how to select all listview items?

If there aren't very many items, this will do it:

foreach (ListViewItem item in myListView.Items)
{
    item.Selected = true;
}

If there are a lot of items, see this answer for how to use LVM_SETITEMSTATE.

Just pass your listview and checkstate to the function.

public void CheckAllItems(ListView lvw, bool check)
{
    lvw.Items.OfType<ListViewItem>().ToList().ForEach(item => item.Checked = check);
}

Related