Cross-thread operation not valid in Windows Forms

Question

Could anyone help me i have a problem I'm trying to get this code to work in the background via threadpool but i cannot seem to get it to work i keep getting this error:

Cross-thread operation not valid: Control 'ListBox3' accessed
from a thread other than the thread it was created on. 

Here is the code i am using:

private void DoWork(object o)
{
    var list = ListBox3;

    var request = createRequest(TxtServer.Text, WebRequestMethods.Ftp.ListDirectory);

    using (var response = (FtpWebResponse)request.GetResponse())
    {
        using (var stream = response.GetResponseStream())
        {
            using (var reader = new StreamReader(stream, true))
            {
                while (!reader.EndOfStream)
                {
                    list.Items.Add(reader.ReadLine());
                    ResultLabel.Text = "Connected";
                }
            }
        }
    }
}

Answer 1

You need to invoke a delegate to update the list. See this MSDN example.

Answer 2

You can access controll by do this

 Invoke(new Action(() => {Foo.Text="Hi";}));

Answer 3

You can also access controls via invoke using the following syntax with an Action delegate:

 Invoke((Action)(() =>
 {
      var myVar = SomeWinFormControl.Property;
 }));

Answer 4

You can't access the control from a separate thread, it has to be from the same thread that the control was created from.

Answer 5

This extension method solves the problem too.

/// <summary>
/// Allows thread safe updates of UI components
/// </summary>
public static void InvokeEx<T>(this T @this, Action<T> action) where T : ISynchronizeInvoke
{
    if (@this.InvokeRequired)
    {
        @this.Invoke(action, new object[] { @this });
    }
    else
    {
        action(@this);
    }
}

Use in your worker thread as follows

InvokeEx(x => x.MyControl.Text = "foo");

Answer 6

I'm assuming that DoWork is launched on another thread. The code accesses ListBox3, which is a GUI control. .NET restricts access to GUI controls to the thread that created them.

Answer 7

You can do this instead as accessing controls from other than UI thread require invoke.

When you start (I assume you use BackgroundWorker), pass in the url from your textbox to RunWorkerAsync(TxtServer.Text) as argument, then:

private void DoWork(object o, DoWorkEventArgs e)
{
    string Url = (string) e.Argument;

    List<of string> tmpList = new List<of string>;

    var request = createRequest(url, WebRequestMethods.Ftp.ListDirectory);

    using (var response = (FtpWebResponse)request.GetResponse())
    {
        using (var stream = response.GetResponseStream())
        {
            using (var reader = new StreamReader(stream, true))
            {
                while (!reader.EndOfStream)
                {
                    list.Add(reader.ReadLine());
                    //ResultLabel.Text = "Connected";
                    //use reportprogress() instead
                }
            }
        }
    }
    e.result = tmpList;
}

Then on your Completed event cast e.result to list and add it to your control.