Quite often you'll find that perfect code sample to fit a need, but it's in that other language. Generally, I can translate with no problems even if it takes some time. But occasionally I'll happen upon a use of .NET that I'm unfamiliar with, but the syntax difference between C# and VB are so different that I have no clue how to translate it.

This happened recently with a great sample app written by Tony Sneed of Developmentor which he used in his TechEd US Developer presentation, Real World LINQ to SQL.

Tony had written a helper class to customize collection behavior in a WCF Client. The class implements the ObservableCollection .NET class. The custom collection will be used generically, but can only be used for classes that implement particular interfaces. In other words, by being generic, it can be a collection of any type; but adding constraints you can define the collection to work with only certain types .

It was the "s" at the end of "interfaces" that stumped me.

Here's what the class declaration looks like in C#.

 public class ChangeTrackingCollection<T> : ObservableCollection<T>
        where T : ITrackable, INotifyPropertyChanged
    {} 

The collection inherits from ObservableCollection<T> and can only be built from classes which implement ITrackable (a custom interface) and INotifyPropertyChanged.

In VB, I'm familiar with applying constraints to generics. But only a single constraint.

Public Class ChangeTrackingCollection(Of T As INotifyPropertyChanged)
    Inherits ObservableCollection(Of T) 

It took me about 45 minutes of fiddling around, then reading online until I finally realized that the term I needed to focus on was "constraint" and then by searching for "multiple constraints" with generics, I easily found an example of how to force multiple constraints in VB right in the MSDN documentation.

The proper syntax is:

Public Class ChangeTrackingCollection(Of T As {INotifyPropertyChanged,ITrackable})
    Inherits ObservableCollection(Of T)

Somehow after all of this, I remembered that so many have recommended Tangible's Instant VB and C# applications for converting. So I downloaded the demo for InstantVB, fed it the C# above and yes, Instantly, it converted to the correct syntax in VB. That's way faster than 45 minutes.

I'm sold.

By the way, Charles Petzold just published an article on ObservableCollection in the September Issue of MSDN Magazine: Dependency Properties And Notifications