How do I stop an async method I am waiting for from outside the method

March 1, 2023

I am using async methods and a SemaphoreSlim object to wait for and detect a click. Eventually, on a different method being called, I want to stop the async method that is being run and continue as if the method completed.

Incase it affects the answer, I am using Windows Forms.

Here is a skeleton of my problem (not my actual code)

private SemaphoreSlim _semaphoreClick = new SemaphoreSlim(0, 1);

static void Main() 
{
    RunProgram();
    SomeFinishedFuncion();
}

async void RunProgram()
{
    object thing = await WaitForSemaphoreSlim();   
}

async Task<object> WaitForSemaphoreSlim() 
{
    object thing;
    // do stuff
    
    await _semaphoreClick.WaitAsync();
        
    // do stuff
    return thing;
}


void OnStopProgram() 
{
    // Do something here that will stop the RunProgram() 
    // which will make Main() continue to SomeFinishedFunction()
}

>Solution :

Use a CancellationTokenSource and CancellationToken:

private SemaphoreSlim _semaphoreClick = new SemaphoreSlim(0, 1);
private CancellationTokenSource cts = new CancellationTokenSource();

static void Main() 
{
    RunProgram(cts.Token);
    SomeFinishedFuncion();
}

async void RunProgram(CancellationToken cToken)
{
    try
    {
        object thing = await WaitForSemaphoreSlim(cToken);
    }
    catch(OperationCanceledException)
    {
         //When a token is cancelled this exception is raised.
    }   
}

async Task<object> WaitForSemaphoreSlim(CancellationToken cToken) 
{
    object thing;
    // do stuff
    
    await _semaphoreClick.WaitAsync(cToken);
        
    // do stuff
    return thing;
}


void OnStopProgram() 
{
    cts.Cancel();
}

Side note: be careful using async void: Avoid Async Void