Question

Here is my sample code:

private void button1_Click(object sender, EventArgs e)
    {
        listBox1.Items.Clear();
        foreach (var i in numbers())
        {
            listBox1.Items.Add(i);
        }
    }

    private List<int> numbers()
    {
        List<int> counts = new List<int>();
        for (int i = 1; i <= 5; i++)
        {
            counts.Add(i);
        }
        return counts;
    }

I want to know if it is possible to not overwrite and remain the previous values from a List<>. In my sample if I click the button it will populate 1,2,3,4,5 in the listbox and when I click the button again I am expecting an output of 1,2,3,4,5,1,2,3,4,5 this values is what I am expecting from my List<> How could I possibly do that? BTW, the listbox there is just for display purposes. Thanks!

Answer 1

With List<int> counts = new List<int>();, you create a new, empty list. This happens anew with each call to numbers().

If I understand your question correctly, you want numbers() to add the five numbers to an existing lists, that hence grows with each call to numbers().

In order to achieve this, do not create a new List<int> with each call of numbers() that you return as a result value; instead create a private field for that list and modify it with each call to numbers():

private List<int> counts = new List<int>();

private List<int> numbers()
{
    for (int i = 1; i <= 5; i++)
    {
        counts.Add(i);
    }
    return counts;
}

Note that numbers() will still return the list, but it’s not a new list; it’s the same list instance every time.