Returning a value from thread

Question

How do I return a value from a thread

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Can use This Code    private Object MyThread Object Data                  Object response   null          Thread newThread   new Thread      gt                        response   MyFunction Data                 MyFunction Is Function that you Define                     newThread Start            newThread Join            return response

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If you don t want to use a BackgroundWorker  and just use a regular Thread  then you can fire an event to return data like this   using System  using System Collections Generic  using System ComponentModel  using System Data  using System Drawing  using System Text  using System Windows Forms  using System Threading   namespace ThreadWithDataReturnExample       public partial class Form1   Form               private Thread thread1   null           public Form1                         InitializeComponent                 thread1   new Thread new ThreadStart this threadEntryPoint                Thread1Completed    new AsyncCompletedEventHandler thread1 Thread1Completed                      private void startButton Click object sender  EventArgs e                        thread1 Start                  Alternatively  you could pass some object               in such as Start someObject                 With apprioriate locking  or protocol where               no other threads access the object until               an event signals when the thread is complete                any other class with a reference to the object                would be able to access that data                But instead  I m going to use AsyncCompletedEventArgs                in an event that signals completion                    void thread1 Thread1Completed object sender  AsyncCompletedEventArgs e                        if  this InvokeRequired                 marshal the call if we are not on the GUI thread                                 BeginInvoke new AsyncCompletedEventHandler thread1 Thread1Completed                     new object     sender  e                               else                                 display error if error occurred                   if no error occurred  process data                 if  e Error    null                     then success                      MessageBox Show  Worker thread completed successfully                        DataYouWantToReturn someData   e UserState as DataYouWantToReturn                      MessageBox Show  Your data my lord      someData someProperty                                      else  error                                       MessageBox Show  The following error occurred     Environment NewLine   e Error ToString                                                         region I would actually move all of this into it s own class             private void threadEntryPoint                                   do a bunch of stuff                    when you are done                    initialize object with data that you want to return                 DataYouWantToReturn dataYouWantToReturn   new DataYouWantToReturn                    dataYouWantToReturn someProperty    more data                      signal completion by firing an event                 OnThread1Completed new AsyncCompletedEventArgs null  false  dataYouWantToReturn                                    lt summary gt                  Occurs when processing has finished or an error occurred                   lt  summary gt              public event AsyncCompletedEventHandler Thread1Completed              protected virtual void OnThread1Completed AsyncCompletedEventArgs e                                  copy locally                 AsyncCompletedEventHandler handler   Thread1Completed                  if  handler    null                                        handler this  e                                            endregion

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A simple solution is to pass a parameter by ref to the function that is running in the thread and change its value in the thread             create a list of threads         List lt Thread gt  threads   new List lt Thread gt                 declare the ref params         bool is1   false          bool is2   false           threads Add new Thread      gt  myFunction someVar  ref is1             threads Add new Thread      gt  myFunction someVar  ref is2              threads ForEach x   gt  x Start                 wait for threads to finish         threads ForEach x   gt  x Join                check the ref params         if   is1                        do something                    if   is2                         do somethign else             If you can t change the function that is running in the tread  you can wrap it another function    bool theirFunction var someVar      return false       void myFunction var someVar ref bool result     result   theirFunction myVar

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My favorite class  runs any method on another thread with just 2 lines of code   class ThreadedExecuter lt T gt  where T   class       public delegate void CallBackDelegate T returnValue       public delegate T MethodDelegate        private CallBackDelegate callback      private MethodDelegate method       private Thread t       public ThreadedExecuter MethodDelegate method  CallBackDelegate callback                this method   method          this callback   callback          t   new Thread this Process             public void Start                 t Start              public void Abort                 t Abort            callback null     can be left out depending on your needs           private void Process                 T stuffReturned   method            callback stuffReturned             usage      void startthework                 ThreadedExecuter lt string gt  executer   new ThreadedExecuter lt string gt  someLongFunction  longFunctionComplete           executer Start              string someLongFunction                 while  workComplete              WorkWork            return resultOfWork            void longFunctionComplete string s                PrintWorkComplete s           Beware that longFunctionComplete will NOT execute on the same thread as starthework   For methods that take parameters you can always use closures  or expand the class

