Q:

How to add SubItems from ListBox to ListView?

Each item in listbox are seperated on the comma. By splitting each line of the listBox i can add only the last items (e.g. STEVEN, D,E,F) but not the other items from the listbox  to the ListView

How can this be solved?
foreach (string item in listBox2.Items)
{
  ListViewItem li = new ListViewItem();
  var a = item.Split(',');
  li.Text = a[0].Trim();
  li.SubItems.Add(a[1].Trim());
  li.SubItems.Add(a[2].Trim());
  li.SubItems.Add(a[3].Trim());
  listView1.Items.Add(li);
}

A:

To use the ListView this way you should set its View property to Details and add Columns to display each string item after doing the split. You can't use your code since the split returns unequal string arrays. Instead, You could do:
foreach (string a in ListBox1.Items)
{
    var arr = a.Replace(" ", "").Split(',');

    ListViewItem lvi = new ListViewItem(arr[0]);

    for(int i = 1; i < arr.Length;i++)
    {
        if(i >= ListView1.Columns.Count )
        {
            ListView1.Columns.Add($"Column {i}");
        }
        lvi.SubItems.Add(arr[i]);
    }
    ListView1.Items.Add(lvi);
}

Good luck.

