passing variables' values

Hi everyone,
I know it must be a silly question but I'm quite confused with this issue:
 
public partial class _Default : System.Web.UI.Page
{
    String sql2 = "";
    DataSet ds;

     protected void Button1_Click(object sender, EventArgs e)
    {
        if (Page.IsValid)
        {
            QueryCommand smd = new QueryCommand(sql2);    // sql2 is NOT null here

            ds = DataService.GetDataSet(smd);
           GridView1.DataSource = ds;          // ds is NOT NULL here

        } 
   ...