C# problem trying to add items in combobox via another form

Question

I have this situation: in my form Order there's a combobox with many products inside. it is expected that the user could add products to the combobox to use in Order, but this is made via another form called ProductAdd, basically made with a textbox where the user types the name of the product and it's added with a button. Since I can't have access to the combobox in the Order form when I'm in the ProductAdd form, I made a method in Order which add into the combobox the product passed.

The string is not added to the combobox in the other form.

This is the method in Order to operate in its combobox

public void addProductInCbb(string newProduct)
  {
      cbbProdotti.Items.Add(newProduct);
  }

This is the method in the other form ProductAdd to add the string in my cbb

private void btnConfirmNewProduct_Click(object sender, EventArgs e)
        {   
            Order o = new Order(new Form1()); //that's because I think I need an instance of Order to call the method... is that correct?
            String newProduct= txtNewProduct.Text; //get product string from txt

            //boring checks to say if product already exists
            bool found = false;
            ArrayList products= o.getProducts();

            foreach(String product in products)
            {
                if (product.Equals(newProduct)) found = true;
            }
            if (!found)
            {
                o.addProductInCbb(newProduct); //method call from Order
                MessageBox.Show("Success!","", MessageBoxButtons.OK, MessageBoxIcon.Information);
            }  
            else MessageBox.Show("Product already exists!", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                
        }

-EDIT- I made the weird "Order o = new Order(new Form1())" constructor because: to call addProductInCbb(string) I ned an Order instance, BUT in turn, the Order constructor ned a Form1 parameter, because when the Order is completed, a PDF is created with all the data from both form1 and Order form... May this cause my issue?

Answer 1

Here are a set of steps to follow to learn, without disturbing your existing code:

• Make a new winforms project
• Ensure it has 2 forms (form1, form2)
• On the first form put a combobox (comboBox1) and a button (button1)
• On the second form, put two text box (textBox1, textbox2) and a button (button1)
• In the constructor of form1, after IinitializeComponent, put:

    var dt = new DataTable();
    dt.Columns.Add("disp");
    dt.Columns.Add("val");
    dt.Rows.Add("first", "1");
    dt.Rows.Add("second", "2");

    comboBox1.DisplayMember = "disp";
    comboBox1.ValueMember = "disp";
    comboBox1.DataSource = dt;

    new Form2(dt).Show();

• Double click the button on form 1 and put a code in the event handler. This is just to demo some other thing, not needed for passing data between forms:

    MessageBox.Show(comboBox1.SelectedValue.ToString());

• Modify the constructor of Form2 to accept a DataTable parameter; store the parameter into a class level variable

    private DataTable _dt;

    public Form2(DataTable dt){
      _dt = dt;
    }

• Double click the button of form2 and put a handler code:

    _dt.Rows.Add(textBox1.Text, textBox2.Text);

• Run the app. You'll notice that whatever you write in tb1 and tb2 on form2, is added to the combo when you click the button. Clicking the button on form1 shows e.g. "1" when "first" is selected in the combo. This is how you show the user some nice string value, but you have some other value, like an int ID, behind it

WHat's happening is: form2 adds data into the shared datatable that both form2 and form1's combo know about (the combo uses the datatable as its data source). When rows are added the binding mechanism ensures that the databound control is notified and it will refresh itself.

It doesn't have to be a datatable, it can be any data containing class that you share, but datatables are fairly convenient for data binding in windows forms apps