PropertyChanged.Fody

Injects code which raises the PropertyChanged event, into property setters of classes which implement INotifyPropertyChanged.

This is an add-in for Fody.

It is expected that all developers using Fody either become a Patron on OpenCollective, or have a Tidelift Subscription. See Licensing/Patron FAQ for more information.

Usage

NuGet installation

Install the PropertyChanged.Fody NuGet package and update the Fody NuGet package:

PM> Install-Package Fody
PM> Install-Package PropertyChanged.Fody

The Install-Package Fody is required since NuGet always defaults to the oldest, and most buggy, version of any dependency.

Add to FodyWeavers.xml

Add <PropertyChanged/> to FodyWeavers.xml

<Weavers>
  <PropertyChanged/>
</Weavers>

Overview

NOTE: All classes that implement INotifyPropertyChanged will have notification code injected into property setters.

Before code:

public class Person : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    
    public string GivenNames { get; set; }
    public string FamilyName { get; set; }
    public string FullName => $"{GivenNames} {FamilyName}";
}

What gets compiled:

public class Person : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    string givenNames;
    public string GivenNames
    {
        get => givenNames;
        set
        {
            if (value != givenNames)
            {
                givenNames = value;
                OnPropertyChanged(InternalEventArgsCache.GivenNames);
                OnPropertyChanged(InternalEventArgsCache.FullName);
            }
        }
    }

    string familyName;
    public string FamilyName
    {
        get => familyName;
        set 
        {
            if (value != familyName)
            {
                familyName = value;
                OnPropertyChanged(InternalEventArgsCache.FamilyName);
                OnPropertyChanged(InternalEventArgsCache.FullName);
            }
        }
    }

    public string FullName => $"{GivenNames} {FamilyName}";

    protected void OnPropertyChanged(PropertyChangedEventArgs eventArgs)
    {
        PropertyChanged?.Invoke(this, eventArgs);
    }
}

internal static class InternalEventArgsCache
{
    internal static PropertyChangedEventArgs FamilyName = new PropertyChangedEventArgs("FamilyName");
    internal static PropertyChangedEventArgs FullName = new PropertyChangedEventArgs("FullName");
    internal static PropertyChangedEventArgs GivenNames = new PropertyChangedEventArgs("GivenNames");
}

(the actual injected type and method names are different)

Notes

Dependent properties — In the above sample, the getter for FullName depends on the getters for GivenName and FamilyName. Therefore, when either GivenName or FamilyName is set, PropertyChanged is raised for FullName as well. This behavior can be configured manually using the AlsoNotifyFor attribute on the source property, or the DependsOn attribute on the target property).
Intercepting the notification call
- Global interception
- Class-level interception — The OnPropertyChanged method will only be injected if there is no such existing method on the class; if there is such a method, then calls to that method will be injected into the setters — see here.
- Property-level interception — For a given property, if there is a method of the form On<PropertyName>Changed, then that method will be called — see here.
To get the before / after values, use the following signature for OnPropertyChanged / On<PropertyName>Changed:
```
public void OnPropertyChanged(string propertyName, object before, object after)
```
To prevent a specific class from having the notification call injection, use the DoNotNotify attribute.
To scope the rewriting only to specific classes, and not the whole Assembly, you can use the FilterType attribute. This changes the general behavior from from opt-out to opt-in. Example: [assembly: PropertyChanged.FilterType("My.Specific.OptIn.Namespace.")]. The string is interpreted as a Regex, and you can use multiple filters. A class will be weaved, if any filter matches.
The INotifyPropertyChanged interface can be automatically implemented for a specific class using the AddINotifyPropertyChangedInterfaceAttribute attribute. Raising an issue about "this attribute does not behave as expected" will result in a RTFM and the issue being closed.
Behavior is configured via attributes, or via options in the Weavers.xml file.

For more information, see the wiki pages.

Icon

Icon courtesy of The Noun Project

gookcedemir/PropertyChanged