INotifyPropertyChanged with C# 9.0 Source Generators

Credit: This article originally appeared at https://jaylee.org/archive/2020/04/29/notify-property-changed-with-rosyln-generators.html, written by Jerome Laban.

In a design meeting far, far away, source generators were designed to be part of C# 6.0, but sadly never came to be. At the time, wanting that source generation feature pretty badly, I went on implementing the specification which later became Uno.SourceGeneration, and it turns out it was the right decision to stick with a very similar API.

I could port the INotifyPropertyChanged (INPC) generator I built a while back that uses Uno.SourceGeneration package, convert it to use Roslyn’s shiny new C# 9.0 feature in a matter of minutes. Amazing!

The INPC Generator

As a quick refresher, the generator works by allowing the creation of a class this way:

partial class MyClass
{
  [GeneratedProperty]
  private string _myProperty;

  partial void OnMyPropertyChanged(string previousValue, string newValue)
  {
    Console.WriteLine($"OnMyPropertyChanged({previousValue}, {newValue}");
  }
}

You’ll notice here that the property is not visible at all, and it will be assumed that the generator will recognize the GeneratedPropertyAttribute and do the rest of the work for the author of the class.

The generator could then produce something like this:

partial class MyClass : INotifyPropertyChanged
{
  public string MyProperty
  {
    get => _myProperty;
    set
    {
      var previous = _myProperty;
      _myProperty = value;
      PropertyChanged?.Invoke(new PropertyChangedEventArgs(nameof(MyProperty)));
      OnMyPropertyChanged(previous, value);
    }
  }

  partial void OnMyPropertyChanged(string previousValue, string newValue);
}

This generator is generating the property on your behalf, and automatically implements the INotifyPropertyChanged as well as a partial method that you can optionally implement to get local notification on changes.

For a more detailed explanation of the generation process, head to this earlier article.

Upgrading the source generator to C# 9.0

First, we’ll need the INPC.Generator Sample Project to use the the C# 9.0 preview:

<PropertyGroup>
  <LangVersion>preview</LangVersion>
</PropertyGroup>

Then change the NuGet packages from Uno.SourceGeneration to Roslyn’s Analyzers:

<ItemGroup>
	<PackageReference Include="Microsoft.CodeAnalysis.CSharp.Workspaces" Version="3.6.0-3.20207.2" PrivateAssets="all" />
	<PackageReference Include="Microsoft.CodeAnalysis.Analyzers" Version="3.0.0-beta2.final" PrivateAssets="all" />
</ItemGroup>

Then we’ll need to adjust the source code to be use the new interfaces:

[Generator]
public class MySourceGenerator : ISourceGenerator
{
    public void Initialize(InitializationContext context)
    {
        // No initialization required for this one
    }

    public void Execute(SourceGeneratorContext context)
    {
        // Generation
    }
}

The execute method in Uno.SourceGeneration has the same signature, and the new SourceGeneratorContext type the only property that really matters Compilation.

The method to provide the generated source has changed a bit, and uses a SourceText instead of a string:

context.AddSource(sanitizedName, SourceText.From(builder.ToString(), Encoding.UTF8));

We’ll need to update the path to the binary, where SourceGenerator becomes Analyzer, in the INPC.Generator.props file:

<ItemGroup>
    <Analyzer Include="$(MSBuildThisFileDirectory)..\bin\$(Configuration)\netstandard2.0\INPC.Generator.dll"
            Condition="Exists('$(MSBuildThisFileDirectory)..\bin')" />
    <Analyzer Include="$(MSBuildThisFileDirectory)..\tools\INPC.Generator.dll"
            Condition="Exists('$(MSBuildThisFileDirectory)..\tools')" />
</ItemGroup>

And finally, we simply need to compile the main program with the C# 9.0 preview.

When running the sample, here’s what happens:

OnIntPropertyChanged(0,42)
OnIntPropertyChanged(,My 42)

The job is done!

In closing

I hope the feature will grow, and provide lots more like access to msbuild properties, or dependencies between generators, and it will finally fix the issues with dynamically generated <Compile /> item groups.

I hope you will give this feature a try, as it opens up a world of possibilities, as we’ve been finding out while developing Uno (Here’s the list of generators), as well as the Uno.CodeGen generators (which some may not be needed anymore with the C# 9.0 records feature!).

Happy source-generation!