How to pass combobox1 items from form1 to combobox2 in form2

Question

I have two combobox (combobox1 in form 1 and combobox2 in form2). I need to import an excel file and the combobox1 gets the sheets name. Then in the form2 (I don't want to import the same file another time) i have combobox2 that have also the same sheets name from the combobox1.

I tried this but i got the error of NULLFUNCTION Expression(Additional Information: The object reference is not set to an instance of an object).

 using (OpenFileDialog openFileDialog = new OpenFileDialog() { Filter = "Excel 97-2003 Workbook|*.xls|Excel Workbook|*.xlsx |fichiers Textes (*.txt)|*.txt|fichiers CSV (*.csv)|*.csv|tous les fichiers (*.*)|*.*" })
            {
                if (openFileDialog.ShowDialog() == DialogResult.OK)
                {
                    TextBox_Emplacement.Text = openFileDialog.FileName;
                    using (var stream = File.Open(openFileDialog.FileName, FileMode.Open, FileAccess.Read))
                    {
                        using (IExcelDataReader reader = ExcelReaderFactory.CreateReader(stream))
                        {
                            DataSet result = reader.AsDataSet(new ExcelDataSetConfiguration()
                            {
                                ConfigureDataTable = (_) => new ExcelDataTableConfiguration() { UseHeaderRow = true }
                            });
                            dataTableCollection = result.Tables; 
                            dataTableCollection1 = result.Tables;
                            sheet.Items.Clear();
                            //cp.radDropDownList1.Items.Clear();
                            foreach (DataTable table in dataTableCollection)
                                sheet.Items.Add(table.TableName);//add sheet to combobox

                           **Form_Copie cp1 = (Form_Copie)Application.OpenForms["Form_Copie"];
                           foreach (DataTable table in dataTableCollection)
                              cp1.radDropDownList1.Items.Add(table.TableName);//add sheet to combobox**

Answer 1

If the item you're trying to pass is just the combobox string value Could pass the value as you load the new form

{
string sheet = comboBox1.Text;
Form frm2 = new Form2(sheet);
frm2.show();
}

Public Form2(string sheet)
{
Initialize();
comboBox2.Text = sheet;
}

Answer 2

There are a few ways this can be done. Overloading the constructor is a good way, but doing this per control can quickly become daunting and messy. If you want to do this for multiple controls I'd suggest using a model that contains properties of Bindings (eg System.ComponentModel.BindingList and/or System.Forms.Binding) as a field in each form. Then overload the constructor and pass in the bindings through the model class.

public class BindingModel {
    public property BindingList<string> ComboBoxBinding {get;set;}
    ... other binding/non-binding properties ...
}

public class Form1{
    private BindingModel _bindingModel = new _bindingModel();

    private void Form1_Shown(object sender, EventArgs e) {
        this.ComboBox1.DataSource = _bindingModel.ComboBoxBinding;
        this.ComboBox1.DataMember = "";

        // the add would go in your excel method
        _bindingModel.ComboBoxBinding.Add("Item 1");
    }

    private void btnOpenForm2_Click(object sender, EventArgs e) {
        var frm = new Form2(_bindingModel);
        frm.Show();
        this.Close();
    }
}

public class Form2{
    private BindingModel _bindingModel = new _bindingModel();
    
    public void Form2(BindingModel bindingModel){
        this._bindingModel = bindingModel;
    }

    // Once the form is shown bind the ComboBox and it will populate the values
    private void Form2_Shown(object sender, EventArgs e){
        this.ComboBox1.DataSource = _bindingModel.ComboBoxBinding;
        this.ComboBox1.DataMember = "";
    }
}

Alternatively it is possible to set the controls items within the calling form:

public class Form1{
    private void btnOpenForm2_Click(object sender, EventArgs e) {
        var frm = new Form2();
        for(int i = 0; i < this.ComboBox1.Count - 1; i++){
            frm.ComboBox1.Items.Add(ComboBox1.Items[i]);
        }
        frm.Show();
        this.Close();
    }
}