Access windows control from Backgroundworker DoWork

Question

Hi, my issue is the following:

I have a windows form in which I've placed a LayoutPanel, when the forms Loads, multiple controls like: textboxes and labels are being added to the LayoutPanel.

Then on a button click, I need to process the data entered by the user on those dynamically created controls. For that purpouse I use a Backgroundworker which is supposed to take those controls and read their data.

My issue es that the Backgroundworker doesn't allows me to access the control from the DoWork Method, but I need to do it that way because I'll be reporting the progress of the operations.

Here are portions of my code to clarify the concept:

private void frmMyForm_Load(object sender, EventArgs e)
    {
       //I add multiple controls, this one is just for example
       LayoutPanel1.add(TextBox1);

       ....
    }

private void bgwBackground_DoWork(object sender, DoWorkEventArgs e)
    {

       foreach (Control controlOut in LayoutPanel1.Controls)
       {
           //do some stuff, this one is just for example
           string myString = controlOut.Name; //-> Here is the error, cant access controls from different Thread.
       }
    }

Setting text is simple just using a delegate, but how about getting the entire parent control to manipulate the child controls (just for getting info, I don't want to set any data, just need to Get Name, Text, stuff like that).

Hope I made myself clear, thank you all.

Answer 1

As you are already aware, accessing control values from any thread other than the UI thread is a big no-no. I'd say one reasonable implementation is to use a .NET synchronization mechanism, such as a WaitHandle, to suspend your background thread while the UI thread updates a thread-safe data structure of your choice.

The idea is that your background thread notifies the UI thread (via the delegate mechanism you are already familiar with) that it needs information, then waits. When the UI is finished populating the shared variable with information, it resets the WaitHandle, and the background worker resumes.

Without writing out and testing all the code, let me give you a few resources:

• WaitHandle.WaitOne documentation with example usage: http://msdn.microsoft.com/en-us/library/kzy257t0.aspx
• My own favorite method of invoking an event on the UI thread: http://www.notesoncode.com/articles/2009/01/24/PowerfulExtensionMethodsPart1.aspx

Answer 2

You can only access Windows Forms controls from the GUI thread. If you want to manipulate them from another thread, you will need to use the Control.Invoke method to pass in a delegate to execute on the GUI thread. In your situation, you should be able to do this:

private void bgwBackground_DoWork(object sender, DoWorkEventArgs e)
{
    foreach (Control controlOut in LayoutPanel1.Controls)
    {
        this.Invoke(new MethodInvoker(delegate {
            // Execute the following code on the GUI thread.
            string myString = controlOut.Name;
        }));
    }
}

I like to define an extension method that allows me to use the cleaner lambda syntax:

// Extension method.
internal static void Invoke(this Control control, Action action) {
    control.Invoke(action);
}

// Code elsewhere.
this.Invoke(() => {
    string myString = controlOut.Name;
});