How do I keep waiting for a list of tasks until one is successful?

Question

0.00/5 (No votes)

See more:

I have a list of tasks that I would like to run in parallel. When one completes, I would like to examine the result of that one task. If it is successful, I would like to end all the other tasks, which I think should be no problem using a CancellationToken. If the task is unsuccessful, I'd like to go on waiting for the other tasks.

Can anyone offer advice on how best to approach this?

Having tried to use async/await, I am thinking this may be a case where I need to go back to something more 'manual', perhaps accumulating results in a suitable concurrent collection and building logic around that.

What I have tried:

I've come up with this so far but I don't know what to do in the case where a task completes but is unsuccessful. I'm thinking I need to remove it from the list of tasks that I wait for but this causes an IndexOutOfRangeException. Also, the double use of await worries me because I don't recall ever reading any code that does that.

public static class Program
{
	private const int NumTasks = 10;
	private readonly static int[] TaskDelays = { 5000, 6000, 6000, 6000, 8000, 6000, 10000, 12000, 1000, 22000 };
	private static List<Task<bool>> _tasks = new();

	static async Task Main( string[] args )
	{
		Debug.WriteLine( "****** START ******" );
		await RunAsync();
		Debug.WriteLine( "****** END ******" );
		Console.ReadKey();
	}

	private static async Task RunAsync()
	{
		for( int i = 0; i < NumTasks; ++i )
			_tasks.Add( ReturnDelayedResultAsync( i ) );
		int numCompletedTasks = 0;
		bool gotSuccess = false;
		while( !gotSuccess && numCompletedTasks < NumTasks )
		{
			Debug.WriteLine( $"Awaiting {_tasks.Count} tasks" + TaskStatusString() );
			bool result = await await Task.WhenAny( _tasks );
			++numCompletedTasks;
			if( result )
			{
				Debug.WriteLine( $"Task successfully connected; number of completed tasks = {numCompletedTasks}." );
				gotSuccess = true;
			}
			else
			{
				// Don't know what to do here.
			}
		}
	}

	private static async Task<bool> ReturnDelayedResultAsync( int index )
	{
		Debug.WriteLine( $"Starting task {index}." );
		await Task.Delay( TaskDelays[ index ] );
		Debug.WriteLine( $"Ending task {index}." );
		return index == 6;
	}

	private static string TaskStatusString()
	{
		StringBuilder sb = new StringBuilder( ": " );
		for( int i = 0; i < NumTasks; ++i )
			sb.Append( i.ToString() + "=" + _tasks[ i ].Status.ToString() + "; " );
		sb.Append( Environment.NewLine );
		return sb.ToString();
	}
}

Posted 9-Nov-21 21:35pm

Patrick Skelton

Updated 9-Nov-21 23:16pm

Add a Solution

1 solution

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Richard Deeming · Accepted Answer · 2021-11-09T23:16:00

Task.WhenAny will return the first task which completed. You can use that to remove it from the list.

C#

while (!gotSuccess && _tasks.Count != 0)
{
    Task<bool> completedTask = await Task.WhenAny(_tasks);
    _tasks.Remove(completedTask);
    ++numCompletedTasks;
    
    if (await completedTask) // Or: if (completedTask.Result)
    {
        Debug.WriteLine( $"Task successfully connected; number of completed tasks = {numCompletedTasks}." );
        gotSuccess = true;
    }
}

If you want to cancel the other tasks once one task has succeeded, then a CancellationToken is the correct approach:

C#

private static async Task<bool> ReturnDelayedResultAsync( int index, CancellationToken cancellationToken )
{
	Debug.WriteLine( $"Starting task {index}." );
	await Task.Delay( TaskDelays[ index ], cancellationToken );
	Debug.WriteLine( $"Ending task {index}." );
	return index == 6;
}

private static async Task RunAsync()
{
    CancellationTokenSource cts = new();
    for( int i = 0; i < NumTasks; ++i )
        _tasks.Add( ReturnDelayedResultAsync( i, cts.Token ) );

    int numCompletedTasks = 0;
    bool gotSuccess = false;
    while( !gotSuccess && _tasks.Count != 0 )
    {
        Debug.WriteLine( $"Awaiting {_tasks.Count} tasks" + TaskStatusString() );
        Task<bool> completedTask = await Task.WhenAny(_tasks);
        _tasks.Remove(completedTask);
        ++numCompletedTasks;
        
        if (await completedTask) // Or: if (completedTask.Result)
        {
            Debug.WriteLine( $"Task successfully connected; number of completed tasks = {numCompletedTasks}." );
            gotSuccess = true;
            cts.Cancel();
        }
    }
}