继承母版页的两个页面之间的传值问题;将A页中Gridview中的一条记录值传到另B页

Question

界面上GirdView中的修改模板列:
<asp:TemplateField HeaderText="修改" ShowHeader="False">
                    <ItemTemplate>
                        <asp:Button ID="Button1" runat="server" CausesValidation="False" CommandName="Select"
                            PostBackUrl="~/Admins/UpdateUsers.aspx" Text="修改" />
                    </ItemTemplate>
                </asp:TemplateField> .cs里的取值代码:
        GridView gv = (GridView)PreviousPage.FindControl("gvusers");
        if (gv != null)
        {
            Label SourceTextBox = (Label)gv.SelectedRow.FindControl("Label1");
            Label Sourceddlcommission = (Label)gv.SelectedRow.FindControl("Label6");
            Label Sourceddlcommission_bjl = (Label)gv.SelectedRow.FindControl("Label7");
            if (Sourceddlcommission != null)
            {
                ddlCommission.SelectedValue = Sourceddlcommission.Text;
            }
            if (Sourceddlcommission_bjl != null)
            {
                ddlCommission_bjl.SelectedValue = Sourceddlcommission_bjl.Text;
            }
        }
之前没有用母版页.都可以实现.继承之后就不行了.调试结果是gv=null;