Gridview DrowdownList selectedindexchanged not Firing

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ASP.NET

Gridview

DropDownList

<asp:GridView runat="server" ID="gridtest">
<columns> <asp:TemplateField>
<itemtemplate>
<asp:DropDownList ID="ddl_list" runat="server" AutoPostBack="true" EnableViewState="true"
OnSelectedIndexChanged="ddl_list_SelectedIndexChanged">
<asp:ListItem Text="1" Value="1">1
<asp:ListItem Text="2" Value="2">2
<asp:ListItem Text="3" Value="3">3

protected void Page_Load(object sender, EventArgs e)
   {
       SqlConnection con = new SqlConnection();
       con.ConnectionString = ConfigurationManager.AppSettings[ConnectionString"];
       if (!Page.IsPostBack)
       {
           con.Open();
           SqlCommand cmd = new SqlCommand("Select * from Mas_exporter;, con);
           cmd.ExecuteNonQuery();
           SqlDataAdapter da = new SqlDataAdapter();
           da = new SqlDataAdapter(cmd);
           DataSet ds = new DataSet();
           da.Fill(ds);
           gridtest.DataSource = ds;
           gridtest.DataBind();
       }

   }
   protected void ddl_list_SelectedIndexChanged(object sender, EventArgs e)
   {
       Response.Write("Success");
   }

This one firing dropdown selectedindexchange Correctly. But in my case i can't bind gridview on PageLoad. i will bind gridview on button click event. If i bind Gridview on button Click event , Drop downlist selectedindexchange didn't firing...what i need to do?