How to Add the Form in Panel from another Form

Hi...
My problem is solved the following code is clear my doubt,Thanks for Vulpes.

public class Form2 : Form
{  
  private void button1_Click(object, EventArgs e)
  {
      Form3 form3 = new Form3();
      form3.TopLevel = false;
      form3.TopMost = true;
      Form1 form1 = (Form1)Application.OpenForms["Form1"];
      Panel panel1 = (Panel)form1.Controls["panel1"];
      panel1.Controls.Add(form3); 
      form3.Show();
  }
} 

And Also i give the thanks for all Comments.

To simply describe what you are doing in VB: When a Button is Clicked on an instance of Form2:

1. take a pre-existing instance of Form3

2. insert that instance of Form3 into a Panel Control in an instance of Form1

It is not a good idea to put a Form inside another Form, or inside a Control in another Form (unless you are using the old MDI architecture, which I suggest you avoid). OriginalGriff is right-on when he advises you to use a UserControl !

To help you get a better understanding of programming in WinForms, we need to know which Form is the "main" Form here: does Form1 create Form2, and Form3 ?

If that's the case, then we can say your goal is:

1. to give an instance of Form2 a way to communicate to Form1 that it needs to insert an instance of a UI into a Panel inside it.

So, we give Form1 a way to subscribe to the Button Click Event on Form2 like this:

In Form2: by creating a publicly acessible reference to its Button:

public partial class Form2 : Form
{
    // create a public property of type 'Button
    public Button f2Button { set; get; }

    public Form2()
    {
        InitializeComponent();
        
        // set the Property so 'Form1 can see it
        f2Button = this.button1;
    }
}

In Form1: by subscribing to the Button's Click Event

public partial class Form2 : Form
{
    public Form1()
    {
        InitializeComponent();
    }
        
    Form2 f2 = new Form2();

    private void Form1_Load(object sender, EventArgs e)
    {
       // subscribe to Form2's 'button1 Click Event
       f2.f2Button.Click += f2Button_Click;
    }

    private void f2Button_Click(object sender, EventArgs e)
    {
       // the code to add some UI to the Panel on Form1 goes here
       MyUserControl uc = new MyUserControl;
       
       panel1.Controls.Add(uc);

       // set position, or Dock, or Anchor of the added UserControl as needed 
    }

}

By constraining what Form2 exposes to other Forms/Classes to only its Button, we are trying to keep the "coupling" (dependency) between Forms/Classes to a minimum.