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Understanding the NotifyPropertyChanged Aspect

This section describes the principles and algorithm on which the NotifyPropertyChangedAttribute aspect is based. It helps developers and architects to understand the behavior and limitation of the aspect.

This topic contains the following sections:

Implementation of the INotifyPropertyChanged interface

The NotifyPropertyChangedAttribute aspect introduces the INotifyPropertyChanged interface to the target class unless the target class already implements the interface. Additionally, the aspect also introduces the OnPropertyChanged(String) method. The aspect always introduces this method as protected and virtual, so it can be overridden in derived classes.

If the target class already implements the INotifyPropertyChanged interface, the aspect requires the class to expose the OnPropertyChanged(String) method.

The aspect uses the OnPropertyChanged(String) to raise the PropertyChanged event. Thanks to this method, the aspect is able to raise the event even when the INotifyPropertyChanged is not implemented by the aspect. This mechanism also allows user code to raise notifications that are not automatically handled by the NotifyPropertyChangedAttribute aspect.

Instrumentation of fields

Although most implementations the INotifyPropertyChanged interface rely on instrumenting the property setter, this strategy has severe limitations: it is unable to handle composite properties, which return a value based on several other fields or properties. Composite properties have no setter, rendering this strategy unusable.

Instead, the NotifyPropertyChangedAttribute aspect instruments all write operations to fields (for instance a FullName property appending FirstName and LastName). It analyzes dependencies between fields and properties and raises a change notification for any property affected by a change in this specific field.

All methods, and not just property setters, can make a change to a field and therefore cause the PropertyChanged event to be raised. Property setters do not have any specific status in the NotifyPropertyChangedAttribute implementation.

Analysis of field-property dependencies

In order to adequately raise the PropertyChanged event, the NotifyPropertyChangedAttribute aspect needs to know which properties are affected by a change of a class field. The field-property dependency map is created at build time by analyzing the source code: the analyzer reads the getter of all properties and check for field references. The map is then serialized inside the assembly and used at run time to raise relevant events when a field has changed.

Dependencies on fields of the current object

Consider the following code snippet:

C#
[NotifyPropertyChanged]              
class Invoice
{
  private decimal _amount;
  private decimal _tax;

  public decimal Amount { get { return this._amount; } set { this._amount = value; } }
  public decimal Tax { get { return this._tax; } set { this._tax = value; } }

  public void Set( decimal amount, decimal tax )
  {
     this._amount = amount;
     this._tax = tax;
  }

  public decimal Total { get { return this._amount + this._tax; } }
}

The result of the analysis for the code snippet above would be the map { _amount => ( Amount, Total ), _tax => ( Tax, Total ) }. Whenever the _amount field is changed, the PropertyChanged event will be raised for properties Amount and Total.

Automatic properties are processed as handwritten properties; in this case, the implicit backing field is taken into account for the dependency analysis.

Recursive analysis of the call graph

Field references are not only looked for in the getter, but in any method invoked from the getter, and recursively.

Consider the following code snippet:

C#
[NotifyPropertyChanged]              
class Invoice
{
  private decimal _amount;
  private decimal _exchangeRate;

  public decimal Amount { get { return this._amount; } set { this._amount = value; } }
  public decimal ExchangeRate { get { return this._exchangeRate; } set { this._exchangeRate = value; } }

  private decimal Convert( decimal amount )
  {
    return amount * this.ExchangeRate;
  }

  public int AmountBase { get { return this.Convert( this.Amount ); } }

}

In the code snippet above, the analyzer starts from the getter of the AmountBase property, follows the call to the Amount property getter, then call to the AmountBase method and recursively follows the ExchangeRate property getter. Therefore, the resulting property map remains { _amount => ( Amount, AmountBase ), _exchangeRate => ( ExchangeRate, AmountBase ) }.

Dependencies on properties of external objects

The NotifyPropertyChangedAttribute aspect does not just handle dependencies between a property and a field of the same class. It also handles dependencies on properties of properties or properties of fields, and recursively. That is, it supports expressions of the form _f.P1.P2.P3 where _f is a field or property and P1, P2 and P3 are properties.

Consider the following code snippet:

C#
[NotifyPropertyChanged]              
class InvoiceModel
{
  private decimal _amount;
  private decimal _tax;

  public decimal Amount { get { return this._amount; } set { this._amount = value; } }
  public decimal Tax { get { return this._tax; } set { this._tax = value; } }
}

[NotifyPropertyChanged]
class InvoiceViewModel
{
  InvoiceModel _model;  

  public InvoiceModel Model { get { return this._model; } }

  public decimal Total { get { return this._model.Amount + this.Model.Tax; } }

}

In the example above, the InvoiceViewModel.Total property is dependent on properties Amount and Tax of the _model field. Therefore, changes in the InvoiceModel._amount field will trigger a change notification for the InvoiceModel.Amount and InvoiceViewModel.Total properties.

