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

Question

I have a scenario   Windows Forms  C    NET    There is a main form which hosts some user control  The user control does some heavy data operation  such that if I directly call the UserControl Load method the UI become nonresponsive for the duration for load method execution  To overcome this I load data on different thread  trying to change existing code as little as I can    I used a background worker thread which will be loading the data and when done will notify the application that it has done its work    Now came a real problem  All the UI  main form and its child usercontrols  was created on the primary main thread  In the LOAD method of the usercontrol I m fetching data based on the values of some control  like textbox  on userControl     The pseudocode would look like this   CODE 1  UserContrl1 LoadDataMethod         if  textbox1 text     MyName      This gives exception                 Load data corresponding to  MyName             Populate a globale variable List lt string gt  which will be binded to grid at some later stage            The Exception it gave was     Cross-thread operation not valid  Control accessed from a thread other than the thread it was created on    To know more about this I did some googling and a suggestion came up like using the following code  CODE 2  UserContrl1 LoadDataMethod         if  InvokeRequired     Line  1               this Invoke new MethodInvoker UserContrl1 LoadDataMethod            return             if  textbox1 text     MyName      Now it wont give an exception             Load data correspondin to  MyName            Populate a globale variable List lt string gt  which will be binded to grid at some later stage           BUT BUT BUT    it seems I m back to square one  The Application again  become nonresponsive  It seems to be due to the execution of line  1 if condition  The loading task is again done by the parent thread and not the third that I spawned   I don t know whether I perceived this right or wrong  I m new to threading   How do I resolve this and also what is the effect of execution of Line 1 if block   The 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 the corresponding data can be fetched from the database

User · Answer

This is not the recommended way to solve this error but you can suppress it quickly, it will do the job . I prefer this for prototypes or demos . add

CheckForIllegalCrossThreadCalls = false

in Form1() constructor .

User · Answer

Controls in  NET are not generally thread safe   That means you shouldn t access a control from a thread other than the one where it lives   To get around this  you need to invoke the control  which is what your 2nd sample is attempting     However  in your case all you ve done is pass the long-running method back to the main thread   Of course  that s not really what you want to do   You need to rethink this a little so that all you re doing on the main thread is setting a quick property here and there

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Action y    declared inside class  label1 Invoke y     label1 Text  text

User · Answer

Follow the simplest  in my opinion  way to modify objects from another thread   using System Threading Tasks  using System Threading   namespace TESTE       public partial class Form1   Form               public Form1                         InitializeComponent                       private void button1 Click object sender  EventArgs e                        Action lt string gt  DelegateTeste ModifyText   THREAD MOD              Invoke DelegateTeste ModifyText   MODIFY BY THREAD                       private void THREAD MOD string teste                        textBox1 Text   teste

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Threading Model in UI  Please read the Threading Model in UI applications  old VB link is here  in order to understand basic concepts  The link navigates to page that describes the WPF threading model  However  Windows Forms utilizes the same idea   The UI Thread   There is only one thread  UI thread   that is allowed to access System Windows Forms Control and its subclasses members  Attempt to access member of System Windows Forms Control from different thread than UI thread will cause cross-thread exception  Since there is only one thread  all UI operations are queued as work items into that thread       If there is no work for UI thread  then there are idle gaps that can be used by a not-UI related computing  In order to use mentioned gaps use System Windows Forms Control Invoke or System Windows Forms Control BeginInvoke methods      BeginInvoke and Invoke methods   The computing overhead of method being invoked should be small as well as computing overhead of event handler methods because the UI thread is used there - the same that is responsible for handling user input  Regardless if this is System Windows Forms Control Invoke or System Windows Forms Control BeginInvoke  To perform computing expensive operation always use separate thread  Since  NET 2 0 BackgroundWorker is dedicated to performing computing expensive operations in Windows Forms  However in new solutions you should use the async-await pattern as described here  Use System Windows Forms Control Invoke or System Windows Forms Control BeginInvoke methods only to update a user interface  If you use them for heavy computations  your application will block      Invoke   System Windows Forms Control Invoke causes separate thread to wait till invoked method is completed      BeginInvoke   System Windows Forms Control BeginInvoke doesn t cause the separate thread to wait till invoked method is completed      Code solution  Read answers on question How to update the GUI from another thread in C    For C  5 0 and  NET 4 5 the recommended solution is here

User · Answer

Same question   how-to-update-the-gui-from-another-thread-in-c  Two Ways    Return value in e result and use it to set yout textbox value in backgroundWorker RunWorkerCompleted event Declare some variable to hold these kind of values in a separate class  which will work as data holder    Create static instance of this class adn you can access it over any thread    Example   public  class data holder for controls         it will hold value for your label     public  string status   string Empty     class Demo       public static  data holder for controls d1   new data holder for controls        static void Main string   args                ThreadStart ts   new ThreadStart perform logic           Thread t1   new Thread ts           t1 Start            t1 Join              your label Text d1 status  --- can access it from any thread             public static void perform logic                   put some code here in this function         for  int i   0  i  lt  10  i                            statements here                     set result in status variable         d1 status    Task done

