S>>>        private volatile bool _changed;

S>>>        private void textBox1_KeyUp(object sender, KeyEventArgs e)
S>>>        {
S>>>            _changed = true;
S>>>            FillItems();
S>>>        }

S>>>        private async void FillItems()
S>>>        {
S>>>            _changed = false;
S>>>            await Task.Delay(500);

S>>>            if (_changed)
S>>>                return;

S>>>            textBox2.Clear();

S>>>            var items = await MakeRequest(textBox1.Text);

S>>>            if (_changed)
S>>>                return;

S>>>            textBox2.Text = string.Join("\r\n", items);
S>>>        }

S>>>        Random _random = new Random();
S>>>        private async Task<List<string>> MakeRequest(string q)
S>>>        {
S>>>            await Task.Delay(_random.Next(100, 5000));
S>>>            return await Task.FromResult(new List<string>()
S>>>            {
S>>>                q + "пункт 1 ",
S>>>                q + "пункт 2",
S>>>                q + "пункт 3",
S>>>            });
S>>>        }
S>>>