How to pass listbox data from form1 to form2 with a button click

Question

I have two Windows Forms. I have read data from the database and load data into Listbox1 on Form1(FrmSelection) . Now what i want is, when a user select a value from Listbox1 on Form1 and click process button, the selected value must be loaded on Form2(FrmProcessOrder) Listbox2 . I have tried different examples but am not get it to work. here are the steps below

1.Run Application

2.FrmSelection Opens with empty listbox,then click process button which opens FrmSelection

3.FrmSelections opens up,this form contains one listbox and data the i have read from the database.

4.Select one item from the listbox

5.Click Addforms button after selecting the item from listbox on FrmSelections.

6. Selected item must appear on FrmProcessorder listbox.

Problem - Am getting error on FrmProcessorder form when i click Addforms button after selecting the item(object not set to an instance of an object)

here is my code below..

Form1(FrmSelection)

    public partial class FrmProcessOrder : Form
{

    public FrmProcessOrder()
    {
        InitializeComponent();        
    }
    private void btnProcess_Click(object sender, EventArgs e)
    {
        Program._FrmSelection = new FrmSelection();
        Program._FrmSelection.Show();
    }

    public void AddList(ListBox _Listing)
    {
        ListBoxForms.Items.AddRange(_Listing.Items); 
    }
}

Form 2(FrmProcessOrder )

   FrmProcessOrder _FrmProcessOrder;
    public FrmSelection()
    {
        InitializeComponent();

        LoadData();
    }

    //Method which reads data from the db and laod data on listview
    public void LoadData()
    {
        _connection = "Data Source=MILESTONE-PC;Initial Catalog=Jack;Integrated Security=True";
        _sql = "Select * from tblJackon";
        _conn = new SqlConnection(_connection);
        _conn.Open();

        _comm = new SqlCommand(_sql, _conn);
        _adapt.SelectCommand = _comm;
        _adapt.Fill(_dataset, _sql);
        DataTable _tbl = _dataset.Tables[0];
        _dtrow = null;

        foreach (DataRow _dataVariable in _tbl.Rows)
        {
            _dtrow = _dataVariable;
            ListBoxSelection.Items.Add((_dtrow["FormNames"]));
        }
    }

    private void btnAddForms_Click(object sender, EventArgs e)
    {
        _FrmProcessOrder.AddList(ListBoxSelection);  // It throws error on this line after select from listbox so that the item must to FrmProcessOrder listbox
        _FrmProcessOrder = new FrmProcessOrder();    
    }

Answer 1

Edit (after the question is edited):

Check this:

FrmProcessOrder _FrmProcessOrder; //no object declared

private void btnAddForms_Click(object sender, EventArgs e)
{
    _FrmProcessOrder.AddList(ListBoxSelection);  // This is null exception
    _FrmProcessOrder = new FrmProcessOrder(); //it should not be put here
}

Your _FrmProcessOrder is used before it is initialized properly. Do this instead:

FrmProcessOrder _FrmProcessOrder = new FrmProcessOrder(); //Now it is declared, only once

private void btnAddForms_Click(object sender, EventArgs e)
{
    _FrmProcessOrder.AddList(ListBoxSelection);  // Now it should be ok
}

---

Original:

One of the simplest way would be to put your input as a field (or create a method call to do it) rather than as a constructor argument.

Change this:

public FrmProcessOrder(ListBox _List)
{
    InitializeComponent();
    ListBoxForms2.Items.AddRange(_List.Items);         
}

Into this

public FrmProcessOrder()
{
    InitializeComponent();
}

public void AddList(ListBox _List){
    ListBoxForms2.Items.AddRange(_List.Items);         
}

Then when you could call your Form Method whenever you need

_FrmProcessOrder.AddList(listBox);

Answer 2

In this line

Application.Run(new FrmProcessOrder());

you're trying to instantiate a new FrmProcessOrder , but you have one constructor like this:

public FrmProcessOrder(ListBox _List)

so you can't instantiate without a parameter ListBox . If you want this you have to add to Form1 the empty constructor:

public FrmProcessOrder()
{

}

Answer 3

I believe you have wrongly copied the code blocks(swapped Form2 & Form1).

This line Application.Run(new FrmProcessOrder()); should be replaced by Application.Run(new FrmSelection()); which would load the data from the database.

The compilation error you are seeing is since you do not have an empty constructor. As per the current code, to instantiate 'FrmProcessOrder' you have to provide an instance of ListBox.