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Here is a simple example using a delegate      void Main        DoIt d1   Doer DoThatThang     DoIt d2   Doer DoThatThang      IAsyncResult r1   d1 BeginInvoke  5  null  null       IAsyncResult r2   d2 BeginInvoke  10  null  null        Thread Sleep  1000        var s1   d1 EndInvoke  r1       var s2   d2 EndInvoke  r2        s1 Dump       You told me 5    s2 Dump       You told me 10    public delegate string DoIt  int x     public class Doer     public static string DoThatThang  int x            return  You told me     x ToString            There s a terrific series on threading at Threading in C

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I m no kind of expert in threading  that s why I did it like this   I created a Settings file and   Inside the new thread   Setting Default ValueToBeSaved  Setting Default Save      Then I pick up that value whenever I need it

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The BackgroundWorker is nice when developing for Windows Forms   Say you wanted to pass a simple class back and forth   class Anything          Number and Text are for instructional purposes only     public int Number   get  set        public string Text   get  set           Data can be any object - even another class     public object Data   get  set        I wrote up a short class that does the following    Create or Clear a list Start a loop In loop  create a new item for the list In loop  create a thread In loop  send the item as a parameter to the thread In loop  start the thread In loop  add thread to list to watch After loop  join each thread After all joins have completed  display the results   From inside the thread routine    Call lock so that only 1 thread can enter this routine at a time  others have to wait  Post information about the item  Modify the item  When the thread completes  the data is displayed on the console    Adding a delegate can be useful for posting your data directly back to your main thread  but you may need to use Invoke if some of the data items are not thread safe   class AnyTask        private object m lock       public AnyTask             m lock   new object                 Something to use the delegate     public event MainDelegate OnUpdate       public void Test Function int count            var list   new List lt Thread gt  count           for  var i   0  i  lt  count  i                  var thread   new Thread new ParameterizedThreadStart Thread Task                var item   new Anything                     Number   i                  Text   String Format  Test Function   0    i                             thread Start item               list Add thread                     foreach  var thread in list                thread Join                         private void MainUpdate Anything item  bool original            if  OnUpdate    null                OnUpdate item  original                        private void Thread Task object parameter            lock  m lock                var item    Anything parameter              MainUpdate item  true               item Text   String Format   0   Thread Task   1    item Text  item Number               item Number   0              MainUpdate item  false                        To test this  create a little Console Application  and put this in the Program cs file      A delegate makes life simpler delegate void MainDelegate Anything sender  bool original    class Program        private const int COUNT   15      private static List lt Anything gt  m list       static void Main string   args            m list   new List lt Anything gt  COUNT           var obj   new AnyTask            obj OnUpdate    new MainDelegate ThreadMessages           obj Test Function COUNT           Console WriteLine            foreach  var item in m list                Console WriteLine   Complete      item Text                     Console WriteLine  Press any key to exit             Console ReadKey               private static void ThreadMessages Anything item  bool original            if  original                Console WriteLine   main method      item Text             else               m list Add item                        Here is a screenshot of what I got with this     I hope others can understand what I ve tried to explain   I enjoy working on threads and using delegates  They make C  a lot of fun   Appendix  For VB Coders  I wanted to see what was involved in writing the code above as a VB Console Application  The conversion involved a few things I didn t expect  so I will update this thread here for those wanting to know how to thread in VB   Imports System Threading  Delegate Sub MainDelegate sender As Anything  original As Boolean   Class Main      Private Const COUNT As Integer   15     Private Shared m list As List Of Anything       Public Shared Sub Main args As String            m list   New List Of Anything  COUNT          Dim obj As New AnyTask           AddHandler obj OnUpdate  New MainDelegate AddressOf ThreadMessages          obj Test Function COUNT          Console WriteLine           For Each item As Anything In m list             Console WriteLine   Complete      item Text          Next         Console WriteLine  Press any key to exit            Console ReadKey       End Sub      Private Shared Sub ThreadMessages item As Anything  original As Boolean          If original Then             Console WriteLine   main method      item Text          Else             m list Add item          End If     End Sub  End Class  Class AnyTask      Private m lock As Object      Public Sub New           m lock   New Object       End Sub       Something to use the delegate     Public Event OnUpdate As MainDelegate      Public Sub Test Function count As Integer          Dim list As New List Of Thread  count          For i As Int32   0 To count - 1             Dim thread As New Thread New ParameterizedThreadStart AddressOf Thread Task               Dim item As New Anything               item Number   i             item Text   String Format  Test Function   0    i              thread Start item              list Add thread          Next         For Each thread As Thread In list             thread Join           Next     End Sub      Private Sub MainUpdate item As Anything  original As Boolean          RaiseEvent OnUpdate item  original      End Sub      Private Sub Thread Task parameter As Object          SyncLock m lock             Dim item As Anything   DirectCast parameter  Anything              MainUpdate item  True              item Text    String  Format   0   Thread Task   1    item Text  item Number              item Number   0             MainUpdate item  False          End SyncLock     End Sub  End Class   Class Anything       Number and Text are for instructional purposes only     Public Property Number   As Integer         Get             Return m Number         End Get         Set value As Integer              m Number   value         End Set     End Property     Private m Number As Integer     Public Property Text   As String         Get             Return m Text         End Get         Set value As String              m Text   value         End Set     End Property     Private m Text As String       Data can be anything or another class     Public Property Data   As Object         Get             Return m Data         End Get         Set value As Object              m Data   value         End Set     End Property     Private m Data As Object End Class

