Winforms threading problem, second thread can't access 1st main forms controls

Question

I have a winforms application, the issue has to do with threading. Since I am calling 'MyCustomCode() which creates a new thread, and calls the method 'SomeMethod()' which then accesses MessageBox.Show(...).

The problem has to do with threading, since the newly created thread is trying to access a control that was created on another thread.

I am getting the error:

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

public TestForm()
{
 InitializeComponent();


 // custom code
 //
 MyCustomCode();


}

public void SomeMethod()
{

 // ***** This causes an error  ****

 MessageBox.Show(this, 
  ex.Message, 
  "Error", 
  MessageBoxButtons.OK, 
  MessageBoxIcon.Error
 );
}



private void InitializeAutoUpdater()
{
     // Seperate thread is spun to keep polling for updates
     ThreadStart ts = new ThreadStart(SomeMethod);
     pollThread = new Thread(ts);
     pollThread.Start();
}

Update

If you look at this example http://www.codeproject.com/KB/cs/vanillaupdaterblock.aspx, the method CheckAndUpdate is calling MessageBox.Show(..) that is what my problem is. I would have thought that code was good to go!

Funny thing is that this code was working just fine on Friday???

Answer 1

You cannot acces UI elements from multiple threads.

One way to solve this is to call the Invoke method of a control with a delegate to the function wich use the UI elements (like the message box). Somethin like:

public delegate void InvokeDelegate();

public void SomeMethod()
{

    button1.Invoke((InvokeDelegate)doUIStuff);


}


void doUIStuff()
{
           MessageBox.Show(this,   
                ex.Message, 
                "Error", 
                MessageBoxButtons.OK, 
                MessageBoxIcon.Error
            );
}

Answer 2

You should NOT use BeginInvoke, you should use Invoke, then once you grasp that, you can look into using BeginInvoke if really needed.

Answer 3

• Use Control.BeginInvoke or Control.Invoke methods

OR

• Use SynchronizationContext

Answer 4

You should check this MSDN article.

Answer 5

    '*******************************************************************
    ' Get a new processor and fire it off on a new thread.
    '*******************************************************************
    fpProc = New Processor(confTable, paramFile, keyCount)
'
    AddHandler fpProc.LogEntry, AddressOf LogEntry_Handler
'    
    Dim myThread As System.Threading.Thread = New System.Threading.Thread(AddressOf fpProc.ProcessEntry)
'      
    myThread.Start()
'
'
'   
    Then in the parent app you have:

       '*************************************************************************
       '     Sub: LogEntry_Handler()
       '  Author: Ron Savage
       '    Date: 08/29/2007
       '
       ' This routine handles the LogEntry events raised by the Processor class
       ' running in a thread.
       '*************************************************************************
       Private Sub LogEntry_Handler(ByVal logLevel As Integer, ByVal logMsg As String) Handles fProc.LogEntry
        writeLogMessage(logMsg);
       End Sub

That's what I do.

Ron

Answer 6

to avoid cross-thread exceptions (InvalidOperationException), here is the code pattern:

protected delegate void someGuiFunctionDelegate(int iParam);

protected void someGuiFunction(int iParam)
{
    if (this.InvokeRequired)
    {
     someGuiFunctionDelegate dlg = new 
            someGuiFunctionDelegate(this.someGuiFunction);
     this.Invoke(dlg, new object[] { iParam });
     return;
    }

    //do something with the GUI control here
}

i agree that this is annoying, but it is an artifact of the fact that windows GUI controls are not thread-safe. The exception can be turned off with a flag somewhere or other, but don't do that as it can lead to extremely hard to find bugs.

Answer 7

The number one rule when multi-threading is you absolutely cannot touch the UI from worker threads. There are many ways to implement multi-threading and it's very hard to get it "right".

Here's a succinct article that should help you out - Updating the UI from a Secondary Thread

And here's a lengthy article that discusses threading in-depth - Multi-threading in .NET

Answer 8

To keep things simple you can look into using the BackGroundWorker class. This class will provide a framework for to handle your threading and progress notification events. Your ui thread will handle the progress event and display the error message you pass back.

Answer 9

Check for InvokeRequired

Answer 10

I praticularly like a recursive call.

public delegate void InvokeDelegate(string errMessage); 

    public void SomeMethod() 
    { 
        doUIStuff("my error message");
    } 


    void doUIStuff(string errMessage) 
    { 
        if (button1.InvokeRequired)
            button1.Invoke((InvokeDelegate)doUIStuff(errMessage)); 
        else
        {
               MessageBox.Show(this,    
                    ex.Message,  
                    errMessage,  
                    MessageBoxButtons.OK,  
                    MessageBoxIcon.Error 
                ); 
        }
    } 

Answer 11

I know this is an older post, but I recently found an elegant solution to this problem using generics and extension methods. This is a combination of the authors works and some comments.

A Generic Method for Cross-thread Winforms Access

http://www.codeproject.com/KB/cs/GenericCrossThread.aspx

public static void Manipulate<T>(this T control, Action<T> action) where T : Control
{
    if (control.InvokeRequired)
    {
        control.Invoke(new Action<T, Action<T>>(Manipulate),
                    new object[] { control, action });
    }
    else
    { action(control); }
}

This can be called in the following manner, for simplicity I used a label.

someLabel.Manipulate(lbl => lbl.Text = "Something");