How can I wait for a thread to finish with NET

Question

I ve never really used threading before in C  where I need to have two threads  as well as the main UI thread  Basically  I have the following  public void StartTheActions          Starting thread 1       Thread t1   new Thread new ThreadStart action1      t1 Start          Now  I want for the main thread  which is calling  StartTheActions  method       to wait for  t1  to finish  I ve created an event in  action1  for this       The I wish  t2  to start       Thread t2   new Thread new ThreadStart action2      t2 Start       So  essentially  how can I have a thread wait for another one to finish  What is the best way to do this

User · Answer

The previous two answers are great and will work for simple scenarios. There are other ways to synchronize threads, however. The following will also work:

public void StartTheActions()
{
    ManualResetEvent syncEvent = new ManualResetEvent(false);

    Thread t1 = new Thread(
        () =>
        {
            // Do some work...
            syncEvent.Set();
        }
    );
    t1.Start();

    Thread t2 = new Thread(
        () =>
        {
            syncEvent.WaitOne();

            // Do some work...
        }
    );
    t2.Start();
}

ManualResetEvent is one of the various WaitHandle's that the .NET framework has to offer. They can provide much richer thread synchronization capabilities than the simple, but very common tools like lock()/Monitor, Thread.Join, etc.

They can also be used to synchronize more than two threads, allowing complex scenarios such as a 'master' thread that coordinates multiple 'child' threads, multiple concurrent processes that are dependent upon several stages of each other to be synchronized, etc.

User · Answer

Add  t1 Join          Wait until thread t1 finishes   after you start it  but that won t accomplish much as it s essentialy the same result as running on the main thread   I can highly recommended reading Joe Albahari s Threading in C  free e-book  if you want to gain an understanding of threading in  NET

User · Answer

Try this   List lt Thread gt  myThreads   new List lt Thread gt      foreach  Thread curThread in myThreads        curThread Start       foreach  Thread curThread in myThreads        curThread Join

User · Answer

If using from  NET 4 this sample can help you  class Program       static void Main string   args                Task task1   Task Factory StartNew      gt  doStuff             Task task2   Task Factory StartNew      gt  doStuff             Task task3   Task Factory StartNew      gt  doStuff             Task WaitAll task1  task2  task3           Console WriteLine  quot All threads complete quot               static void doStuff                    Do stuff here          From  Create multiple threads and wait all of them to complete

User · Answer

Here s a simple example that waits for a tread to finish  within the same class   It also makes a call to another class in the same namespace   I included the  quot using quot  statements so it can execute as a Windows Forms form as long as you create button1  using System  using System Collections Generic  using System Threading Tasks  using System Windows Forms  using System Threading   namespace ClassCrossCall        public partial class Form1   Form               int number   0     This is an intentional problem  included                            for demonstration purposes         public Form1                         InitializeComponent                       private void Form1 Load object sender  EventArgs e                        button1 Text    quot Initialized quot                      private void button1 Click object sender  EventArgs e                        button1 Text    quot Clicked quot               button1 Refresh                Thread Sleep 400               List lt Task gt  taskList   new List lt Task gt                 taskList Add Task Factory StartNew      gt  update thread 2000                 taskList Add Task Factory StartNew      gt  update thread 4000                 Task WaitAll taskList ToArray                 worker update button this  number                      public void update thread int ms                           It s important to check the scope of all variables             number   ms     This could be either 2000 or 4000  Race condition              Thread Sleep ms                        class worker               public static void update button Form1 form  int number                        form button1 Text     quot  number  quot

User · Answer

You want the Thread Join   method  or one of its overloads

User · Answer

I would have your main thread pass a callback method to your first thread  and when it s done  it will invoke the callback method on the mainthread  which can launch the second thread  This keeps your main thread from hanging while its waiting for a Join or Waithandle  Passing methods as delegates is a useful thing to learn with C  anyway

User · Answer

I took a little different approach  There is a counter option in previous answers  and I just applied it a bit differently  I was spinning off numerous threads and incremented a counter and decremented a counter as a thread started and stopped  Then in the main method I wanted to pause and wait for threads to complete I did  while  threadCounter  gt  0        Thread Sleep 500      Make it pause for half second so that we don   t spin the CPU out of control     This is documented in my blog post  http   www adamthings com post 2012 07 11 ensure-threads-have-finished-before-method-continues-in-c

User · Answer

I can see five options available  1  Thread Join As with Mitch s answer  But this will block your UI thread  however you get a Timeout built in for you   2  Use a WaitHandle ManualResetEvent is a WaitHandle as jrista suggested  One thing to note is if you want to wait for multiple threads  WaitHandle WaitAll   won t work by default  as it needs an MTA thread  You can get around this by marking your Main   method with MTAThread - however this blocks your message pump and isn t recommended from what I ve read   3  Fire an event See this page by Jon Skeet about events and multi-threading  It s possible that an event can become unsubcribed between the if and the EventName this EventArgs Empty  - it s happened to me before   Hopefully these compile  I haven t tried  public class Form1   Form       int  count       void ButtonClick object sender  EventArgs e                ThreadWorker worker   new ThreadWorker            worker ThreadDone    HandleThreadDone           Thread thread1   new Thread worker Run           thread1 Start              count   1             void HandleThreadDone object sender  EventArgs e                   You should get the idea this is just an example         if   count    1                        ThreadWorker worker   new ThreadWorker                worker ThreadDone    HandleThreadDone               Thread thread2   new Thread worker Run               thread2 Start                  count                         class ThreadWorker               public event EventHandler ThreadDone           public void Run                            Do a task              if  ThreadDone    null                  ThreadDone this  EventArgs Empty                       4  Use a delegate public class Form1   Form       int  count       void ButtonClick object sender  EventArgs e                ThreadWorker worker   new ThreadWorker             Thread thread1   new Thread worker Run           thread1 Start HandleThreadDone             count   1             void HandleThreadDone                    As before - just a simple example         if   count    1                        ThreadWorker worker   new ThreadWorker                 Thread thread2   new Thread worker Run               thread2 Start HandleThreadDone                 count                         class ThreadWorker                  Switch to your favourite Action lt T gt  or Func lt T gt          public void Run object state                           Do a task              Action completeAction    Action state              completeAction Invoke                       If you do use the  count method  it might be an idea  to be safe  to increment it using Interlocked Increment ref  count  I d be interested to know the difference between using delegates and events for thread notification  the only difference I know are events are called synchronously   5  Do it asynchronously instead The answer to this question has a very clear description of your options with this method   Delegate Events on the wrong thread The event delegate way of doing things will mean your event handler method is on thread1 thread2 not the main UI thread  so you will need to switch back right at the top of the HandleThreadDone methods     Delegate example if  InvokeRequired        Invoke new Action HandleThreadDone        return

User · Answer

When I want the UI to be able to update its display while waiting for a task to complete  I use a while-loop that tests IsAlive on the thread       Thread t   new Thread      gt  someMethod parameters        t Start        while  t IsAlive                Thread Sleep 500           Application DoEvents

[c#] How can I wait for a thread to finish with .NET?

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