DOM009: Class Defines a Member Reserved by the NotifyPropertyChanged Aspect


Severity	Error
Reported by	PostSharp.Patterns.Model
Message	`Class "<Class>" or one of its base classes already defines the ChildPropertyChanged event or the OnChildPropertyChanged(string) method, whose names the [NotifyPropertyChanged] aspect reserves. Rename the member.`

Cause

The NotifyPropertyChangedAttribute aspect propagates the changes of child objects to their parents. For this purpose, it adds a ChildPropertyChanged event and an OnChildPropertyChanged method to the class, so these names are reserved for the aspect.

PostSharp reports DOM009 on a class with the aspect when the class, or one of its base classes, already defines an event named ChildPropertyChanged, of any type, or a method named OnChildPropertyChanged with a single string parameter. The check does not apply to a class whose base class has the aspect, because the members then come from the aspect of the base class.

In the following example, Order defines its own ChildPropertyChanged event.

Full screen

using System;
using System.ComponentModel;
using PostSharp.Patterns.Model;

namespace Errors.DOM009_Error;

// The class defines a ChildPropertyChanged event, a name that the aspect reserves.
[NotifyPropertyChanged]
public class Order
{
    public decimal Total { get; set; }

    public event EventHandler? ChildPropertyChanged;

    public void ChangeLine()
    {
        this.ChildPropertyChanged?.Invoke( this, EventArgs.Empty );
    }
}

public static class Program
{
    public static void Main()
    {
        Order order = new Order();
        ( (INotifyPropertyChanged) order ).PropertyChanged += ( _, e ) => Console.WriteLine( $"{e.PropertyName} changed." );
        order.ChildPropertyChanged += ( _, _ ) => Console.WriteLine( "A line changed." );
        order.Total = 100;
        order.ChangeLine();
    }
}

Full screen

error DOM009: DOM009_Error.cs(9,14): Class "Errors.DOM009_Error.Order" or one of its base classes already defines the ChildPropertyChanged event or the OnChildPropertyChanged(string) method, whose names the [NotifyPropertyChanged] aspect reserves. Rename the member.

Resolution

Rename the event or the method, in the class or in the base class that defines it.

In the following example, the event is renamed LineChanged.

Full screen

using System;
using System.ComponentModel;
using PostSharp.Patterns.Model;

namespace Errors.DOM009_Fixed;

[NotifyPropertyChanged]
public class Order
{
    public decimal Total { get; set; }

    // The event has a name that the aspect does not use.
    public event EventHandler? LineChanged;

    public void ChangeLine()
    {
        this.LineChanged?.Invoke( this, EventArgs.Empty );
    }
}

public static class Program
{
    public static void Main()
    {
        Order order = new Order();
        ( (INotifyPropertyChanged) order ).PropertyChanged += ( _, e ) => Console.WriteLine( $"{e.PropertyName} changed." );
        order.LineChanged += ( _, _ ) => Console.WriteLine( "A line changed." );
        order.Total = 100;
        order.ChangeLine();
    }
}

Full screen

Total changed.
A line changed.

Cause

Resolution

See Also