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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    quot  quot       if textbox1 InvokeRequired                textbox1 Invoke new MethodInvoker delegate   name   textbox1 text                 if name     quot MyName quot                    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   quot MyName quot      lt  lt       Now it wont give exception                   Load data correspondin to  quot MyName quot            Populate a globale variable List lt string gt  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

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Here is an alternative way if the object you are working with doesn t have    InvokeRequired    This is useful if you are working with the main form in a class other than the main form with an object that is in the main form  but doesn t have InvokeRequired  delegate void updateMainFormObject FormObjectType objectWithoutInvoke  string text    private void updateFormObjectType FormObjectType objectWithoutInvoke  string text        MainForm Invoke new updateMainFormObject UpdateObject   objectWithoutInvoke  text      public void UpdateObject ToolStripStatusLabel objectWithoutInvoke  string text        objectWithoutInvoke Text   text      It works the same as above  but it is a different approach if you don t have an object with invokerequired  but do have access to the MainForm

User · Answer

Simple and re-usable way to work around this problem   Extension Method  public static class FormExts       public static void LoadOnUI this Form frm  Action action                if  frm InvokeRequired  frm Invoke action           else action Invoke              Sample Usage  private void OnAnyEvent object sender  EventArgs args        this LoadOnUI      gt                label1 Text               button1 Text

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You only want to use Invoke or BeginInvoke for the bare minimum piece of work required to change the UI  Your  heavy  method should execute on another thread  e g  via BackgroundWorker  but then using Control Invoke Control BeginInvoke just to update the UI  That way your UI thread will be free to handle UI events etc   See my threading article for a WinForms example - although the article was written before BackgroundWorker arrived on the scene  and I m afraid I haven t updated it in that respect  BackgroundWorker merely simplifies the callback a bit

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I know its too late now  However even today if you are having trouble accessing cross thread controls  This is the shortest answer till date  P  Invoke new Action      gt                                        label1 Text    WooHoo                             This is how i access any form control from a thread

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I find the check-and-invoke code which needs to be littered within all methods related to forms to be way too verbose and unneeded  Here s a simple extension method which lets you do away with it completely   public static class Extensions       public static void Invoke lt TControlType gt  this TControlType control  Action lt TControlType gt  del           where TControlType   Control                       if  control InvokeRequired                  control Invoke new Action      gt  del control                 else                 del control             And then you can simply do this   textbox1 Invoke t   gt  t Text    A      No more messing around - simple

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Simply use this   this Invoke  MethodInvoker delegate                               YourControl Property  value     runs thread safe

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For example to get the text from a Control of the UI thread   Private Delegate Function GetControlTextInvoker ByVal ctl As Control  As String  Private Function GetControlText ByVal ctl As Control  As String     Dim text As String      If ctl InvokeRequired Then         text   CStr ctl Invoke              New GetControlTextInvoker AddressOf GetControlText   ctl       Else         text   ctl Text     End If      Return text End Function

User · Answer

I have had this problem with the FileSystemWatcher and found that the following code solved the problem   fsw SynchronizingObject   this  The control then uses the current form object to deal with the events  and will therefore be on the same thread

User · Answer

The cleanest  and proper  solution for UI cross-threading issues is to use SynchronizationContext  see Synchronizing calls to the UI in a multi-threaded application article  it explains it very nicely

User · Answer

I have had this problem with the FileSystemWatcher and found that the following code solved the problem   fsw SynchronizingObject   this  The control then uses the current form object to deal with the events  and will therefore be on the same thread

User · Answer

this Invoke new MethodInvoker delegate                                 your code here

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I found a need for this while programming an iOS-Phone monotouch app controller in a visual studio winforms prototype project outside of xamarin stuidio  Preferring to program in VS over xamarin studio as much as possible  I wanted the controller to be completely decoupled from the phone framework  This way implementing this for other frameworks like Android and Windows Phone would be much easier for future uses   I wanted a solution where the GUI could respond to events without the burden of dealing with the cross threading switching code behind every button click  Basically let the class controller handle that to keep the client code simple  You could possibly have many events on the GUI where as if you could handle it in one place in the class would be cleaner  I am not a multi theading expert  let me know if this is flawed   public partial class Form1   Form       private ExampleController MyController controller       public Form1                           InitializeComponent            controller   new ExampleController MyController  ISynchronizeInvoke  this           controller Finished    controller Finished             void controller Finished string returnValue                label1 Text   returnValue              private void button1 Click object sender  EventArgs e                controller SubmitTask  Do It              The GUI form is unaware the controller is running asynchronous tasks   public delegate void FinishedTasksHandler string returnValue    public class MyController       private ISynchronizeInvoke  syn       public MyController ISynchronizeInvoke syn      syn   syn         public event FinishedTasksHandler Finished        public void SubmitTask string someValue                System Threading ThreadPool QueueUserWorkItem state   gt  submitTask someValue               private void submitTask string someValue                someValue   someValue         DateTime Now ToString            System Threading Thread Sleep 5000     Finished someValue   This causes cross threading error if called like this           if  Finished    null                        if   syn InvokeRequired                                 syn Invoke Finished  new object     someValue                               else                               Finished someValue

