Cannot find reference using delegate-event mechanism

Question

Cannot find reference using delegate-event mechanism

Rajkumar Rao 100

Hi ,

i am stuck at a point where i am trying to reference of a control which is a panel type and which is in a form named as form1 .
below is the complete code

   internal class Operations
    {
        
        //paneldelegate part 
        public delegate void PanelAccessDelegate(Panel panel);

        public static event PanelAccessDelegate PanelAccessChanged;

        public static void UpdatePanel(Panel panel)
        {
            PanelAccessChanged?.Invoke(panel);
        }
    }

public partial class form1: Form
    {
        public form1() 
        { 
            InitializeComponent();
        }         
       private void form1_Load(object sender, EventArgs e) 
        {            
           Operations.UpdatePanel(this.panel1); // ok   
   openpanel.findpanel<Form2>();  // open form2 right away        
}    
 } 
}

 public static class openpanel
    {       

        // access panel 
       static Panel form1panel;
        public static void getpanelinstancehere(Panel param)
        {
            form1panel= param;
        }

        public static void findpanel<T>() where T : Form, new()
        {
            Operations.PanelAccessChanged += getpanelinstancehere;
           
            foreach (Form oldForm in form1panel.Controls.OfType<Form>().ToArray())

I am getting error at form1panel.controls . The error says that Object reference not set to an instance of an object.'

and when i check , the formpanel1 was null

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Answer 1

KOZ6.0 6,810

The Operations.PanelAccessChanged event must be tied to openpanel.getpanelinstancehere before Operations.UpdatePanel is executed.

internal class Operations
{
    // ■ Add
    static Operations() {
        PanelAccessChanged += openpanel.getpanelinstancehere;
    }

    public delegate void PanelAccessDelegate(Panel panel);
    public static event PanelAccessDelegate PanelAccessChanged;
    public static void UpdatePanel(Panel panel) {
        PanelAccessChanged?.Invoke(panel);
    }
}

public static class openpanel
{
    static Panel form1panel;
    public static void getpanelinstancehere(Panel param) {
        form1panel = param;
    }

    public static void findpanel<T>() where T : Form, new() {
        // ■ Delete
        //Operations.PanelAccessChanged += getpanelinstancehere; 
        foreach (Form oldForm in form1panel.Controls.OfType<Form>().ToArray()) {
        }
    }
}

Rajkumar Rao 100 Reputation points

2023-10-16T05:15:24.91+00:00

@KOZ6.0 , not working .
KOZ6.0 6,810 Reputation points

2023-10-16T05:28:33.89+00:00

What didn't work?

Rajkumar Rao 100

@KOZ6.0 , sorry for coming late .

What i am looking is the way by which i can access the panel1 from the form1 anywhere in my project . I found it extremly hard to get the things work as i am a new learner . I forgot to mention that Form2. which also looks for the same panel to perform some operations . now the problem is that , here and there i got a null reference error for panel1 . Below is the complete code .

  internal class Operations
    {
        
        //paneldelegate part 
        public delegate void PanelAccessDelegate(Panel panel);

        public static event PanelAccessDelegate PanelAccessChanged;

        public static void UpdatePanel(Panel panel)
        {
            PanelAccessChanged?.Invoke(panel);
        }
    }

public partial class form1: Form
    {
        public form1() 
        { 
            InitializeComponent();
        }         
       private void form1_Load(object sender, EventArgs e) 
        {            
           Operations.UpdatePanel(this.panel1); // ok   
   openpanel.findpanel<Form2>();  // open form2 right away        
}    
 } 
}

public static class openpanel
    {       

        // access panel 
       static Panel form1panel;
        public static void getpanelinstancehere(Panel param)
        {
            form1panel= param;
        }

        public static void findpanel<T>() where T : Form, new()
        {
            Operations.PanelAccessChanged += getpanelinstancehere;
           
            foreach (Form oldForm in form1panel.Controls.OfType<Form>().ToArray())

            oldForm.close ();
          }

 public partial class Form2 : Form
    {
          public Form2 ()    
     {            
       InitializeComponent();     
      }


  protected override bool ProcessCmdKey(ref Message msg, Keys keyData)              {        
     if (keyData == (Keys.Escape))  
           {       
          // how to access panel1 here also

KOZ6.0 6,810 Reputation points

2023-10-16T14:43:47.4366667+00:00

It doesn't include the fix I suggested. :( This code doesn't work.

Operations.UpdatePanel is called in form1_Load, but at this time there is nothing in Operations.PanelAccessChanged event, so PanelAccessChanged?.Invoke(panel); does nothing.

That's why openpanel.form1panel is null.

KOZ6.0 6,810

Why is this a troublesome process? Isn't this simple?

public static class OpenPanel
{
    static Panel mainMasterPanel;

    public static void SetPanelInstance(Panel panel) {
        mainMasterPanel = panel;
    }

    public static void RecreateCenterForm<T>() where T : Form, new() {
        foreach (Form oldForm in mainMasterPanel.Controls.OfType<Form>().ToArray()) {
            using (oldForm) {
                mainMasterPanel.Controls.Remove(oldForm);
            }
        }
        var newForm = new T {
            TopLevel = false,
            FormBorderStyle = FormBorderStyle.None,
            ControlBox = false,
            ShowInTaskbar = false
        };
        mainMasterPanel.Controls.Add(newForm);
        int x = (mainMasterPanel.Width - newForm.Width) / 2;
        int y = (mainMasterPanel.Height - newForm.Height) / 2;
        newForm.Location = new Point(x, y);
        newForm.Show();
        newForm.Focus();
    }
}

public partial class Form1 : Form
{
    public Form1() {
        InitializeComponent();
    }

    private void Form1_Load(object sender, EventArgs e) {
        OpenPanel.SetPanelInstance(MainMasterPanel);
        OpenPanel.RecreateCenterForm<Form2>();
    }
}

Rajkumar Rao 100

@KOZ6.0 , Thanks for the guidance .Although I had marked you answer as the Accepted , Just one last query . if I also want to access the same panel in form2

public partial class Form2 : Form
    {
          public Form2 ()    
     {            
       InitializeComponent();     
      }


  protected override bool ProcessCmdKey(ref Message msg, Keys keyData)              {        
     if (keyData == (Keys.Escape))  
           {       
          // how to access panel1 here also

how do i access in here . by making the

public static class OpenPanel
{
    static Panel mainMasterPanel;

to

public static class OpenPanel
{
   public static Panel mainMasterPanel;

KOZ6.0 6,810 Reputation points

2023-10-17T09:24:49.05+00:00
You can create a method like the one below and call it.

public static Panel GetPanelInstance() { return mainMasterPanel; }

I don't think it's a good idea to expose field variables directly. This is because you cannot respond flexibly when you want to add or change features. If you want to make it public, it would be better to make it a property.

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Cannot find reference using delegate-event mechanism

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