WPF FormattedText "The system cannot find the file specified" exception in a service

Question

We are using the WPF FormattedText object to determine text size in a service that grabs the latest news headlines from an RSS feed. The text retrieved needs to be in a specified canvas size. The service runs the code every 10 seconds and uses up to 2 threads if one takes longer than that. I'm using TaskFactory (which I've overridden the LimitedConcurrencyLevelTaskScheduler to limit to the amount of threads I specified).

This works great, except after several days (the length is variable), we start to get the following exceptions. The same code was working fine before we started using TPL to make it mult-threaded.

I need help figuring out what this is caused by. A few thoughts I'm looking into are: thread collisions holding on to a TTF file, memory issue, the dispatcher (see the stack trace) isn't playing nicely with the TaskFactory, other?? We don't have good profiling setup, but we've looked at the TaskManager when the exception is occurring and memory usage looks normal. My next attempt is to use the TextBlock object and see if the exception is avoided.

Error Message: The system cannot find the file specified Error Source: WindowsBase Error Target Site: UInt16 RegisterClassEx(WNDCLASSEX_D)

Exception Stack Trace:

at MS.Win32.UnsafeNativeMethods.RegisterClassEx(WNDCLASSEX_D wc_d) at MS.Win32.HwndWrapper..ctor(Int32 classStyle, Int32 style, Int32 exStyle, Int32 x, Int32 y, Int32 width, Int32 height, String name, IntPtr parent, HwndWrapperHook[] hooks) at System.Windows.Threading.Dispatcher..ctor() at System.Windows.Threading.Dispatcher.get_CurrentDispatcher() at System.Windows.Media.TextFormatting.TextFormatter.FromCurrentDispatcher(TextFormattingMode textFormattingMode) at System.Windows.Media.FormattedText.LineEnumerator..ctor(FormattedText text) at System.Windows.Media.FormattedText.DrawAndCalculateMetrics(DrawingContext dc, Point drawingOffset, Boolean getBlackBoxMetrics) at System.Windows.Media.FormattedText.get_Metrics() at (my method using the FormattedText, which is in a loop)

   private static Size GetTextSize(string txt, Typeface tf, int size)
    {
        FormattedText ft = new FormattedText(txt, new CultureInfo("en-us"), System.Windows.FlowDirection.LeftToRight, tf, (double)size, System.Windows.Media.Brushes.Black, null, TextFormattingMode.Display);
        return new Size { Width = ft.WidthIncludingTrailingWhitespace, Height = ft.Height };
    }

EDIT: so far I've tried placing a lock around the code that calls this function, and calling it inside the CurrentDispatcher.Invoke method like so:

 return (Size)Dispatcher.CurrentDispatcher.Invoke(new Func<Size>(() =>
    {
      FormattedText ft = new FormattedText(txt, new CultureInfo("en-us"), System.Windows.FlowDirection.LeftToRight, tf, (double)size, System.Windows.Media.Brushes.Black, null, TextFormattingMode.Display);
       return new Size { Width = ft.WidthIncludingTrailingWhitespace, Height = ft.Height };
      }));

EDIT: I've found links to others having similar, but not the exact problem. http://www.eggheadcafe.com/software/aspnet/31783898/problem-creating-an-bitmapsource-from-an-hbitmap-in-threaded-code.aspx ~having a similar problem, but no answers

System.Windows.Media.DrawingVisual.RenderOpen() erroring after a time ~having a similar problem, but no answers

http://connect.microsoft.com/VisualStudio/feedback/details/361469/net-3-5-sp1-breaks-use-of-wpf-under-iis# ~ similar exception, but we're not using 3.5SP1 or IIS 7.

I've also submitted this through the Microsoft Connect site (please vote for it if you are having a similar problem). https://connect.microsoft.com/WPF/feedback/details/654208/wpf-formattedtext-the-system-cannot-find-the-file-specified-exception-in-a-service

EDIT: Response from Microsoft: "WPF objects need to be created on Dispatcher threads, not thread-pool threads. We usually recommend dedicating a thread to run the dispatcher loop to service requests to create objects and return them frozen. Thanks, WPF Team" ~ How would I implement this?

EDIT: final solution thanks to NightDweller

if(Application.Current == null) new Application();
(Size)Application.Current.Dispatcher.CurrentDispatcher.Invoke(new Func<Size>(() =>
        {
...});

EDIT: When I deployed the change (new Application();), I got an error logged " Cannot create more than one System.Windows.Application instance in the same AppDomain." Error Source: PresentationFramework Error Target Site: Void .ctor()

Answer 1

As you already know from the info you provided, All UI elements (FormattedText is one) have to be created on the UI thread.

The code you are looking for is:

return (Size)Application.Current.Dispatcher.CurrentDispatcher.Invoke(new Func<Size>(() =>
    {
      FormattedText ft = new FormattedText(txt, new CultureInfo("en-us"), System.Windows.FlowDirection.LeftToRight, tf, (double)size, System.Windows.Media.Brushes.Black, null, TextFormattingMode.Display);
       return new Size { Width = ft.WidthIncludingTrailingWhitespace, Height = ft.Height };
      }));

Notice the Application.Current - you want the "Application" dispatcher which is the dispatcher for the UI thread in WPF applications. Your current code actually creates a dispatcher for the current thread so you didn't really change the executing thread (see here regarding the dispatcher)