Question

Currently I am sending one Array List from form2 to form 1 and it works fine.

Form1 form2 = new Form1(this, SampleArrayList); //pass form reference and an arraylist
form2.Show();
this.Hide();

And on form1 I associate SampleArrayList with local Array List.

Form2 formParent;
ArrayList SampleArrayList;
public MainForm(Form2 par, ArrayList _SampleArrayList)
{            
       InitializeComponent();
       this.formParent = par;
       this.SampleArrayList = _SampleArrayList;
}

However I want to avoid creating new instance of Form1

form2 = new Form1(this, SampleArrayList);

I want to send Array List to currently running instance of Form1. What would be best way to do this. Thank you

Answer 1

Quote of OP in the comments above:

Form1 is created first, clicking add button creates Form2. Clicking
submit button hides Form2 and creates new instance of Form1 along with
old instance which I didn’t want to dispose of. I want to click submit
button and go back to running instance of Form1.

That’s really a larger problem then. Here’s a nice solution that I like.

public partial class Form1 : Form
{
    private string dataFromThisForm; //can be whatever
    private void button1_Click(object sender, EventArgs e)
    {
        Form2 otherForm = new Form2();
        //pass some data to other form
        otherForm.SomeData = dataFromThisForm; 

        this.Hide();
        otherForm.Show();

        //when the other form is closed
        otherForm.FormClosed += (sender2, e2) =>
        {
            this.Show();

            string newData = otherForm.NewData;
        };

        //when the other form is hidden.
        otherForm.VisibleChanged += (sender2, e2) =>
        {
            this.Show();

            string newData = otherForm.NewData;
        };
    }
}
public partial class Form2 : Form
{
    public Form2()
    {
        InitializeComponent();
        //Use SomeData to populate controls.
    }

    public string SomeData { get; set; } //data passed in from other form

    public string NewData { get; private set; }  //data to expose to other form

    private void button1_Click(object sender, EventArgs e)
    {
        NewData = "SomeDataToPassToForm1";
        //this.Close();
        this.Hide();
    }
}

A few notes:

• You’re passing around an ArrayList, rather than strings as I did in
  this example. An ArrayList is a mutable reference type, whereas
  string is immutable. This means that you can just modify the
  ArrayList passed to Form2, and those changes will be reflected in
  the variable in Form1 since they both point to the same underlying
  ArrayList. I left this code in here as it covers the general case
  though.
• You say that you hide Form2 when you click the submit button.
  Normally in this design you would close it since it won’t be needed
  anymore. If you really don’t plan to use it again I suggest closing
  it. If you really do plan to show that form again then just hiding
  it is fine.
• If you close Form2 on submit it will fire the FormClosing event, if
  you just hide it it will fire the Visible event. You should probably
  remove one of those two event handlers in my code depending on
  whether you actually close it or hide it. If you sometimes do one and
  sometimes the other then feel free to leave both. You won’t actually
  harm anything (other than confusing people) if you leave both in even
  if you only use one.