Cancel task in c#

Question

I´m still learning to code, and I'm making a new project with MDI forms (C# and Visual Studio 2019). In mdichild, I launched a task, but if the form is unloaded, the task still remains. I would like to know how cancel the task, even on a cancel button click.

The code:

private async void BuscaActualizaciones()
{
    await Task.Run(() => DoLongThing());
}

private void DoLongThing()
{
    //some hard stuff
}

private void BtnBuscar_Click(object sender, EventArgs e)
{
    //In here i launch the task with hard stuff
    BuscaActualizaciones();
}

This code works perfectly, but I need to cancel in some events, and I don't know how.

I tried some home-made tricks, and read on Google about task cancellation, but all of them used Task in other way that I don't understand. I'm still learning, and it is my first time with tasks.

Answer 1

As others have pointed out, the correct solution will use a CancellationToken. However, you don't want to pass it to Task.Run; you want to pass it to DoLongThing, as such:

CancellationTokenSource cts = new ();

private async void BuscaActualizaciones()
{
  await Task.Run(() => DoLongThing(cts.Token)); 
}

private void DoLongThing(CancellationToken token)
{
  ...
  token.ThrowIfCancellationRequested();
}

private void CancelButton_Click(object sender, EventArgs e)
{
  cts.Cancel();
}

Polling for cancellation (ThrowIfCancellationRequested) is common for CPU-bound methods that need to periodically check if they're canceled. I have a blog post that goes into more details on the subject.