ILoggable

A place to keep my thoughts on programming

July 8, 2007 .net , , , , ,

INotifyPropertyChanged and Cross-thread exceptions

I’m currently reading Adam Nathan‘s WPF Unleashed. It’s a good read and has me ready for doing some serious WPF work. However, the things I have on my plate are a already Windows Forms and can’t really take a late in the project UI architecture change, so my production use of WPF will have to wait a little longer. I originally thought WPF was horribly over-engineered, but going through Adam’s book, the decisions that led to the sometimes odd APIs make sense. It’s clear that toolability of Xaml, separation of logic and code and allowing the greatest amount of design without any code are what led to the cumbersome manual C# syntax. But once you mix in Expression Blend, you won’t be doing any of those nasty things unless you’re writing your own custom controls. Ok, this is a lovely tangent, but hardly the subject of this entry…

While reading I came across INotifyPropertyChanged. Since it’s meant for binding in general, I thought this might be a perfect fit for a design issue I was having on a WinForms project. Basically, I have an object that manages a native process and can get manipulated via remoting. The UI in the program really just exists for status updates on this management object. The solution for keeping the UI in sync was either a timer that polled the object’s properties or create an event for each property change, manually subscribe to them in the form and update the UI that way. However, INotifyPropertyChanged provided a simple methodology for tracking changes in all objects and let the implementing object be used for data-binding. I’m usually against code that uses strings to reference object members, like data-binding is wont to do, since it falls outside of compile-time checking and is liable to get out of sync in refactoring, but data-binding is convenient and so prevalent in .NET, that I decided to use it for this.

Implementing INotifyPropertyChanged on my object was dead simple and the syntax for binding my labels to property changes was even easier. Perfect, another process simplified. Or so I thought. What happened next is what I personally would describe as a bug or at least mis-feature in data binding, but at least according to the docs, it’s by design.

INotifyPropertyChangeddoes not work for asynchronous operations

What happened was this: I made changes to my object via the remoting proxy and my client UI died with an InvalidOperationException(Cross-thread operation not valid…). On it’s face this is straight forward: My object is on a background thread, being manipulated by remoting. My UI is on the UI thread, so tweaking the UI by the background thread is a no-no. This is confirmed by the MSDN documentation which says:

If your data source implements the INotifyPropertyChanged and you are performing asynchronous operations you should not make changes to the data source on a background thread. Instead, you should read the data on a background thread and merge the data into a list on the UI thread.

That to me is bad advice and a lazy way of saying that data binding doesn’t do its due diligence on binding operations. Why? Well, the purpose of something like INotifyPropertyChanged is Separation of Concerns, decoupling the observer of the property data from the observable object. But what the above says is that the observed object is responsible for knowing how it might be observed.

The logical thing is that your INotifyPropertyChanged object neither knows nor should care that it’s data may be used to update the UI on another thread. The party that’s responsible for that bit of house-keeping is the one that does the actual updating of the UI controls, i.e. data-binding. Data-binding is intimately aware of the UI object it’s tied to and it’s the one that isactually updating the UI in response to an incoming event. It seems logical and practical that it would do the necessary check on Control.InvokeRequired and perform said invoke. Instead telling you that your object better live on the UI thread is just not very good advice, imho.

Where does that leave us?

Well, even if the business object wanted to invoke on the UI thread, it couldn’t unless it was aware of the UI thread, and that once again violates the Separation of Concerns that should exist between business logic and UI. So that leaves us with two options: a) change the way binding works to the way described above or b) put a proxy between the binding and the INotifyPropertyChanged implementor.

Since you can assign a new BindingContext to a control, a) may be an option, and it’s one I’m going to investigate next. However for the time being, going down the path of b) was simpler.

I hardcoded a proxy for my business object, had it implement INotifyPropertyChanged as well and created a factory that would take both the form and the proxied object to create a new proxy. The proxy then subscribed to the proxied PropertyChanged event and used its reference to the form to make sure it was invoked on the UI thread.

public class MyBusinessObject : INotifyPropertyChanged
{
  [...]
  public int MyProperty
  {
    get { [...] }
  }
  [...]
}

public class MyBusinessObjectProxy : INotifyPropertyChanged
{
  Control bindingControl;
  MyBusinessObject bindingSource;

  private MyBusinessObjectProxy(Control bindingControl, MyBusinessObject bindingSource)
  {
    this.bindingControl = bindingControl;
    this.bindingSource = bindingSource;
    bindingSource.PropertyChanged += new PropertyChangedEventHandler(bindingSource_PropertyChanged);
  }

  void bindingSource_PropertyChanged(object sender, PropertyChangedEventArgs e)
  {
    if (PropertyChanged != null)
    {
      if (bindingControl.InvokeRequired)
      {
        bindingControl.BeginInvoke(new PropertyChangedEventHandler(bindingSource_PropertyChanged), sender, e);
        return;
      }
      PropertyChanged(this, new PropertyChangedEventArgs(e.PropertyName));
    }
  }

  public int MyProperty
  {
    get { return bindingSource.MyProperty; }
  }
   
  #region INotifyPropertyChanged Members

  public event PropertyChangedEventHandler PropertyChanged;

  #endregion
}

That works beautifully, but since it’s specific to my particular object that’s a lot of hand-coding. This whole proxy/adapter scheme screams for a generic implementation. But now we need to intercept requests of properties we may not know exist. Back in the perl days (and most dynamic languages feature this as well) I would have just created an AUTOLOAD method that catches all calls to methods that don’t exist. It didn’t seem like this would exist in C#, since that violates the whole compile time checking of a static language like C#. But there is Reflection, so maybe I was wrong. However, after checking with Oren Eini, who clearly has a much greater understanding of how to do funky things with C#, it turns out that such a facility indeed does not exist.

Next up was creating a proxy object that could be cast to the original type and handle the binding calls that way. I built a prototype based on Castle‘s Dynamic Proxy, which proxied the properties just fine, but apparently the event subscription doesn’t work, because the proxy’s PropertyChanged event never acquired any subscribers. So until I can figure out what happens to events on Dynamic Proxy, I’m stuck with statically built proxies.

In the meantime, I guess I’m going to look at BindingContext to see if the problem can be solved generically at that end, while trying to figure out why Dynamic Proxy isn’t working. Stay tuned.

2 to “INotifyPropertyChanged and Cross-thread exceptions”

  1. Anonymous says...

    Nice How To to the AsyncOperationManager: http://www.codeproject.com/KB/cpp/SyncContextTutorial.aspx

  2. Anonymous says...

    "Data-binding is intimately aware of the UI object it's tied to and it's the one that isactually updating the UI in response to an incoming event. It seems logical and practical that it would do the necessary check on Control.InvokeRequired and perform said invoke."

    Amen!

Leave a comment