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INotifyPropertyChanged attributes

  • Article

The INotifyPropertyChanged type is an attribute that allows inserting MVVM support code into existing types. Along with other related attributes (ObservableObject and ObservableRecipient), its purpose is to support developers in cases where the same functionality from these types was needed, but the target types were already implementing from another type. Since C# does not allow multiple inheritance, these attributes can instead be used to have the MVVM Toolkit generator add the same code right into those types, sidestepping this limitation.

Note

In order to work, annotated types need to be in a partial class. If the type is nested, all types in the declaration syntax tree must also be annotated as partial. Not doing so will result in a compile errors, as the generator will not be able to generate a different partial declaration of that type with the requested additional code.

Note

These attributes are only meant to be used in cases where the target types cannot just inherit from the equivalent types (eg. from ObservableObject). If that is possible, inheriting is the recommended approach, as it will reduce the binary size by avoiding creating duplicated code into the final assembly.

Platform APIs: INotifyPropertyChanged, ObservableObject, ObservableRecipient

How to use them

Using any of these attributes is pretty straightforward: just add them to a partial class and all the code from the corresponding types will automatically be generated into that type. For instance, consider this:

[INotifyPropertyChanged]
public partial class MyViewModel : SomeOtherType
{    
}

This will generate a complete INotifyPropertyChanged implementation into the MyViewModel type, complete with additional helpers (such as SetProperty) that can be used to reduce verbosity. Here is a brief summary of the various attributes:

  • INotifyPropertyChanged: implements the interface and adds helper methods to set properties and raise the events.
  • ObservableObject: adds all the code from the ObservableObject type. It is conceptually equivalent to INotifyPropertyChanged, with the main difference being that it also implements INotifyPropertyChanging.
  • ObservableRecipient: adds all the code from the ObservableRecipient type. In particular, this can be added to a type inheriting from ObservableValidator to combine the two.

Examples

  • Check out the sample app (for multiple UI frameworks) to see the MVVM Toolkit in action.
  • You can also find more examples in the unit tests.