Passing data between forms

Below code working perfect on my machine.

private void button1_Click(object sender, EventArgs e)
{
    Form1 f1 = new Form1();
    f1.listBox1.Items.Add(textBox1.Text );//ListBox1 : Modifier property made public
    f1.ShowDialog();
}

Ok, If you are Calling Sequence is like, Form1->Form2 and Form2 updates the value of Form1 then you have to use ParentForm() or Delegate to update the previous form.

Let's assume Form1 calls Form2. Please look at the code:

Form1:

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

    private void button1_Click(object sender, EventArgs e)
    {
        Form2 frm = new Form2();
        frm.Show();
        frm.VisibleChanged += formVisibleChanged;


    }

    private void formVisibleChanged(object sender, EventArgs e)
    {
        Form2 frm = (Form2)sender;
        if (!frm.Visible)
        {
            this.listBox1.Items.Add(frm.ReturnText);
            frm.Dispose();
        }


    }

}

Form2:

 public partial class Form2 : Form
{

    public string ReturnText { get; set; }

    public Form2()
    {
        InitializeComponent();

    }

    private void button1_Click(object sender, EventArgs e)
    {
        this.ReturnText = this.textBox1.Text;
        this.Visible = false;

    }


}

The answer is to declare public property on Form2 and when form gets hidden. Access the same instance and retrieve the value.

Try adding a parameter to the constructor of the second form (in your example, Form1) and passing the value that way. Once InitializeComponent() is called you can then add the parameter to the listbox as a choice.

public Form1(String customItem)
{
  InitializeComponent();
  this.myListBox.Items.Add(customItem);
}

// In the original form's code:
Form1 frm = new Form1(this.textBox.Text);