using System;

using System.ComponentModel;

using System.Threading.Tasks;

public class Program

{

    public static async Task Main()

    {

        var foo = new Foo();

        foo.Bar = "Init Value";

        foo.CancelPropertyChanging += Foo_CancelPropertyChanging;

​

        foo.Bar = "Hello World";

        foo.Bar = "Hello World 2";

        Console.WriteLine(foo.Bar);

        Console.WriteLine(foo.Bar == "Hello World 2" ? "Error" : "Correct");

​

        var fooAsync = new Foo();

        fooAsync.Bar = "Init Value";

        fooAsync.CancelPropertyChanging += Foo_AsyncCancelPropertyChanging;

​

        fooAsync.Bar = "Hello World";

        fooAsync.Bar = "Hello World 2";

        Console.WriteLine(fooAsync.Bar);

        Console.WriteLine(fooAsync.Bar == "Hello World 2" ? "Error" : "Correct");

​

        //wait for the Async event handlers to fire

        await Task.Delay(3_000);

        void Foo_CancelPropertyChanging(object? sender, CancelPropertyChangingEventArgs e)

        {

            Console.WriteLine($"Changing Sync - OldValue: {e.OldValue} | NewValue: {e.NewValue}");

​

            if (Convert.ToString(e.NewValue) == "Hello World 2")

                e.Cancel = e.Cancel || true;

        }

        async void Foo_AsyncCancelPropertyChanging(object? sender, CancelPropertyChangingEventArgs e)

        {

            await Task.Delay(1_000);

            Console.WriteLine($"Changing Async - OldValue: {e.OldValue} | NewValue: {e.NewValue}");

​

            if (Convert.ToString(e.NewValue) == "Hello World 2")

                e.Cancel = e.Cancel || true;

        }

    }

}

​

public class Foo

{

    private string? _bar;

    public event EventHandler<CancelPropertyChangingEventArgs>? CancelPropertyChanging;

    public string? Bar

    {

        get => _bar;

        set

        {

            if (CancelPropertyChanging is { } cancelPropertyChanging)

            {

                var eventArgs = new CancelPropertyChangingEventArgs()

                {

                    Cancel = false,

                    NewValue = value,

                    OldValue = _bar,

                    PropertyName = nameof(Bar),

                };

                cancelPropertyChanging(this, eventArgs);

                if (eventArgs.Cancel)

                    return;

            }

​

            _bar = value;

        }

    }

​

    public override string ToString() => Bar ?? "";

}

​

public class CancelPropertyChangingEventArgs : CancelEventArgs

{

    public object? OldValue { get; set; }

    public object? NewValue { get; set; }

    public string PropertyName { get; set; } = "";

}