C++ CLI Background Worker Runtime Error

Question

1.00/5 (2 votes)

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I seem to be having an issue where my program forces an error.
http://puu.sh/mUq0l/600ed167c1.png

I am not certain to how to fix it as I only have experience with the standard multi threading in C++.

[code]

C#

void setText(int data)
	{
		pingOutbox->Text = Convert::ToString(data);
		pingOutbox->Refresh();
		cout << data << endl;
	}

	void mainPingThing()
	{
		int speed;
		setText(runPing());
		Sleep(2500);
	}

	private: System::Void backgroundWorker1_DoWork_1(System::Object^  sender, System::ComponentModel::DoWorkEventArgs^  e) 
	{
		while (true) { mainPingThing(); }
	}

[/code]
This is the basic layout of what I have done. I do not know what corrections have to be made. Any help would be much appreciated as Microsoft's craziness knows no limits.

Posted 3-Feb-16 5:26am

Member 10657083

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Comments

Philippe Mori 3-Feb-16 13:49pm

You should write the error message directly in the question. We don't like to open external links if we don't have to...

Sergey Alexandrovich Kryukov 6-Feb-16 11:44am

Excuse me, why posting a png somewhere and make use loading that page, if this is a pure textual information we could read as a part of your question? I understand that it would require you to type those 5 lines, but this is still better than the whole post with a png...

—SA

2 solutions

Solution 1

It's way to easy to blame Microsoft in all sins, including your own. At the same time, the exception message is crystal clear and explains what you did wrong: tried to works with some object in some non-UI thread, but this object is part of UI and was created and added to UI in a UI thread. This is invalid operation. All such operations should be done in a UI thread. Can you work with them in a non-UI thread? Of course you can.

You cannot call anything related to UI from non-UI thread. Instead, you need to use the method Invoke or BeginInvoke of System.Windows.Threading.Dispatcher (for both Forms or WPF) or System.Windows.Forms.Control (Forms only).

You will find detailed explanation of how it works and code samples in my past answers:
Control.Invoke() vs. Control.BeginInvoke(),
Problem with Treeview Scanner And MD5.

See also more references on threading:
.NET event on main thread,
How to get a keydown event to operate on a different thread in vb.net,
Control events not firing after enable disable + multithreading.

[EDIT]

The mechanism I described above is universal in both System.Windows.Forms and WPF UI and can be used in any thread. But I forgot to mention the mechanism specific to System.ComponentModel.BackgroundWorker. Philippe Mori reminded me about it in his comment to this solution:
BackgroundWorker.ReportProgress Method (System.ComponentModel)[^],
BackgroundWorker.ProgressChanged Event (System.ComponentModel)[^].

Basically, you can directly call System.ComponentModel.BackgroundWorker.ReportProgress in any thread. This call is abstracted from any kind of UI. The delegation to the UI thread happens if you handle the event System.ComponentModel.BackgroundWorker.ProgressChanged; and this is done in a UI-specific way; naturally, in the event handler, you can address UI elements directly.

—SA

Posted 3-Feb-16 5:56am

Sergey Alexandrovich Kryukov

Updated 3-Feb-16 9:24am

v3

Comments

Philippe Mori 3-Feb-16 13:17pm

Or since he is already using a background worker, use ReportProgress member with an extra argument. And Refresh would also be useless.

Sergey Alexandrovich Kryukov 3-Feb-16 15:15pm

Ah, good point, thank you very much. I just updated the answer and credited your comment.
—SA

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Philippe Mori · Accepted Answer · 2016-02-03T07:41:00

Solution 2

As you are already using a background worker, the easiest way to fix the problem would be to use it properly!

C++

private: System::Void backgroundWorker1_DoWork_1(
    System::Object^  sender, System::ComponentModel::DoWorkEventArgs^  e) 
{
    while (true)
    {
        int data = runPing();
        backgroundWorker1->ReportProgress(0, Convert::ToString(data));
        Sleep(2500);
    }
}

And connect an handler for ProgressChanged event and then move some of your code to it:

C++

void backgroundWorker1_ProgressChanged(object sender,
    ProgressChangedEventArgs e)
{
    // I forgotten to change a . to ->. In C++/cli, you use the arrows for members 
    // while in C#, it is the dot.
    auto textData = e->UserData.ToString()UserState->ToString();
    pingOutbox->Text = textData;

    // Not necessary. The UI will update itself if the main thread is responding.
    // pingOutbox->Refresh();    

    cout << textData << endl;
}

It would probably also be possible to leave cout in the background loop but it might mess up the outputed text if multiple threads are writing to it.

