[Solved] Why is the Windows Forms UI blocked when executing Task with ContinueWith?

Question

Since you already create (and save) your tasks, the easiest fix would be to await them for each iteration of your while loop:

while (run)
{
    action2();

    foreach (Task t in continutask)
        await t;
}

That way, when all pings completed (successful or not) you start the entire process again – without delay.

One more thing: You could add a textBox1.ScrollToEnd(); to PrintResult

Since there is a lot of room for improvement, below is a rewritten and simplified example. I’ve removed a lot of unused variables (e.g. seqnum) and made the PingStart method completely asynchronous. I also replaced your ListBox with a TextBox for easier testing, so you might want to revert that in your code.

This still isn’t the cleanest of all possible implementations (mainly because of the global run) but it should show you how to do things “more async” 🙂

private async void buttonStart_Click(object sender, EventArgs e)
{
    // If the ping loops are already running, don't start them again
    if (run)
        return;

    run = true;

    // Get all IPs (in my case from a TextBox instead of a ListBox
    string[] ips = txtIPs.Text.Split(new[] {"\r\n"}, StringSplitOptions.RemoveEmptyEntries);

    // Create an array to store all Tasks
    Task[] pingTasks = new Task[ips.Length];

    // Loop through all IPs
    for(int i = 0; i < ips.Length; i++)
    {
        string ip = ips[i];

        // Launch and store a task for each IP
        pingTasks[i] = Task.Run(async () =>
            {
                // while run is true, ping over and over again
                while (run)
                {
                    // Ping IP and wait for result (instead of storing it an a global array)
                    var result = await PingStart(ip);

                    // Print the result (here I removed seqnum)
                    PrintResult(result.Item2);

                    // This line is optional. 
                    // If you want to blast pings without delay, 
                    // you can remove it
                    await Task.Delay(1000);
                }
            }
        );
    }

    // Wait for all loops to end after setting run = false.
    // You could add a mechanism to call isPing.SendAsyncCancel() instead of waiting after setting run = false
    foreach (Task pingTask in pingTasks)
        await pingTask;
}

// (very) simplified explanation of changes:
// async = this method is async (and therefore awaitable)
// Task<> = This async method returns a result of type ...
// Tuple<bool, string> = A generic combination of a bool and a string
// (-)int seqnum = wasn't used so I removed it
private async Task<Tuple<bool, string>> PingStart(string ip)
{
    Ping isPing = new Ping();
    const int timeout = 2000;
    const string data = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
    var buffer = Encoding.ASCII.GetBytes(data);
    PingOptions options = new PingOptions {DontFragment = false};

    // await SendPingAsync = Ping and wait without blocking
    PingReply reply = await isPing.SendPingAsync(ip, timeout, buffer, options);
    string rtt = reply.RoundtripTime.ToString();

    bool success = reply.Status == IPStatus.Success;
    string text;

    if (success)
    {
        text = $"{ip}" + " Success!" + $" rtt: [{rtt}]" + $"Thread: {Thread.CurrentThread.GetHashCode()} Is pool thread: {Thread.CurrentThread.IsThreadPoolThread}";
    }
    else
    {
        text = $"{ip}" + $" Not Successful! Status: {reply.Status}" + $"Thread: {Thread.CurrentThread.GetHashCode()} Is pool thread: {Thread.CurrentThread.IsThreadPoolThread}";
    }

    // return if the ping was successful and the status message
    return new Tuple<bool, string>(success, text);
}

This way you will have a loop for each IP that will continue independently of each other until run is set to false.

Accepted Answer