C# – Wait for two threads to finish

c++multithreading

If you have one main thread that starts two other threads.
what is the cleanest way to make the primary thread wait for the two other threads?

I could use bgndworker and sleep spinner that checks for both the bgnd workers's IsBusy, but I would think there's a better way.

EDIT
Some more requirements:

The main thread has some other work to do (e.g. GUI).
The two spawned threads should be able to report exceptions and return result values

Best Solution

Quick example using Thread.Join();

        Thread t1 = new Thread(new ThreadStart(delegate()
        {
            System.Threading.Thread.Sleep(2000);
        }));

        Thread t2 = new Thread(new ThreadStart(delegate()
        {
            System.Threading.Thread.Sleep(4000);
        }));

        t1.Start();
        t2.Start();

        t1.Join();
        t2.Join();

EDIT Another 3example using Wait Handles:

            ManualResetEvent[] waitHandles = new ManualResetEvent[]{
            new ManualResetEvent(false),
            new ManualResetEvent(false)
        };

        Thread t1 = new Thread(new ParameterizedThreadStart(delegate(object state)
        {
            ManualResetEvent handle = (ManualResetEvent)state;
            System.Threading.Thread.Sleep(2000);
            handle.Set();
        }));

        Thread t2 = new Thread(new ParameterizedThreadStart(delegate(object state)
        {
            ManualResetEvent handle = (ManualResetEvent)state;
            System.Threading.Thread.Sleep(4000);
            handle.Set();
        }));

        t1.Start(waitHandles[0]);
        t2.Start(waitHandles[1]);

        WaitHandle.WaitAll(waitHandles);

        Console.WriteLine("Finished");

Related Solutions

C# – Cross-thread operation not valid: Control accessed from a thread other than the thread it was created on

As per Prerak K's update comment (since deleted):

I guess I have not presented the question properly.

Situation is this: I want to load data into a global variable based on the value of a control. I don't want to change the value of a control from the child thread. I'm not going to do it ever from a child thread.

So only accessing the value so that corresponding data can be fetched from the database.

The solution you want then should look like:

UserContrl1_LOadDataMethod()
{
    string name = "";
    if(textbox1.InvokeRequired)
    {
        textbox1.Invoke(new MethodInvoker(delegate { name = textbox1.text; }));
    }
    if(name == "MyName")
    {
        // do whatever
    }
}

Do your serious processing in the separate thread before you attempt to switch back to the control's thread. For example:

UserContrl1_LOadDataMethod()
{
    if(textbox1.text=="MyName") //<<======Now it wont give exception**
    {
        //Load data correspondin to "MyName"
        //Populate a globale variable List<string> which will be
        //bound to grid at some later stage
        if(InvokeRequired)
        {
            // after we've done all the processing, 
            this.Invoke(new MethodInvoker(delegate {
                // load the control with the appropriate data
            }));
            return;
        }
    }
}

C++ – How to propagate exceptions between threads

C++11 introduced the exception_ptr type that allows to transport exceptions between threads:

#include<iostream>
#include<thread>
#include<exception>
#include<stdexcept>

static std::exception_ptr teptr = nullptr;

void f()
{
    try
    {
        std::this_thread::sleep_for(std::chrono::seconds(1));
        throw std::runtime_error("To be passed between threads");
    }
    catch(...)
    {
        teptr = std::current_exception();
    }
}

int main(int argc, char **argv)
{
    std::thread mythread(f);
    mythread.join();

    if (teptr) {
        try{
            std::rethrow_exception(teptr);
        }
        catch(const std::exception &ex)
        {
            std::cerr << "Thread exited with exception: " << ex.what() << "\n";
        }
    }

    return 0;
}

Because in your case you have multiple worker threads, you will need to keep one exception_ptr for each of them.

Note that exception_ptr is a shared ptr-like pointer, so you will need to keep at least one exception_ptr pointing to each exception or they will be released.

Microsoft specific: if you use SEH Exceptions (/EHa), the example code will also transport SEH exceptions like access violations, which may not be what you want.

Best Solution

Related Solutions

C# – Cross-thread operation not valid: Control accessed from a thread other than the thread it was created on

C++ – How to propagate exceptions between threads

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