inotifypropertychanging.md

uid	title	product	categories	summary
inotifypropertychanging	Implementing INotifyPropertyChanging	postsharp	PostSharp;AOP;Metaprogramming	The document provides instructions on implementing the INotifyPropertyChanging interface with PostSharp, which signals before a property value changes, by creating an 'OnPropertyChanging' method.

Implementing INotifyPropertyChanging

By convention, the xref:System.ComponentModel.INotifyPropertyChanged.PropertyChanged event must be raised after the property value has changed. However, some components need to be signaled before the property value will be changed. This is the role of the xref:System.ComponentModel.INotifyPropertyChanging interface.

Because the xref:System.ComponentModel.INotifyPropertyChanging interface is not portable (in Xamarin, it is even a part of a different namespace), PostSharp cannot introduce it. However, if you implement the xref:System.ComponentModel.INotifyPropertyChanging interface yourself in your code, PostSharp will signal the xref:System.ComponentModel.INotifyPropertyChanging.PropertyChanging event. To make that work, you need to create an OnPropertyChanging method with the right signature.

To add the INotifyPropertyChanging interface to a class:

1. Make your class implement xref:System.ComponentModel.INotifyPropertyChanging and add the xref:System.ComponentModel.INotifyPropertyChanging.PropertyChanging event.

2. Add the OnPropertyChanging method with exactly the following signature, and invoke the xref:System.ComponentModel.INotifyPropertyChanging.PropertyChanging event.

   protected void OnPropertyChanging( string propertyName )
   {
       if ( this.PropertyChanging != null )
       {
         this.PropertyChanging( this, new PropertyChangingEventArgs ( propertyName ) );
       }
   }

3. Add the xref:PostSharp.Patterns.Model.NotifyPropertyChangedAttribute aspect to your class as described in xref:inotifypropertychanged.

Note

The contract between your class and the xref:PostSharp.Patterns.Model.NotifyPropertyChangedAttribute is only the OnPropertyChanging method. As long as this method exists in the class, it will be invoked by the aspect before the value of a property changes.

Example

The following example demonstrates how to implement xref:System.ComponentModel.INotifyPropertyChanging.

[NotifyPropertyChanged]                
public class MyClass : INotifyPropertyChanging
{
    public event PropertyChangingEventHandler PropertyChanging;
    
    protected void OnPropertyChanging( string propertyName )
    {
       if ( this.PropertyChanging != null )
       {
          this.PropertyChanging( this, new PropertyChangingEventArgs ( propertyName ) );
       }
    }
    
    public string MyProperty { get; set; }
}