Question

How can I call a method on a form from a method called from an external class from a backgroundWorker? I believe that delegates are somehow the answer to this question, but after spending time reading, I still am confused by this problem.

This is in Visual Studio 2008, the backgroundWorker is run from the form and calls ExternalClass.Method. The form is in namespace ProgramName and the ExternalClass is using ProgramName. When i declare public delegate MyDelegate in the namespace ProgramName in the file of my windows.form I can create an instance of MyDelegate and call it in a method of my form (but this does not help me), but if I try to create an instance of MyDelegate and call it from a method of my external class I cannot access the method of the windows.form, even though it is public.

thanks

yes, I want to pass progress reports (int percent, string status) back from ExternalClass.Method. Can you explain a bit more about that CSharpAtl (or anyone)?

Answer 1

Note that your question (afaik) is not just about the backgroundwiorker but just as much about how to break a circular reference between classes. This is a standard problem with a standard solution.

You can pass a delegate (referring to a Form-method) around just as any object so also to a Backgroundworker. And the Bgw can pass it to the external method. A delegate includes a reference to the object (in this case the Form).

Note that since you are on another thread you will need to use Control.Invoke inside the delegate, or use the Bgw ReportProgress event.

public partial class Form1 : Form
{
    private void ReportProgresshandler(int percent, string state)
    {
        backgroundWorker1.ReportProgress(percent);  // also does the Invoke
    }

    private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
    {
        var ex = new ExampleClass();
        ex.Dowork(ReportProgresshandler);    
    }
}

and something like

class ExampleClass
{
    delegate void ReportDelegate(int percent, string status);

    public void Dowork(ReportDelegate report)
    {
        report(0, "starting");
    }

}