Error in declare new Form Part 2

Form8's constructor need two arguments: a reference to frmTreeView and a reference to Form7.

If you're instantiating Form8 from frmTreeView, it therefore follows that the latter needs a reference to Form7.

If this was passed in frmTreeView's constructor, then you should have stored it in a field. Otherwise, you can get it as follows:

Form7 f7 = (Form7)Application.OpenForms["Form7"];
Form8 f8 = new Form8(this, f7);

In the follow  part I Have  in frmTreeView Form I have

 Form8 f8 = new Form8(this);
                            if (f8.ShowDialog() == DialogResult)
                            {
                                f8.Dispose();

                            }

the this stands for data of frmTreeView. I think after the this i need ti put the info of Form7 but when i do it it give me a problem. What code i'm missing to add ?

ok I did it as follow  
        private frmTreeView getport1;
        private Form7 f7;
      
        public Form8(frmTreeView form, Form7 form7)
        {
            InitializeComponent();
            getport1 = form;
            f7 = form7;
          
        }

but from frmTreeView1 Form i need to open Form8 and i have as follow:

  Form8 f8 = new Form8(this);
                            if (f8.ShowDialog() == DialogResult)
                            {
                                f8.Dispose();

                            }
It give me a error message at the red part and is as follow:
 
No overload for method 'Form8' takes '1' arguments   

what can be the problem?

Ok , I tried this part already

public partial class Form13 : Form
    {
        private Form11 _parent11;
        private Form12 _parent12;

        public Form13(Form11 form11, Form12 form12)
        {
            InitializeComponent();
            _parent11 = form11;
            _parent12 = form12;
        }

but i will double check again. Can be i did some stupid mistake.

I will keep you posted

If you needed to access both Form1 and Form2 from Form3 then, using your original approach, you'd need to pass references to the first two forms to Form3's constructor:

    public partial class Form12 : Form
    {
        private Form11 _parent;

        public Form12(Form11 form)
        {
            InitializeComponent();
            _parent = form;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string halo = _parent.textBox1.Text + " Form2";
            MessageBox.Show(halo); 
        }

        private void button2_Click(object sender, EventArgs e)
        {
            Form13 f = new Form13(_parent, this); 

            f.ShowDialog();
            f.Dispose();
        }
    }

    public partial class Form13 : Form
    {
        private Form11 _parent11;
        private Form12 _parent12;

        public Form13(Form11 form11, Form12 form12)
        {
            InitializeComponent();
            _parent11 = form11;
            _parent12 = form12;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string halo = _parent11.textBox1.Text + " Form3";
            MessageBox.Show(halo);
            _parent12.Text = "Hello from Form 3"; // change Form2's caption
        }
    }

However, there's no need to pass any parameters using the alternative approach:

// in form3's button_Click

Form11 f11 = (Form11)Application.OpenForms["Form11"];
string halo = f11.Controls["textBox1"].Text + " Form3";
MessageBox.Show(halo);

Form12 f12 = (Form12)Application.OpenForms["Form12"];
f12.Text = "Hello from Form 3";

I'm assuming that:

1. You have another button on Form12 (button2 say) which, when pressed, opens up Form13.

2. Form13 has a button (button1 say) which, when pressed, accesses the Text property of textBox1 on Form11.

You could then do that like this:

    public partial class Form12 : Form
    {
        private Form11 _parent;

        public Form12(Form11 form)
        {
            InitializeComponent();
            _parent = form;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string halo = _parent.textBox1.Text + " Form2";
            MessageBox.Show(halo); 
        }

        private void button2_Click(object sender, EventArgs e)
        {
            Form13 f = new Form13(_parent);          
            f.ShowDialog();
            f.Dispose();
        }
    }

    public partial class Form13 : Form
    {
        private Form11 _parent;

        public Form13(Form11 form)
        {
            InitializeComponent();
            _parent = form;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string halo = _parent.textBox1.Text + " Form3";
            MessageBox.Show(halo); 
        }
    }

Incidentally, it's not actually necessary to pass references between forms or to change the accessibility of the textbox to public. Instead you can do it like this:

// in form2's button_Click

Form11 f = (Form11)Application.OpenForms["Form11"];
string halo = f.Controls["textBox1"].Text + " Form2";
MessageBox.Show(halo);

// in form3's button_Click

Form11 f = (Form11)Application.OpenForms["Form11"];
string halo = f.Controls["textBox1"].Text + " Form3";
MessageBox.Show(halo);