class Program
{
    private static event EventHandler<int> MyEvent;

    static void OnMyEvent(object sender, int i)
    {
        Console.WriteLine($"tid: #{Thread.CurrentThread.ManagedThreadId}, value: {i}");
    }

    static void Main(string[] args)
    {
        MyEvent += async (sender, i) =>
        {
            var tcs = new TaskCompletionSource<int>();
            tcs.SetResult(i*i);
            var arg = await tcs.Task;
            OnMyEvent(sender, arg);
        };

        foreach (var i in Enumerable.Range(0, 10))
        {
            MyEvent(null, i);
        }
    }
}