The NotifyPropertyChangedAttribute aspect automatically subscribes to the PropertyChanged event of the child object, and unsubscribes whenever the value of the field in the parent object (_model in our example) is modified. However, the parent object does not unsubscribe upon disposal because the NotifyPropertyChangedAttribute makes no assumption that the IDisposable interface has been implemented. Therefore, the implementation of the INotifyPropertyChanged of the external object must hold weak references to clients of the PropertyChanged event.

Recursive dependencies to external objects are handled thanks to an auxiliary interface named INotifyChildPropertyChanged. This interface is implemented by the NotifyPropertyChangedAttribute aspect. It is considered an implementation detail and cannot be implemented manually. Classes that do not implement the INotifyChildPropertyChanged interface can only participate as terminal dependencies, i.e. they can be leaves but not intermediate nodes.

Limitations

The design goal of the NotifyPropertyChangedAttribute aspect is to be able to handle the majority of use cases in real-world source code while requiring only an acceptable amount of compilation time. The dependency analysis algorithm imposes several limitations:

  • Calls to virtual methods (other than through the base keyword), abstract methods, interface methods or delegates are not supported.

  • Calls to static methods or methods of external classes are not supported unless they are decorated with the PureAttribute custom attribute, or unless the method is a property getter in a supported dependency chain.

  • Valuations of properties method return values are not supported. Only properties of fields or properties are supported.

  • Dependencies on properties of variables are not supported if the variable is assigned in a loop (while, for, ...) or in an exception handling block.

See Handling Corner Cases to learn how to cope with these limitations.

Raising notifications

Simplistic implementations of the INotifyPropertyChanged interface signal a change notification immediately after a property has been changed. However, this strategy may cause subtle errors in client code.

Consider the following code:

C#
[NotifyPropertyChanged]
class Invoice
{
  public decimal Amount { get; private set; }
  public decimal Tax { get; private set; }
  public decimal Total { get; private set; }

  public void Set( decimal amount, decimal tax )
  {
    /* 1 */ this.Amount = amount;
    /* 2 */ this.Tax = tax;
    /* 3 */ this.Total = amount + tax;
  }

}

As a class invariant, the assumption Total == Amount + Tax should always be true.

However, suppose that the PropertyChanged event is raised immediately after the Amount property is set at line 1 of the Set method. Clearly, for a client subscribing to this event, the class invariant would be broken at this specific moment.

Therefore, it is not safe to raise change notifications immediately after a change has been achieved. It is necessary to wait until the object can be safely observed by external code, when all class invariants are valid again (i.e. when the object state is consistent). A common best practice in object-oriented programming is to ensure that class invariants are valid before the control flow goes back from the current object to the caller. Typically, it means that a private or protected method can exit with an inconsistent object state, but public and internal methods must guarantee that the object state is consistent upon exit.

The NotifyPropertyChangedAttribute aspect relies on this best practice and raises the property change notifications just before the control flow exits the current object, that is, just before the last public or internal method in the call stack for the current object exits.

Besides avoiding to expose invalid object state, this strategy also avoids the same property to be notified for change several times during the execution of a single public method, which a potentially great positive performance impact.

To solve this problem, NotifyPropertyChangedAttribute aspect uses the following strategy:

  1. Instead of causing immediate change notifications, field changes are buffered into a thread-local storage named the accumulator.

  2. Calls to public and methods are instrumented so the aspect can detect when the control flow exits the object. At this moment, the accumulator is flushed and all change notifications are triggered.

It is possible to flush the accumulator at any time by invoking the NotifyPropertyChangedServices.RaiseEventsImmediate(Object) method.

You can suspend and resume notifications using the NotifyPropertyChangedServices.SuspendEvents() and NotifyPropertyChangedServices.ResumeEvents() methods.

Remarks

The NotifyPropertyChangedAttribute aspect never evaluates property getters at run time. This decision is deliberate and aims at avoiding possible side-effects (lazy-initialization, logging, etc.). Therefore, it is possible that the algorithms emit false positives, i.e. change notifications for properties whose values did not actually change.

The algorithm heuristically detects dependency cycles. If a cycle is detected, an exception is thrown instead of allowing for an infinite update cycle.

All notifications are invoked on the thread on which the change is being made. The accumulator that buffers the changes is a thread-local storage.