.net - Listening to Events in Main Form from Another Form in C#

Question

I have an application that has a main form and uses an event handler to process incoming data and reflect the changes in various controls on the main form. This works fine.

I also have another form in the application. There can be multiple instances of this second form running at any given time.

What I'd like to do is have each instance of this second form listen to the event handler in the main form and update controls on its instance of the second form.

How would I do this?

Here's some sample code. I want to information from the_timer_Tick event handler to update each instance of SecondaryForm.

public partial class Form1 : Form
{
    Timer the_timer = new Timer();
    public Form1()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        the_timer.Tick += new EventHandler(the_timer_Tick);
        the_timer.Interval = 2000;
        the_timer.Enabled = true;
    }

    void the_timer_Tick(object sender, EventArgs e)
    {
        // I would like code in here to update all instances of SecondaryForm
        // that happen to be open now.
        MessageBox.Show("Timer ticked");
    }

    private void stop_timer_button_Click(object sender, EventArgs e)
    {
        the_timer.Enabled = false;
    }

    private void start_form_button_Click(object sender, EventArgs e)
    {
        SecondaryForm new_form = new SecondaryForm();
        new_form.Show();
    }
}

Answer 1

class SecondForm
{
  private FirstForm firstForm;

  public SecondForm()
  {
    InitializeComponent();
    // this means unregistering on form closing, uncomment if is necessary (anonymous delegate)
    //this.Form_Closing += delegate { firstForm.SomeEvent -= SecondForm_SomeMethod; };
  }

  public SecondaryForm(FirstForm form) : this()
  {
    this.firstForm = form; 
    firstForm.Timer.Tick += new EventHandler(Timer_Tick);
  }

  // make it public in case of external event handlers registration
  private void Timer_Tick(object sender, EventArgs e)
  {
    // now you can access firstForm or it's timer here
  }
}

class FirstForm
{
  public Timer Timer
  {
    get
    {
      return this.the_timerl
    }
  }

  public FirstForm()
  {
    InitializeComponent();
  }

  private void Button_Click(object sender, EventArgs e)
  {
    new SecondForm(this).ShowDialog(); // in case of internal event handlers registration (in constructor)
    // or
    SecondForm secondForm = new SecondForm(this);
    the_timer.Tick += new EventHandler(secondForm.Timer_tick); // that method must be public
  }