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class Program       static void Main string   args                string returnValue   null         new Thread                 gt                            returnValue  test                   Start            Console WriteLine returnValue           Console ReadKey               public static string test                 return  Returning From Thread called method

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With the latest  NET Framework  it is possible to return a value from a separate thread using a Task  where the Result property blocks the calling thread until the task finishes     Task lt MyClass gt  task   Task lt MyClass gt  Factory StartNew      gt            string s    my message         double d   3 14159        return new MyClass   Name   s  Number   d            MyClass test   task Result    For details  please see http   msdn microsoft com en-us library dd537613 v vs 110  aspx

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I would use the BackgroundWorker approach and return the result in e Result   EDIT   This is commonly associated with WinForms and WPF  but can be used by any type of  NET application   Here s sample code for a console app that uses BackgroundWorker   using System  using System Threading  using System ComponentModel  using System Collections Generic  using System Text   namespace BGWorker       class Program               static bool done   false           static void Main string   args                        BackgroundWorker bg   new BackgroundWorker                bg DoWork    new DoWorkEventHandler bg DoWork               bg RunWorkerCompleted    new RunWorkerCompletedEventHandler bg RunWorkerCompleted               bg RunWorkerAsync                 while   done                                Console WriteLine  Waiting in Main  tid     Thread CurrentThread ManagedThreadId                   Thread Sleep 100                                    static void bg RunWorkerCompleted object sender  RunWorkerCompletedEventArgs e                        Console WriteLine  Completed  tid     Thread CurrentThread ManagedThreadId               done   true                     static void bg DoWork object sender  DoWorkEventArgs e                        for  int i   1  i  lt   5  i                                  Console WriteLine  Work Line      i      tid     Thread CurrentThread ManagedThreadId                   Thread Sleep 500                                     Output   Waiting in Main  tid 10 Work Line  1  tid 6 Waiting in Main  tid 10 Waiting in Main  tid 10 Waiting in Main  tid 10 Waiting in Main  tid 10 Waiting in Main  tid 10 Work Line  2  tid 6 Waiting in Main  tid 10 Waiting in Main  tid 10 Waiting in Main  tid 10 Waiting in Main  tid 10 Waiting in Main  tid 10 Work Line  3  tid 6 Waiting in Main  tid 10 Waiting in Main  tid 10 Waiting in Main  tid 10 Waiting in Main  tid 10 Waiting in Main  tid 10 Work Line  4  tid 6 Waiting in Main  tid 10 Waiting in Main  tid 10 Waiting in Main  tid 10 Waiting in Main  tid 10 Work Line  5  tid 6 Waiting in Main  tid 10 Waiting in Main  tid 10 Waiting in Main  tid 10 Waiting in Main  tid 10 Waiting in Main  tid 10 Waiting in Main  tid 10 Completed  tid 6   2014 UPDATE  See  Roger s answer below   https   stackoverflow com a 24916747 141172  He points out that you can use a Task that returns a Task lt T gt   and check Task lt T gt  Result

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One of the easiest ways to get a return value from a thread is to use closures  Create a variable that will hold the return value from the thread and then capture it in a lambda expression  Assign the  return  value to this variable from the worker thread and then once that thread ends you can use it from the parent thread   void Main       object value   null     Used to store the return value   var thread   new Thread           gt              value    Hello World      Publish the return value           thread Start      thread Join      Console WriteLine value      Use the return value here

