Another .NET Blog

In this article, I'll show yet another use of PostSharp, this time focusing on the ConfigurationSection class. If you look in the MSDN at the articles which explain how to use the ConfigurationSection, you may have noticed that the code you need to write is repetitive, tedious, and error-prone:

public class PageAppearanceSection : ConfigurationSection
{
	[ConfigurationProperty("remoteOnly", DefaultValue = "false", IsRequired = false)]
	public Boolean RemoteOnly
	{
		get { return (Boolean)this["remoteOnly"]; }
		set	{ this["remoteOnly"] = value; }
	}

	[ConfigurationProperty("font")]
	public FontElement Font
	{
		get	{ return (FontElement)this["font"]; }
		set	{ this["font"] = value; }
	}

	[ConfigurationProperty("color")]
	public ColorElement Color
	{
		get	{ return (ColorElement)this["color"]; }
		set	{ this["color"] = value; }
	}
}

As you can see, you have to put the ConfigurationPropertyAttribute on top of each property, and to make calls to an indexer to the Get and Set methods of the properties, using the same name you used in the attribute.

Doesn't this repetition look like to be a good candidate for an aspect?

One problem of the INotifyPropertyChanged aspect concerns automatic notification of read-only properties which depend on other properties.

Indeed, say you have this class, on which the aspect is applied:

[NotifyPropertyChanged]
public class Test
{
    public string FirstName { get; set; }
    public string LastName { get; set; }

    public string FullName { get { return FirstName + " " + LastName; } }
}

If you change the values of FirstName or LastName, the OnPropertyChanged event will be fired, and the view will be able to display the new values of the properties. But if a control of your view is bound to the FullName property, it won't be updated. A solution would be to declare FullName as an automatic property, and update it in the setters of FirstName and LastName. Not very practical.

In this article, I will show you how to extend the aspect, in order to make it fire the event for dependent properties when a "parent" property is modified.

Here, I won't explain what are Caliburn and PostSharp. Their respective documentations, examples and samples already do that very well. But I will show you how to create an aspect which will be applied by PostSharp on the view models of a WPF assembly.

And as a result, on runtime, each time a public property will be changed, the event PropertyChanged of the interface INotifyPropertyChanged will be fired, and the WPF view will be able to refresh its state.

If you already know PostSharp, or have been on their website, you have certainly noticed that they already propose an aspect which intends to implement the interface INotifyPropertyChanged, and to call PropertyChanged when the value of a property is changed.

And if you already know Caliburn, you know that the classes you must inherit from to implement your view models inherit themselves from an abstract class named PropertyChangedBase, which implements INotifyPropertyChanged, and which proposes a function named NotifyOfPropertyChange, to which you give as a parameter the name of the modified property.

The aspect I'm going to show you will allow us to mix these 2 methods.