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

Question

The issue I'm noticing is this line of code:

tempList.Add(orderables);

In this full code:

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

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

    for (int i = 0; i < fpSpread1.ActiveSheet.RowCount; i++)
    {
        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);
}

When I put my breakpoint on the line mentioned above (tempList.Add(orderables); ), the first time it adds the item correctly to the templist and it will have one item in it. The second time it will add the correct item to the list but if I hover over tempList and want to see its contents, although it has two items, both of them are the same - they are both now the second item that was added to the list. It has overwritten the first one.

I can't figure out what is going wrong with this and why it is happening.

tempList.Clear(); empties the list – Pencho Ilchev May 17 '12 at 22:38 — Pencho Ilchev, May 17 '12 at 22:38

score 8 · Accepted Answer · edited Sep 18 '18 at 18:14

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,
};

Thanks. That fixed it. For my learning purposes: I was thinking one object is enough because every time I am assigning new values to it and passing it to the list...so Is it because it is a Reference Type? and that's the reason? — Bohn, May 17 '12 at 22:42
Precisely. Reference types (classes) differ from value types (structs) because the latter are copied on assignment (e.g. when passed to the `List.Add` method) whilst the former only have their _reference_ (pointer) copied, but would still point to the same object (instance). Thus, changes to an object’s state through any of the variables referencing it would then “appear” on all other variables as well. — Douglas, May 17 '12 at 22:46

score 2 · Answer 2 · answered May 17 '12 at 22:38

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.

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

2 Answers2

Linked

Related