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A thread isn t a method - you don t normally  return  a value   However  if you re trying to fetch a value back from the results of some processing  you have many options  the two main ones being    You can synchronize a shared piece of data  and set it appropriately    You can also pass the data back in some form of callback    It really depends on how you re creating the thread  and how you want to use it  as well as the language framework tools you re using

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It depends on how do you want to create the thread and available  NET version    NET 2 0    A  You can create the Thread object directly  In this case you could use  closure  - declare variable and capture it using lambda-expression   object result   null  Thread thread   new System Threading Thread      gt           Some work        result   42      thread Start    thread Join    Console WriteLine result     B  You can use delegates and IAsyncResult and return value from EndInvoke   method   delegate object MyFunc        MyFunc x   new MyFunc      gt           Some work        return 42      IAsyncResult asyncResult   x BeginInvoke null  null   object result   x EndInvoke asyncResult     C  You can use BackgroundWorker class  In this case you could use captured variable  like with Thread object  or handle RunWorkerCompleted event   BackgroundWorker worker   new BackgroundWorker    worker DoWork     s  e    gt          Some work        e Result   42     worker RunWorkerCompleted     s  e    gt          e Result  returned  from thread     Console WriteLine e Result      worker RunWorkerAsync       NET 4 0    Starting with  NET 4 0 you could use Task Parallel Library and Task class to start your threads  Generic class Task lt TResult gt  allows you to get return value from Result property     Main thread will be blocked until task thread finishes    because of obtaining the value of the Result property  int result   Task Factory StartNew      gt          Some work        return 42    Result     NET 4 5    Starting with  NET 4 5 you could also use async await keywords to return value from task directly instead of obtaining Result property   int result   await Task Run      gt          Some work        return 42        Note  method  which contains the code above shoud be marked with asynckeyword   For many reasons using of Task Parallel Library is preferable way of working with threads

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Simply use the delegate approach   int val  Thread thread   new Thread      gt    val   Multiply 1  2       thread Start      Now make Multiply function that will work on another thread   int Multiply int x  int y        return x   y

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I came across this thread when also trying to obtain the return value of a method that gets executed within a Thread  I thought I would post my solution that works    This solution uses an class to store both the method to be executed  indirectly  and stores the returning value  The class can be used for any function and any return type  You just instantiate the object using the return value type and then pass the function to call via a lambda  or delegate      C  3 0 Implementation    public class ThreadedMethod lt T gt         private T mResult      public T Result                get   return mResult            private set   mResult   value               public ThreadedMethod                      If supporting  net 3 5     public void ExecuteMethod Func lt T gt  func                Result   func Invoke                 If supporting only 2 0 use this and        comment out the other overload     public void ExecuteMethod Delegate d                Result    T d DynamicInvoke              To use this code you can use a Lambda  or a delegate   Here is the example using lambdas   ThreadedMethod lt bool gt  threadedMethod   new ThreadedMethod lt bool gt     Thread workerThread   new Thread  unused    gt                               threadedMethod ExecuteMethod      gt                                   SomeMethod      workerThread Start    workerThread Join    if  threadedMethod Result    false           do something about it          VB NET 2008 Implementation    Anyone using VB NET 2008 can t use lambdas with non-value returning methods  This affects the ThreadedMethod class  so we ll make ExecuteMethod return the value of the function  This doesn t hurt anything   Public Class ThreadedMethod Of T       Private mResult As T     Public Property Result   As T         Get             Return mResult         End Get         Private Set ByVal value As T              mResult   value         End Set     End Property      Sub New       End Sub       If supporting  net 3 5      Function ExecuteMethod ByVal func As Func Of T   As T         Result   func Invoke           Return Result     End Function       If supporting only 2 0 use this and        comment out the other overload      Function ExecuteMethod ByVal d As  Delegate   As T         Result   DirectCast d DynamicInvoke    T          Return Result     End Function  End Class

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Threads do not really have return values  However  if you create a delegate  you can invoke it asynchronously via the BeginInvoke method  This will execute the method on a thread pool thread  You can get any return value from such as call via EndInvoke    Example   static int GetAnswer        return 42          Func lt int gt  method   GetAnswer  var res   method BeginInvoke null  null      provide args as needed var answer   method EndInvoke res     GetAnswer will execute on a thread pool thread and when completed you can retrieve the answer via EndInvoke as shown

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ThreadStart delegates in C  used to start threads have return type  void    If you wish to get a  return value  from a thread  you should write to a shared location  in an appropriate thread-safe manner  and read from that when the thread has completed executing

[c#] Returning a value from thread?

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