Saturday, October 24, 2009

Improving type safety for INotifyPropertyChanged without expressions

Type safety is important. Isn’t it one of the reasons why you should use a static typed language like C#?  But if you’re using the INotifyPropertyChanged interface, you allready have left that type safety. Why? Because it expects you to pass on the propertyname. Easiest way to do this is just using a string parameter.

   1:  public event PropertyChangedEventHandler PropertyChanged;
   3:      private void NotifyPropertyChanged(String propertyName)
   4:      {
   5:          if (PropertyChanged != null)
   6:          {
   7:              PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
   8:          }
   9:      }

But what if you refactor your code and change the name of the property? The string value will remain the same and the compiler will not warn you. But your code will no longer work.

A solution could be the use of expression to pass on the propertyname in a type safe way.

   1:  protected void NotifyPropertyChanged(Expression<Func<T,object>> x)   
   2:  {   
   3:          var body = x.Body as MemberExpression;   
   4:          string propertyName =  body.Member.Name;   
   6:          if this.PropertyChanged;   != null)   
   7:          {     
   8:              this.PropertyChanged;   (this, new PropertyChangedEventArgs(propertyName));   
   9:          }   
  10:  }   

There is, however, an alternative that works on previous versions of .NET and doesn’t involve expression trees. It essentially involves creating a delegate of the target method, and using the delegate properties to get to the corresponding MethodInfo as mentioned on Daniel Cazzulino’s  blog. I didn’t tried it out yet, but as soon as I created an example, I’ll update this post.

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