C Wait until condition is true

Question

I am trying to write a code that executes when a condition is met  Currently  I am using while   loop  which I know is not very efficient  I am also looking at AutoResetEvent   but i don t know how to implement it such that it keeps checking until the condition is true    The code also happens to live inside an async method  so may be some kind of await could work    private async void btnOk Click object sender  EventArgs e               Do some work         Task lt string gt  task   Task Run      gt  GreatBigMethod             string GreatBigMethod   await task              Wait until condition is false         while   isExcelInteractive                          Console WriteLine  Excel is busy                          Do work         Console WriteLine  YAY             private bool isExcelInteractive                 try                       Globals ThisWorkbook Application Interactive   Globals ThisWorkbook Application Interactive              return true     Excel is free                   catch                       return false     Excel will throw an exception  meaning its busy                   I need to find a way to keep checking isExcelInteractive   without CPU stuck in a loop    Note  There is no event handler in Excel that would be raised when it is not in edit mode

User · Answer

Ended up writing this today and seems to be ok. Your usage could be:

await TaskEx.WaitUntil(isExcelInteractive);

code (including the inverse operation)

public static class TaskEx
{
    /// <summary>
    /// Blocks while condition is true or timeout occurs.
    /// </summary>
    /// <param name="condition">The condition that will perpetuate the block.</param>
    /// <param name="frequency">The frequency at which the condition will be check, in milliseconds.</param>
    /// <param name="timeout">Timeout in milliseconds.</param>
    /// <exception cref="TimeoutException"></exception>
    /// <returns></returns>
    public static async Task WaitWhile(Func<bool> condition, int frequency = 25, int timeout = -1)
    {
        var waitTask = Task.Run(async () =>
        {
            while (condition()) await Task.Delay(frequency);
        });

        if(waitTask != await Task.WhenAny(waitTask, Task.Delay(timeout)))
            throw new TimeoutException();
    }

    /// <summary>
    /// Blocks until condition is true or timeout occurs.
    /// </summary>
    /// <param name="condition">The break condition.</param>
    /// <param name="frequency">The frequency at which the condition will be checked.</param>
    /// <param name="timeout">The timeout in milliseconds.</param>
    /// <returns></returns>
    public static async Task WaitUntil(Func<bool> condition, int frequency = 25, int timeout = -1)
    {
        var waitTask = Task.Run(async () =>
        {
            while (!condition()) await Task.Delay(frequency);
        });

        if (waitTask != await Task.WhenAny(waitTask, 
                Task.Delay(timeout))) 
            throw new TimeoutException();
    }
}

Example usage: https://dotnetfiddle.net/Vy8GbV

User · Answer

You can use an async result and a delegate for this   If you read up on the documentation it should make it pretty clear what to do   I can write up some sample code if you like and attach it to this answer   Action isExcelInteractive   IsExcelInteractive   private async void btnOk Click object sender  EventArgs e        IAsyncResult result   isExcelInteractive BeginInvoke ItIsDone  null       result AsyncWaitHandle WaitOne        Console WriteLine  YAY        static void IsExcelInteractive       while  something is false     do your check here             if something is true            return true          Thread Sleep 1      void ItIsDone IAsyncResult result       this isExcelInteractive EndInvoke result       Apologies if this code isn t 100  complete  I don t have Visual Studio on this computer  but hopefully it gets you where you need to get to

User · Answer

You can use thread waiting handler  private readonly System Threading EventWaitHandle waitHandle   new System Threading AutoResetEvent false   private void btnOk Click object sender  EventArgs e           Do some work     Task lt string gt  task   Task Run      gt  GreatBigMethod         string GreatBigMethod   await task          Wait until condition is false     waitHandle WaitOne        Console WriteLine  Excel is busy        waitHandle Reset            Do work     Console WriteLine  YAY         then some other job need to set your handler  void isExcelInteractive            Do your check    waitHandle Set       Update  If you want use this solution  you have to call isExcelInteractive   continuously with specific interval   var actions   new    isExcelInteractive       gt  Thread Sleep 25    foreach  var action in actions                                              action

