ArrayList in ViewState Add & Read Foreach?

I would like to add items to an ArrayList on Gridview checkbox click events to save the Gridview ID column value. How do I code it?

(ArrayList)ViewState["arrayListInViewState"];

protected void CheckBox1_OnCheckChanged(Object sender, EventArgs e)
{
      GridViewRow rw = (GridViewRow)((Control)sender).NamingContainer;
      ArrayList.Add(GridView1.DataKeys[rw.RowIndex]["id"].ToString());
            (ArrayList)ViewState["arrayListInViewState]"
}

protected void txtData_OnTextChanged(object sender, EventArgs e)
{
      GridViewRow rw = (GridViewRow)((Control)sender).NamingContainer;
      ArrayList.Add(GridView1.DataKeys[rw.RowIndex]["id"].ToString());
            (ArrayList)ViewState["arrayListInViewState]"
}

Also unsure how to foreach the arraylist later

protected void btnSubmit_Click(object sender, EventArgs e)
{
    //foreach...
}