Try this !!
<asp:DropDownList ID="ddl" runat="server" AutoPostBack="true" OnSelectedIndexChanged="ddl_SelectedIndexChanged">
</asp:DropDownList>
<asp:GridView runat="server" ID="grdview">
</asp:GridView>
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
con.Open();
string query = "select ruleid from tbl_Rules";
SqlDataAdapter da = new SqlDataAdapter(query, con);
con.Close();
DataTable dt = new DataTable();
da.Fill(dt);
ddl.DataSource = dt;
ddl.DataTextField = "RuleId";
ddl.DataValueField = "RuleId";
ddl.DataBind();
ddl.Items.Insert(0, new ListItem("----Select----", "0"));
}
}
protected void ddl_SelectedIndexChanged(object sender, EventArgs e)
{
string query = "select ruleid from tbl_Rules where ruleid=" + ddl.SelectedValue;
SqlDataAdapter da = new SqlDataAdapter(query, con);
con.Close();
DataTable dt = new DataTable();
da.Fill(dt);
grdview.DataSource = dt;
grdview.DataBind();
}