How to cancel Task C#

Question

I have a Task which retrieves some data from a Web Service. The Webservice should only get called when I open a page (this is Xamarin.Forms) and it should only run if there

Answer 1

You're doing the right check in a wrong place:

var token = tokenSource.Token;
task = Task.Run(async () =>
{
    while (!token.IsCancellationRequested)
    {
        await GetDataAsync();
    }
}

Here you start the task, and after that start it again and again in a loop. The right thing to do is check the token inside your GetData, and futher to api variable, if possible. In this case you'll be able to cancel the service call as you need.

Side notes:

• make your event handlers async, and do not wrap your work in Task.Run
• make your GetData Task<T> so you can actually return status to the calling code, then you can remove BeginInvokeOnMainThread, as await will restore the context after async operation

So your code will be something like this:

protected override async void OnAppearing()
{
    base.OnAppearing();

    if (task != null && (task.Status == TaskStatus.Running || task.Status == TaskStatus.WaitingToRun || task.Status == TaskStatus.WaitingForActivation))
    {
        Debug.WriteLine("Task has attempted to start while already running");
    }
    else
    {
        Debug.WriteLine("Task has began");

        var token = tokenSource.Token;
        PageLoading();
        var r = await GetDataAsync(token);
        if (r == "OK")
        {
            GetDataSuccess();
        }
   }      
}

public async Task GetDataAsync(CancellationToken token)
{
    WebAPI api = new WebAPI();
    if (token.IsCancellationRequested)
    {
        token.ThrowIfCancellationRequested();
    }

    try
    {
        return await api.GetProfileStatus(token);
    }
    catch (Exception e)
    {
        // log exception and return error
        return "Error";
    }
}