User · Answer

Along the same lines as previous answers  but a very short addition that Allows to use all Control properties without having cross thread invokation exception   Helper Method       lt summary gt      Helper method to determin if invoke required  if so will rerun method on correct thread      if not do nothing       lt  summary gt       lt param name  c  gt Control that might require invoking lt  param gt       lt param name  a  gt action to preform on control thread if so  lt  param gt       lt returns gt true if invoke required lt  returns gt  public bool ControlInvokeRequired Control c  Action a        if  c InvokeRequired  c Invoke new MethodInvoker delegate               a                 else return false       return true      Sample Usage     usage on textbox public void UpdateTextBox1 String text          Check if invoke requied if so return - as i will be recalled in correct thread     if  ControlInvokeRequired textBox1       gt  UpdateTextBox1 text    return      textBox1 Text   ellapsed       Or any control public void UpdateControl Color c  String s          Check if invoke requied if so return - as i will be recalled in correct thread     if  ControlInvokeRequired myControl       gt  UpdateControl c  s    return      myControl Text   s      myControl BackColor   c

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A new look using Async Await and callbacks   You only need one line of code if you keep the extension method in your project          lt summary gt      A new way to use Tasks for Asynchronous calls      lt  summary gt  public class Example            lt summary gt          No more delegates  background workers etc  just one line of code as shown below         Note it is dependent on the XTask class shown next           lt  summary gt      public async void ExampleMethod                   Still on GUI Original Thread here           Do your updates before the next line of code         await XTask RunAsync      gt                          Running an asynchronous task here               Cannot update GUI Thread here  but can do lots of work                       Can update GUI Original thread on this line               lt summary gt      A class containing extension methods for the Task class      Put this file in folder named Extensions     Use prefix of X for the class it Extends      lt  summary gt  public static class XTask            lt summary gt          RunAsync is an extension method that encapsulates the Task Run using a callback          lt  summary gt           lt param name  Code  gt The caller is called back on the new Task  on a different thread  lt  param gt           lt returns gt  lt  returns gt      public async static Task RunAsync Action Code                await Task Run      gt                        Code                        return            You can add other things to the Extension method such as wrapping it in a Try Catch statement  allowing caller to tell it what type to return after completion  an exception callback to caller   Adding Try Catch  Auto Exception Logging and CallBack           lt summary gt          Run Async          lt  summary gt           lt typeparam name  T  gt The type to return lt  typeparam gt           lt param name  Code  gt The callback to the code lt  param gt           lt param name  Error  gt The handled and logged exception if one occurs lt  param gt           lt returns gt The type expected as a competed task lt  returns gt       public async static Task lt T gt  RunAsync lt T gt  Func lt string T gt  Code  Action lt Exception gt  Error               var done    await Task lt T gt  Run      gt                        T result   default T               try                              result   Code  Code Here                              catch  Exception ex                                Console WriteLine  Unhandled Exception      ex Message                   Console WriteLine ex StackTrace                   Error ex                             return result                       return done            public async void HowToUse                  We now inject the type we want the async routine to return         var result    await RunAsync lt bool gt   code    gt                 write code here  all exceptions are logged via the wrapped try catch               return what is needed            return someBoolValue                    error   gt                 exceptions are already handled but are sent back here for further processing                    if  result                          we can now process the result because the code above awaited for the completion before               moving to this statement

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There are two options for cross thread operations   Control InvokeRequired Property    and second one is to use   SynchronizationContext Post Method   Control InvokeRequired is only useful when working controls inherited from Control class while SynchronizationContext can be used anywhere  Some useful information is as following links  Cross Thread Update UI    Net   Cross Thread Update UI using SynchronizationContext    Net

User · Answer

You need to look at the Backgroundworker example  http   msdn microsoft com en-us library system componentmodel backgroundworker aspx Especially how it interacts with the UI layer   Based on your posting  this seems to answer your issues

[c#] Cross-thread operation not valid: Control accessed from a thread other than the thread it was created on

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