nest ".ValueChanged.cs" file.

CodeFixProvider.cs

    [ExportCodeFixProvider(LanguageNames.CSharp, Name = nameof(ValueChangedGaneratorCodeFixProvider)), Shared]
    public class ValueChangedGaneratorCodeFixProvider : CodeFixProvider
    {
        // snip.
        
        public sealed override async Task RegisterCodeFixesAsync(CodeFixContext context)
        {
            var root = await context.Document.GetSyntaxRootAsync(context.CancellationToken).ConfigureAwait(false);

            var diagnostic = context.Diagnostics.First();
            var diagnosticSpan = diagnostic.Location.SourceSpan;

            var declaration = root.FindToken(diagnosticSpan.Start).Parent.AncestorsAndSelf().OfType<ClassDeclarationSyntax>().First();

            context.RegisterCodeFix(
                new ValueChangedGaneratorCodeAction(context.Document.FilePath, "Generate properties", c => GenerateValueChanged(context.Document, declaration, true, c)),
                diagnostic);

            context.RegisterCodeFix(
                new ValueChangedGaneratorCodeAction(context.Document.FilePath, "Generate properties (> C# 5)", c => GenerateValueChanged(context.Document, declaration, false, c)),
                diagnostic);
        }

        public class ValueChangedGaneratorCodeAction : CodeAction
        {
            public override string Title { get; }
            public override string EquivalenceKey { get; }
            private Func<CancellationToken, Task<Solution>> CreateChangedSolution { get; }
            public string FilePath { get; }

            public ValueChangedGaneratorCodeAction(string filePath, string title, Func<CancellationToken, Task<Solution>> createChangedSolution, string equivalenceKey = null)
            {
                FilePath = filePath;
                Title = title;
                CreateChangedSolution = createChangedSolution;
                EquivalenceKey = equivalenceKey;
            }

            protected override Task<Solution> GetChangedSolutionAsync(CancellationToken cancellationToken)
            {
                return CreateChangedSolution(cancellationToken);
            }

            protected override async Task<IEnumerable<CodeActionOperation>> ComputeOperationsAsync(CancellationToken cancellationToken)
            {
                var result = await base.ComputeOperationsAsync(cancellationToken).ConfigureAwait(false);
                return result.Concat(new[] { new ValueChangedGaneratorCodeActionOperation(Title, FilePath) });
            }
        }

        public class ValueChangedGaneratorCodeActionOperation : CodeActionOperation
        {
            public override string Title { get; }
            public string FilePath { get; }

            public ValueChangedGaneratorCodeActionOperation(string title, string filePath)
            {
                Title = title;
                FilePath = filePath;
            }

            public override void Apply(Workspace workspace, CancellationToken cancellationToken)
            {
                var dte = Microsoft.VisualStudio.Shell.ServiceProvider.GlobalProvider.GetService(typeof(EnvDTE._DTE)) as EnvDTE.DTE;
                var projectItem = dte.Solution.FindProjectItem(FilePath);
                var generatedFilename = FilePath.Substring(0, FilePath.Length - ".cs".Length) + ".ValueChanged.cs";
                projectItem.ProjectItems.AddFromFile(generatedFilename);
            }
        }
        
        // snip.
        
      }
    }