必须使用Control.Invoke与在单独线程上创建的控件进行交互

Question

Below is the method to start a thread in compact framework 3.5

public ScanEntry(string scanId)
{
   InitializeComponent();
    _scanId = scanId;
    //reader = deviceFactory.Create();
    //reader.YMEvent += new ScanEventHandler(reader_Reading);
    //reader.Enable();
 }


private void CasesEntry_Load(object sender, EventArgs e)
{
      caseCounterLabel.Text = cases.Count.ToString();
      scanIdValueLabel.Text = _scanId;
}



internal void menuItemNewScan_Click(object sender, EventArgs e)
{
       System.Threading.ThreadStart threadDelegate = new System.Threading.ThreadStart(ScanEvents);
       System.Threading.Thread newThread = new System.Threading.Thread(threadDelegate);
       newThread.Start();
}

which calls the below method on thread

private void ScanEvents()
{

  try
  {

     //some other codes     
      if (scanIdValueLabel.InvokeRequired)
     {
          scanIdValueLabel.Invoke((Action)(() => scanIdValueLabel.Text = "value"));
      }  


     attributeNode = docEventFile.CreateNode(XmlNodeType.Element, "Attribute", string.Empty);
     XMLUtils.CreateAttribute(docEventFile, attributeNode, "name", "SCANID");
     XMLUtils.CreateAttribute(docEventFile, attributeNode, "value", scanIdValueLabel.Text);
     attributeSetNode.AppendChild(attributeNode);
     //some other codes
  }
  catch(Execption e)
  {
     Message.Show(e.Message);
  }
}

Errors:

TextAlign = 'scanIdValueLabel.TextAlign' threw an exception of type 'System.NotSupportedException' 
base {System.SystemException} = {"Control.Invoke must be used to interact with controls created on a separate thread."}

In Line

XMLUtils.CreateAttribute(docEventFile, attributeNode, "value", scanIdValueLabel.Text);

I am getting Control.Invoke must be used to interact with controls created on a separate thread at this line

XMLUtils.CreateAttribute(docEventFile, attributeNode, "value", scanIdValueLabel.Text);

I have googled and tried with that solutions but not worked for me.Can any one help me in doing this.

Thanks

Answer 1

When you're dealing with Winforms, WPF, Silverlight there's the following sentence which is very important:

The UI elements can only be accessed by the UI thread. WinForms, WPF, Silverlight doesn't allow access to controls from multiple threads.

However, there is a solution which can be found here :

Update: I've created a sample application to make some things clear:

I've created a form first with a button and a label on it. The label is not visible because it doesn't contain text, but it's right underneath the button.

Scenario 1: Updating without threads:

private void btnStartThread_Click(object sender, EventArgs e)
{
    lblMessage.Text = "Button has been clicked.";
}

Off course this is not a problem. It's some standard code:

Scenario 2: Updating with threads:

private void btnStartThread_Click(object sender, EventArgs e)
{
    System.Threading.ThreadStart threadDelegate = new System.Threading.ThreadStart(ScanEvents);
    System.Threading.Thread newThread = new System.Threading.Thread(threadDelegate);
    newThread.Start();
}

private void ScanEvents()
{
    lblMessage.Text = "Exected in another thread.";
}

This will fail because I'm MODIFYING the controls on my form from another thread:

Now, I will modify the code so that I'm changing the label with an action through an invoke on the label.

private void ScanEvents()
{
    if (lblMessage.InvokeRequired)
    {
        lblMessage.Invoke((Action)(() => lblMessage.Text = "This text was placed from within a thread."));
    }
}

This will make the text change.

So, I hope that it helps. If not, please shout :-)