async void event handlers and reentrancy

Form1.cs

// https://twitter.com/noseratio/status/1466350790952456192?s=20
// dotnet new winforms -n app
// paste this into Form1.cs 

namespace app;

public partial class Form1 : Form
{
    int _count = 0;
    Button _button;

    readonly SemaphoreSlim _semaphoreSlim = new(initialCount: 1, maxCount: 1);
    CancellationTokenSource? _cancellationSource;

    public Form1()
    {
        InitializeComponent();
        _button = new Button() { Text = "Click me", AutoSize = true };
        _button.Click += Button_Click;
        this.Controls.Add(_button);
    }

    private async void Button_Click(object? sender, EventArgs e)
    {
        try
        {
            // cancel the previous click handler workflow
            _cancellationSource?.Cancel();
            _cancellationSource = new();
            await _semaphoreSlim.WaitAsync(_cancellationSource.Token);
            try
            {
                // simualte async operation
                await Task.Delay(1000, _cancellationSource.Token);
                // update the UI (a ViewModel in real life)
                _button.Text = $"Clicked {++_count} times";
            }
            finally
            {
                _semaphoreSlim.Release();
            }
        }
        catch (OperationCanceledException)
        {
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
    }
}