How to clear all data in a listBox?

What about

listbox1.Items.Clear();

If it is bound to a Datasource it will throw an error using ListBox1.Items.Clear();

In that case you will have to clear the Datasource instead. e.g., if it is filled with a Datatable:

  _dt.Clear();   //<-----Here's the Listbox emptied.
  _dt = _dbHelper.dtFillDataTable(_dt, strSQL);

  lbStyles.DataSource = _dt;
  lbStyles.DisplayMember = "YourDisplayMember";
  lbStyles.ValueMember = "YourValueMember";