Generic Implementation of INotifyPropertyChanged on ADO.NET Data Services (Astoria) Proxies with T4 Code Generation

Last Week Mike Flasko from the ADO.NET Data Services (Astoria) Team blogged about what’s coming in V1.5 which will ship prior to VS 2010. I applaud these out of band releases.

One of the new features is support for two-way data binding in the client library generated proxy classes. These classes currently do not implement INotifyPropertyChanged events nor project into ObservableCollections out of the box.

Last week at the MVP Summit I had the chance to see a demo of this and other great things coming down the road from the broader Data Programmability Team. It seems like more and more teams are turning to T4 Templates for code generation which is great for our extensibility purposes. At first I was hopeful that the team had implemented these proxy generation changes via changing to T4 templates along with a corresponding “better” template.  Unfortunately, this is not the case and we won’t see any T4 templates in v1.5. It’s too bad – would it really have been that much more work to invest the time in implementing T4 templates than to add new switches to datasvcutil and new code generation (along with testing that code).

Anyway, after seeing some other great uses of T4 templates coming from product teams for VS 2010, I thought I would invest some of my own time to see if I couldn’t come up with a way of implementing INotifyPropertyChanged all on my own. The problem with the existing code gen is that while there are partial methods created and called for each property setter (i.e. FoobarChanged() ), there is no generic event fired that would allow us to in turn raise a InotifyPropertyChanged.PropertyChanged event. So you can manually added this for each and every property on every class – but it’s tedious.

I couldn’t have been the first person to think of doing this, and after a bit of googling, I confirmed that. Alexey Zakharov’s post on generating custom proxies with T4 has been completely ripped off, er, inspirational in this derivative work. What I didn’t like about Alexy’s solution was that it completely over wrote the proxy client. I would have preferred a solution that just implemented the partial methods in a partial class to fire the PropertyChanged event. This way, any changes, improvements, etc. to the core MS codegen can still be expected down the road. Of course, Alexey’s template is a better solution if there are indeed other things that you want to customize about the template in its entirely should you find that what you need to accomplish can’t be done with a partial class.

What I did like about Alexey’s solution is that it uses the service itself to query the service meta data directly. I had planned on using reflection to accomplish the same thing but in hindsight, that would be difficult to generate a partial class of a class I’m currently reflecting on in the same project (of course). Duh.

So what do you need to do to get this solution working?

1. Add the MetadataHelper.tt file to the project where you have your reference/proxies to the data service. You will want to make sure there is no custom tool associated with this file – it’s just included as a reference in the next one. This file wraps up all the calls to get the meta data I’ve made a couple of small changes to Alexey’s -- Added support for Byte and Boolean (typo in AZ’s).
2. Copy the DataServiceProxy.tt file to the same project. If you have more than one data service, you’ll want one of these files for each reference. So for starters you may want to rename it accordingly. You are going to need to edit this bad boy as well.
3. There are two options you’ll need to specify inside of the proxy template. The MetadataUri should be the uri to your service suffixed with $metadata. I’ve found that if your service is secured with integrated authentication, then the the metadata helper won’t pass those credentials along so for the purposes of code generation you’d best leave anonymous access on. Secondly is the Namespace. You will want to use the same namespace used by your service reference. You might have to do a Show All Files and drill into the Reference.cs file to see exactly what that is. 
   

var options = new {
    MetadataUri = "http://localhost/ObjectSharpSample.Service/SampleDataService.svc/$metadata",
    Namespace = "ObjectSharp.SampleApplication.ServiceClient.DataServiceReference"
    };

That’s it. When you save your file, should everything work, you’ll have a .cs file generate that implements through a partial class an INotifyProxyChanged interface. Something like…..

public partial class Address : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    private void OnPropertyChanged(string property)
    {
        var handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(property));
        }
    }

    partial void OnAddressIdChanged()
    {
        OnPropertyChanged("AddressId");
    }
    partial void OnAddressLine1Changed()
    {
        OnPropertyChanged("AddressLine1");
    }
}