User · Answer

This implementation is totally based on Sinaesthetic s  but adding CancellationToken and keeping the same execution thread and context  that is  delegating the use of Task Run   up to the caller depending on whether condition needs to be evaluated in the same thread or not   Also  notice that  if you don t really need to throw a TimeoutException and breaking the loop is enough  you might want to make use of cts CancelAfter   or new CancellationTokenSource millisecondsDelay  instead of using timeoutTask with Task Delay plus Task WhenAny   public static class AsyncUtils            lt summary gt              Blocks while condition is true or task is canceled           lt  summary gt           lt param name  ct  gt              Cancellation token           lt  param gt           lt param name  condition  gt              The condition that will perpetuate the block           lt  param gt           lt param name  pollDelay  gt              The delay at which the condition will be polled  in milliseconds           lt  param gt           lt returns gt               lt see cref  Task    gt            lt  returns gt      public static async Task WaitWhileAsync CancellationToken ct  Func lt bool gt  condition  int pollDelay   25                try                       while  condition                                  await Task Delay pollDelay  ct  ConfigureAwait true                                   catch  TaskCanceledException                           ignore  Task Delay throws this exception when ct IsCancellationRequested   true                In this case  we only want to stop polling and finish this async Task                            lt summary gt              Blocks until condition is true or task is canceled           lt  summary gt           lt param name  ct  gt              Cancellation token           lt  param gt           lt param name  condition  gt              The condition that will perpetuate the block           lt  param gt           lt param name  pollDelay  gt              The delay at which the condition will be polled  in milliseconds           lt  param gt           lt returns gt               lt see cref  Task    gt            lt  returns gt      public static async Task WaitUntilAsync CancellationToken ct  Func lt bool gt  condition  int pollDelay   25                try                       while   condition                                  await Task Delay pollDelay  ct  ConfigureAwait true                                   catch  TaskCanceledException                           ignore  Task Delay throws this exception when ct IsCancellationRequested   true                In this case  we only want to stop polling and finish this async Task                            lt summary gt              Blocks while condition is true or timeout occurs           lt  summary gt           lt param name  ct  gt              The cancellation token           lt  param gt           lt param name  condition  gt              The condition that will perpetuate the block           lt  param gt           lt param name  pollDelay  gt              The delay at which the condition will be polled  in milliseconds           lt  param gt           lt param name  timeout  gt              Timeout in milliseconds           lt  param gt           lt exception cref  TimeoutException  gt              Thrown after timeout milliseconds          lt  exception gt           lt returns gt               lt see cref  Task    gt            lt  returns gt      public static async Task WaitWhileAsync CancellationToken ct  Func lt bool gt  condition  int pollDelay  int timeout                if  ct IsCancellationRequested                        return                     using  CancellationTokenSource cts   CancellationTokenSource CreateLinkedTokenSource ct                         Task waitTask       WaitWhileAsync cts Token  condition  pollDelay               Task timeoutTask    Task Delay timeout  cts Token               Task finishedTask   await Task WhenAny waitTask  timeoutTask  ConfigureAwait true                if   ct IsCancellationRequested                                cts Cancel                                  Cancel unfinished task                 await finishedTask ConfigureAwait true      Propagate exceptions                 if  finishedTask    timeoutTask                                        throw new TimeoutException                                                              lt summary gt              Blocks until condition is true or timeout occurs           lt  summary gt           lt param name  ct  gt              Cancellation token          lt  param gt           lt param name  condition  gt              The condition that will perpetuate the block           lt  param gt           lt param name  pollDelay  gt              The delay at which the condition will be polled  in milliseconds           lt  param gt           lt param name  timeout  gt              Timeout in milliseconds           lt  param gt           lt exception cref  TimeoutException  gt              Thrown after timeout milliseconds          lt  exception gt           lt returns gt               lt see cref  Task    gt            lt  returns gt      public static async Task WaitUntilAsync CancellationToken ct  Func lt bool gt  condition  int pollDelay  int timeout                if  ct IsCancellationRequested                        return                     using  CancellationTokenSource cts   CancellationTokenSource CreateLinkedTokenSource ct                         Task waitTask       WaitUntilAsync cts Token  condition  pollDelay               Task timeoutTask    Task Delay timeout  cts Token               Task finishedTask   await Task WhenAny waitTask  timeoutTask  ConfigureAwait true                if   ct IsCancellationRequested                                cts Cancel                                  Cancel unfinished task                 await finishedTask ConfigureAwait true      Propagate exceptions                 if  finishedTask    timeoutTask                                        throw new TimeoutException

User · Answer

Try this void Function         while  condition                 await Task Delay 1            This will make the program wait until the condition is not true  You can just invert it by adding a  quot   quot  infront of the condition so that it will wait until the condition is true

User · Answer

At least you can change your loop from a busy-wait to a slow poll   For example       while   isExcelInteractive                  Console WriteLine  Excel is busy            await Task Delay 25

User · Answer

After digging a lot of stuff  finally  I came up with a good solution that doesn t hang the CI    Suit it to your needs   public static Task WaitUntil lt T gt  T elem  Func lt T  bool gt  predicate  int seconds   10        var tcs   new TaskCompletionSource lt int gt         using var cancellationTokenSource   new CancellationTokenSource TimeSpan FromSeconds seconds                  cancellationTokenSource Token Register      gt                        tcs SetException                  new TimeoutException   Waiting predicate  predicate  for  elem GetType    timed out                  tcs TrySetCanceled                         while  cancellationTokenSource IsCancellationRequested                        try                               if   predicate elem                                         continue                                              catch Exception e                                tcs TrySetException e                              tcs SetResult 0               break                     return tcs Task

User · Answer

you can use SpinUntil which is buildin in the  net-framework  Please note  This method causes high cpu-workload

[c#] C# Wait until condition is true

code (including the inverse operation)

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