RxPreemption.cs

private IObservable<string[]> DictionaryLookup(string key) 
{
    ...
}
 
private string FormatAsHtml(string[] results)
{
    ...
}
 
var textBox = new TextBox();

...
 
var keyUp = Observable.FromEvent<KeyEventArgs>(textBox, "KeyUp")
    .Select(ev => ((TextBox)ev.Sender).Text);
 
var query = keyUp.Throttle(TimeSpan.FromMilliseconds(500)).Let(_keyUp =>
    from key in _keyUp
    from d in DictionaryLookup(key).TakeUntil(_keyUp)
    select FormatAsHtml(d));

var query2 = keyUp.Throttle(TimeSpan.FromMilliseconds(500))
    .Select(key => DictionaryLookup(key))
    .Switch()
    .Select(d => FormatAsHtml(d));

Throttling events in RxNet