How to cancel await Task.Delay()?

If you're going to poll, poll on a CancellationToken:

public async Task TaskDelayTestAsync(CancellationToken token)
{
  for (int i = 0; i < 100; i++)
  {
    textBox1.Text = i.ToString();
    await Task.Delay(TimeSpan.FromSeconds(1), token);
  }
}

For more information, see the cancellation documentation.

Use the overload of Task.Delay which accepts a CancellationToken

public async Task TaskDelayTest(CancellationToken token)
{
    while (LoopCheck)
    {
        token.throwIfCancellationRequested();
        for (int i = 0; i < 100; i++)
        {
            textBox1.Text = i.ToString();
            await Task.Delay(1000, token);
        }
    }
}

var tokenSource = new CancellationTokenSource();
TaskDelayTest(tokenSource.Token);
...
tokenSource.Cancel();

Just a slight comment about having a cancellation token, and using a try-catch to stop it throwing an exception - your iteration block might fail due to a different reason, or it might fail due to a different task getting cancelled (e.g. from an http request timing out in a sub method), so to have the cancellation token not throw an exception you might want a bit more complicated catch block

public async void TaskDelayTest(CancellationToken token)
{
    while (!token.IsCancellationRequested)
    {
        for (int i = 0; i < 100; i++)
        {
            try
            {
                textBox1.Text = i.ToString();
                await DoSomethingThatMightFail();
                await Task.Delay(1000, token);
            }
            catch (OperationCanceledException) when (token.IsCancellationRequested)
            {
                //task is cancelled, return or do something else
                return;
            }
            catch(Exception ex)
            {
                 //this is an actual error, log/throw/dostuff here
            }
        }
    }
}