplateContent(UncommonField`1 templateDataField, FrameworkElement container)
   at System.Windows.FrameworkElement.ApplyTemplate()
   at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
   at System.Windows.UIElement.Measure(Size availableSize)
   at System.Windows.ContextLayoutManager.UpdateLayout()
   at System.Windows.UIElement.UpdateLayout()
   at System.Windows.Interop.HwndSource.SetLayoutSize()
   at System.Windows.Interop.HwndSource.set_RootVisualInternal(Visual value)
   at System.Windows.Interop.HwndSource.set_RootVisual(Visual value)
   at System.Windows.Window.SetRootVisual()
   at System.Windows.Window.SetRootVisualAndUpdateSTC()
   at System.Windows.Window.SetupInitialState(Double requestedTop, Double requestedLeft, Double requestedWidth, Double requestedHeight)
   at System.Windows.Window.CreateSourceWindow(Boolean duringShow)
   at System.Windows.Window.CreateSourceWindowDuringShow()
   at System.Windows.Window.SafeCreateWindowDuringShow()
   at System.Windows.Window.ShowHelper(Object booleanBox)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.DispatcherOperation.InvokeImpl()
   at System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Windows.Threading.DispatcherOperation.Invoke()
   at System.Windows.Threading.Dispatcher.ProcessQueue()
   at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
   at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
   at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
   at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
   at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
   at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
   at System.Windows.Threading.Dispatcher.Run()
   at System.Windows.Application.RunDispatcher(Object ignore)
   at System.Windows.Application.RunInternal(Window window)
   at System.Windows.Application.Run(Window window)
   at System.Windows.Application.Run()
   at MarkupExtParam.App.Main() in d:\Docs\Projects\_Bugs\SubclassingBinding\obj\Debug\App.g.cs:line 0
   at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
   at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
   at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
   at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()

If I don't use DataTemplate and put <TextBlock Text="{my:Ext {StaticResource foo}}"/> outside, everything works fine.

I've tried debugging with .NET Refelector's generated PDBs (public sources don't seem to include any internal classes), but unsuccessfully, because Reflector can't generate proper PDBs for heavily optimized assemblies, so breakpoints don't work, most variables and arguments are unavailable etc. I understood that XAML parser tries to assign StaticResourceHolder (subclass of StaticResourceExtension) to Foo and fails (can't understand why it happens though).

Question: How to fix the problem? Maybe I'm just doing something wrong?

Answer 1

Problem solved by adding the following attribute to the Foo class:

[TypeConverter(typeof(TypeConverter))]

Apparently specifying a type converter becomes mandatory in this case. If you need a different type converter for the class, don't forget to call base methods (Convert*, CanConvert*). A way to apply TypeConverter attribute to a constructor argument doesn't seem to exist.