How to reset a CancellationToken properly?

You need to recreate the CancellationTokenSource - there is no way to "reset" this once you set it.

This could be as simple as:

private void Button_Click(object sender, RoutedEventArgs e)
{
    if (button.Content == "Start")
    {
        button.Content = "Stop";
        cts.Dispose(); // Clean up old token source....
        cts = new CancellationTokenSource(); // "Reset" the cancellation token source...
        DoWork(cts.Token);
    }
    else
    {
        button.Content = "Start";
        cts.Cancel();
    }
}

I had the same problem and I figured it out that the best way to solve it is to create cancellation token source newly just before calling the method.

this is what I do on my start button click:

cancellationTokenSource = new CancellationTokenSource();
cancellationToken = cancellationTokenSource.Token;
Task.Factory.StartNew(StartUpload, cancellationToken);

I change the caption for the same button to cancel and when a click occurs on cancel, I call

cancellationTokenSource.Cancel();

Here is the full code:

if (button3.Text != "&Start Upload")
{
    cancellationTokenSource.Cancel();
}
else
{
    try
    {
        cancellationTokenSource = new CancellationTokenSource();
        cancellationToken = cancellationTokenSource.Token;
        Task.Factory.StartNew(StartUpload, cancellationToken);
    }
    catch (AggregateException ex)
    {
        var builder = new StringBuilder();
        foreach (var v in ex.InnerExceptions)
            builder.Append("\r\n" + v.InnerException);
        MessageBox.Show("There was an exception:\r\n" + builder.ToString());
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
}

Now, with .NET 6 you can use TryReset() method which Attempts to reset the CancellationTokenSource to be used for an unrelated operation. see this issue and CancellationTokenSource.cs for more details.