How to fix cross thread exception when raising a custom event?

Question

0.00/5 (No votes)

See more:

I created a dll named HttpDownloader. It has three events. ProgressChanged,DownloadCompleted and DownloadCancelled. When I use the ProgressChanged event to write the progress to a listview in Form.cs it throws an InvalidOperationException which "cross thread operations not allowed" is written in its text message.

How can I fix this without invoking control in Form.cs . I copied my code parts that I think it's necessary to be understanded below.

C#

void Download(int startRange, bool overWrite)
{
//some code lines....
while ((bytesRead = str.Read(buffer, 0, buffer.Length)) > 0)
            {
                if (state == DownloadState.Cancelled | state == DownloadState.Paused) break;
                state = DownloadState.Downloading;
                file.Write(buffer, 0, bytesRead);
                file.Flush();
                bytesReceived += bytesRead;
                speedBytes += bytesRead;
                this.Progress = progress = (int)(bytesReceived * 100.0 / contentLength);
                speed = (int)(speedBytes / 1.0 / stpWatch.Elapsed.TotalSeconds);

            }
//some code lines....
}

public int Progress
        {
            get { return progress; }
            private set
            {
                progress = value;
                if (DownloadProgressChanged != null)
                    DownloadProgressChanged.Invoke(this, new DownloadProgressEventArgs(progress, speed));
            }
        }
public async void StartDownloadAsync()
        {
            if (state != DownloadState.Started & state != DownloadState.Completed & state != DownloadState.Cancelled)
                return;

            state = DownloadState.Started;
            await Task.Run(() =>
            {
                Download(0, false);
            });

        }
void downloader_QueueProgressChanged(object sender, EventArgs e)
        {
            listViewDwlds.Items[0].Text = downloader.CurrentProgress + "%";
        }

{

Posted 30-Oct-15 7:54am

Amt-Coder

Add a Solution

Comments

Richard Deeming 30-Oct-15 14:27pm

You solve the problem by using the Invoke method in the event handler.

That's how every other event handler solves it; what's so special about yours that you can't use it?

Amt-Coder 30-Oct-15 14:37pm

i think it is not professional. WebClient class has a DownloadProgressChanged event and DownloadFileTaskAsync method like my dll. But in its progress event it's not necessary to use invoke. I want to fix the problem like this class but i dont know how.

2 solutions

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Sergey Alexandrovich Kryukov · Accepted Answer · 2015-10-30T09:21:00

It does not matter if event is custom or not. You cannot directly use methods and properties of the elements of the UI from a non-UI thread. Instead, you can use UI thread invocation mechanism. 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:
.NET event on main thread[^],
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.

—SA

Amt-Coder · Accepted Answer · 2015-11-09T13:30:00

I have just found a better solution for this problem using AsyncOperationManager class. A code sample is below.

C#

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }
    private void button1_Click(object sender, EventArgs e)
    {
        Counter myCounter = new Counter(1000);
        myCounter.IndexValueChanged += myCounter_IndexValueChanged;
        myCounter.StartCountAsync();
    }
    void myCounter_IndexValueChanged(object sender, IndexValueChangedEventArgs e)
    {
        textBox1.Text = e.Index.ToString();
    }
}
class Counter
{
    public delegate void IndexValueChangedEventHandler(object sender, IndexValueChangedEventArgs e);
    public event IndexValueChangedEventHandler IndexValueChanged;

    int _maxNumber;
    int _index;

    public Counter(int maxNumber)
    {
        _maxNumber = maxNumber;
    }

    public async void StartCountAsync()
    {
        //We create an instance of AsyncOperation class
        AsyncOperation asyncCountOperation = AsyncOperationManager.CreateOperation(null);

        await Task.Run(() =>
        {
            for (int i = 0; i < _maxNumber; i++)
            {
                _index = i;
                //We use post method with method will be invoked
                asyncCountOperation.Post(new SendOrPostCallback(delegate
                    {
                        if (IndexValueChanged != null)
                            IndexValueChanged(this,
                                           new IndexValueChangedEventArgs(_index));
                    }), null);
                /*If we raise event without Post method like below it throws an error because of cross thread calls
                 * if (IndexValueChanged != null)
                            IndexValueChanged(this,
                                           new IndexValueChangedEventArgs(_index));
                */
            }
        });
    }
}
class IndexValueChangedEventArgs
{
    int indexNum;
    public IndexValueChangedEventArgs(int index)
    {
        indexNum = index;
    }
    public int Index
    {
        get { return indexNum; }
    }
}

How to fix cross thread exception when raising a custom event?

2 solutions

Solution 1

Solution 2

Add your solution here

Preview 0