Unable to use "Add Range" to copy a list in a Class to another list in other Class

Question

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    class mylist
    {
        public List<string> list1 = new List<string>();
    }

    class mylist2
    {
        public List<string> list2 = new List<string> { "aton", "electron" };
        public void listconvert()
        {
            mylist ml = new mylist();
            ml.list1.AddRange(list2);
            MessageBox.Show("do you kn ow");
        }
    }

    private void button1_Click(object sender, EventArgs e)
    {
        mylist2 ml2 = new mylist2();
        ml2.listconvert();
        mylist ml = new mylist();

        for (int i = 0; i < ml.list1.Count; i++)
        {
            textBox1.AppendText(ml.list1[i].ToString() + "\n");
        }
    }
}

Answer 1

I'll narrow it down to these lines:

        mylist2 ml2 = new mylist2();
        ml2.listconvert();
        mylist ml = new mylist();

The third line in that snippet is working with a different instance of your mylist type than the second line in that snippet. When you then go on to interator over the ml variable, you haven't done anything with that list yet.

Maybe you want this:

public class mylist
{
    public List<string> list1 = new List<string>();
}

public class mylist2
{
    public List<string> list2 = new List<string> { "aton", "electron" };

    public void listconvert(List<string> other)
    {
        other.AddRange(list2);
        //MessageBox.Show("do you kn ow");
    }
}

private void button1_Click(object sender, EventArgs e)
{
    mylist ml = new mylist();
    mylist2 ml2 = new mylist2();

    ml2.listconvert(ml);   
    for (int i = 0; i < ml.list1.Count; i++)
    {
        textBox1.AppendText(ml.list1[i].ToString() + "\n");
    }
}

Answer 2

Your appear to have two issues.

First, this code creates a list but doesn't store a reference anywhere so it is effectively lost:

    public void listconvert()
    {
        mylist ml = new mylist();
        ml.list1.AddRange(list2);
        MessageBox.Show("do you kn ow");
    }

Second, this code creates an empty list and then tries to add the elements of this empty list to the text box:

        mylist ml = new mylist();

        for (int i = 0; i < ml.list1.Count; i++)
        {
            textBox1.AppendText(ml.list1[i].ToString() + "\n");
        }

You probably want to change your code to be like this:

    class mylist2
    {
        public List<string> list2 = new List<string> { "aton", "electron" };
        public mylist listconvert()
        {
            mylist ml = new mylist();
            ml.list1.AddRange(list2);
            MessageBox.Show("do you kn ow");
            return ml;
        }
    }

    private void button1_Click(object sender, EventArgs e)
    {
        mylist2 ml2 = new mylist2();
        mylist ml = ml2.listconvert();

        for (int i = 0; i < ml.list1.Count; i++)
        {
            textBox1.AppendText(ml.list1[i].ToString() + "\n");
        }
    }