避免刷新页面

我必须避免页面刷新。每次单击提交按钮，页面都会刷新。我该如何避免这种情况？

protected void Button1_Click(object sender, EventArgs e)
{
    string firstname = DropDownList1.SelectedItem.Text;
    if (firstname == "All")
    {
        da = new SqlDataAdapter(query, con);
    }
    dt = new DataTable();
    dt = ds.DataTable1;
    da.Fill(dt);
    rdc.Load(Server.MapPath("CrystalReport.rpt"));
    rdc.SetDataSource(ds);
    CrystalReportViewer1.ReportSource = rdc;
    CrystalReportViewer1.RefreshReport();
}