Update
By the way, reading official documentation would help: How to: Implement a Form That Uses a Background Operation[^]

Also, it is very easy to search help using Google once you have the error message and you won't have to wait for someone to answer!

Posted 3-Feb-16 7:41am

Philippe Mori

Updated 6-Feb-16 4:09am

v8

Comments

Sergey Alexandrovich Kryukov 3-Feb-16 15:25pm

5ed.
—SA

Member 10657083 4-Feb-16 6:18am

What does UserData refer to?

Philippe Mori 4-Feb-16 8:38am

It is the data specified in the second argument of ReportProgress. Thus you can pass any object you want in addition to a percentage and the event will be generated in the UI thread (assuming that the worker was started from that thread).

Member 10657083 5-Feb-16 9:46am

error C2039: 'UserData': is not a member of 'System::ComponentModel::ProgressChangedEventArgs'

Philippe Mori 5-Feb-16 10:38am

In fact, it is UserState. Usually, IntelliSense would told me such thing but here on Code Project, there are no such thing.

Philippe Mori 6-Feb-16 9:54am

You have to be sure that RunWorkerAsync is called from the UI thread. Usually, this is not a problem as the code is executed in a button clicked handler or something similar.

I don't understand why you commented out the call to ReportProgress with 2 arguments and used the one with a single argument instead.

If you do that, then you also have to update code in backgroundWorker1_ProgressChanged handler as you send the data as the progress percentage and not as user state.

I have no idea of what safe_cast does. The System::Object ^ you send to ReportProgress in second parameter is the one you get in UserState.

Thus if you send a System::String ^ you might call UserState->ToString() (In my sample code, I forgot to change . for ->. as it is C++/CLI) Otherwise, you would do a

dynamic_cast<TargetType ^><code>.

Philippe Mori 6-Feb-16 10:04am

By the way, it is useless to dump that much code... Code in a comment is unreadable. Most of the code is already written either in question or my answer.

Also, does not works doesn't tell much. What is the problem. It does not compile? The handler is not called? You don't get expected data in the handler? No one can really guess.

On the other hand, we assume that the code is otherwise standard background worker inside a form and started from an event handler.

Member 10657083 6-Feb-16 23:02pm

The Full code
http://pastebin.com/P7EJFg5s
The code that causes the error and the error it's self
http://pastebin.com/PEBZVvy1

Member 10657083 7-Feb-16 6:02am

To go into more detail the progress report function does not do anything.

Philippe Mori 7-Feb-16 8:47am

You have not connected the event... That why it is never called! And you haven't fixed the dot for an arrow. All that is already explained in answer and comment. It would help if you would read more carefully...

Member 10657083 8-Feb-16 8:33am

Clearly that is wrong
auto textData = e->UserState->ToString();
error C2228: left of '.ToString' must have class/struct/union

From what I know this
backgroundWorker1->ReportProgress(0,data);
this meant to call the ProgressChanged function.

Philippe Mori 8-Feb-16 9:09am

Yes, ReportProgress would eventually calls ProgressChanged handler but you have to hook the event as you have done for DoWork event handler. Like any other event handler (for ex. button clicked), they are never connected automatically.

If you have an older version of Visual Studio, you can use Windows Forms designer from C++/cli project. Otherwise, you have to do it manually. Simply look at generated code that initialize the background worker.

Member 10657083 10-Feb-16 5:21am

Ok that just makes no sense,look at this.
https://msdn.microsoft.com/en-us/library/system.componentmodel.backgroundworker.progresschanged%28v=vs.110%29.aspx?cs-save-lang=1&cs-lang=cpp#code-snippet-2

Philippe Mori 10-Feb-16 9:00am

Since you were able to add an handler for DoWork event, you should be able to do it for ProgressChanged. Or do you modify someone else code and even don't have any idea how it was done. Maybe, that's the reason why there is a backgroundWorker1_DoWork and a backgroundWorker1_DoWork_1 method.

Is it that hard to look code between lines 172 and 175 at
http://pastebin.com/P7EJFg5s and see that there are no line to attach an event Handler for ProgressChanged.

With all provided information, it is very easy to fix the problem. If you still cannot understand, then use Google (there are a lot of example on the web even some in C++/cli). And if you don't have basin understanding of WinForms development, then take some tutorials or a course.

Member 10657083 11-Feb-16 4:48am

Ok I think I will proceed from that thank you.