Run method Safe cancellation?

Question

I want to call a function in a different thread than my GUI.

I used the below code to trigger the function:

private void button1_Click(object sender, EventArgs e)
{
    tokensource = new CancellationTokenSource();
    var token = tokensource.Token;
    Task.Run(()=>foo() , token);
}

private void foo()
{
    // Uses some resources
}

private void button2_Click(object sender, EventArgs e)
{
    tokenSource.Cancel();
}

How can I safely close the occupied resources in foo() when the task is cancelled?

Answer 1

You need to pass the token to the function as well. The cancellation token passed to Task.Run won't abort an already running task, it will prevent a scheduled task from running.

Inside foo, you can check the token for cancellation and return, or throw an exception. You can use a using block to dispose resources safely. Eg:

private void foo(CancellationToken token)
{
    using(var reader=new StreamReader(somePath)
    {
            string line;
            // Read the line if no cancellation was requested
            while (!token.IsCancellationRequested && (line = sr.ReadLine()) != null) 
            {
                Console.WriteLine(line);
            }
    }
}

This code reads a line only if cancellation wasn't requested and returns quietly otherwise

You can also throw an OperationCancelledException by calling CancellationToken.ThrowIfCancellationRequested

private void foo(CancellationToken token)
{
    using(var reader=new StreamReader(somePath)
    {
            string line;
            // Read the line if no cancellation was requested
            while ((line = sr.ReadLine()) != null) 
            {
                token.ThrowIfCancellationRequested();
                Console.WriteLine(line);
            }
    }
}

This will throw an exception that will be raised in the calling code when the task's result is retrieved, eg when using await Task.Run(..) or Task.Run(..).Wait()