Suppressing False Positives

The NotifyPropertyChangedAttribute aspect, when applied to a class, raises the PropertyChanged event every time it detects a possible change of a property, even when the actual value of the property doesn't change. By default, the aspect doesn’t keep track of the property values because that would require the aspect to invoke property getters arbitrarily outside of the developer’s control. And when property getters have any side effects (lazy-initialization, logging, etc.), invoking them randomly is not a safe behavior.

In certain scenarios, such as rich client applications with many UI controls, redundant event notifications are not desired because they cause excessive UI updates and can degrade the application responsiveness. You can avoid these redundant event notifications by suppressing false positives in the NotifyPropertyChangedAttribute aspect.

This topic contains the following sections:

Example of a false positive

The following Calc class has two integer fields a and b, and a property Sum that returns the sum of these two numbers. The Main method creates an instance of the Calc class and changes the fields from (a=1, b=2) to (a=2, b=1). There are two PropertyChanged event notifications shown in the output, even though the actual Sum value doesn’t change in the second case.

C#
[NotifyPropertyChanged]
class Calc
{
    private int a, b;

    public int Sum
    {
        get { return this.a + this.b; }
    }

    public void Update(int a1, int b1)
    {
        this.a = a1;
        this.b = b1;
    }
}

static void Main()
{
    Calc calc = new Calc();
    ((INotifyChildPropertyChanged) calc).PropertyChanged +=
        (sender, eventArgs) =>
        {
            Console.WriteLine("Property {0} changed. New value = {1}.",
                eventArgs.PropertyName,
                sender.GetType().GetProperty(eventArgs.PropertyName).GetValue(sender));
        };

    calc.Update(1, 2);
    calc.Update(2, 1);
}

Output:

Property Sum changed. Value = 3.
Property Sum changed. Value = 3.
How to suppress false positives

To suppress false positive event notifications, set the PreventFalsePositives property to true when you apply the aspect to the target element.

C#
[NotifyPropertyChanged(PreventFalsePositives = true)]
class Calc
{
    // ...
}

If you run the test code snippet again after this change, you can see that there’s only one change notification now.

Property Sum changed. Value = 3.
Caution note Caution

Do not suppress false positive notifications in your class when the property getters in the class have side effects. In this case reset the PreventFalsePositives property to its default value of false.

By enabling the PreventFalsePositives option of the NotifyPropertyChangedAttribute aspect you can reduce the number of events raised in your application and improve your UI responsiveness.

See Also