Issue with List.Add() only saving the last added item [duplicate]

Answer 1

The problem is that you only have one instance of orderables and you keep changing the same instance and re-adding it to the list. Each reference in the list points to the same object. Move the orderables declaration inside the inner for loop and it will fix the problem.

Answer 2

You need to instantiate the Orderables within the for loop; otherwise, you keep reusing the same instance in all iterations (and overwriting its properties each time).

AssociatedComboItems ai = new AssociatedComboItems();
List<Orderables> tempList = new List<Orderables>();

foreach (var t in comboBox1.Items)
{
    ai.ComboBoxItem = t.ToString();

    for (int i = 0; i < fpSpread1.ActiveSheet.RowCount; i++)
    {
        Orderables orderables = new Orderables();  // ← Instantiate here
        orderables.Display = fpSpread1.ActiveSheet.Cells[i, 1].Text;
        orderables.ShowInDSR = (bool)fpSpread1.ActiveSheet.Cells[i, 0].Value;
        orderables.DisplayOrder = i;
        tempList.Add(orderables);
    }

    ai.AssociatedItems = tempList;
    tempList.Clear();
    if(AssociatedItems == null)
    AssociatedItems = new List<AssociatedComboItems>();
    AssociatedItems.Add(ai);
}

Unrelated to the question: You might find object initializer syntax to be cleaner:

Orderables orderables = new Orderables
{
    Display = fpSpread1.ActiveSheet.Cells[i, 1].Text,
    ShowInDSR = (bool)fpSpread1.ActiveSheet.Cells[i, 0].Value,
    DisplayOrder = i,
};