关于wpf SelectionChanged事件，给combox赋不上值

    private void vcategory1099Combox_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            System.Data.DataRowView dv = vcategory1099Combox.SelectedValue as System.Data.DataRowView;
            if (dv != null)
            {
                string value = dv.Row["vcategoryname"].ToString();
                if (value.Equals(this.FindResource("addaNew").ToString()))
                {
                    Accounting.pop.add_category_1099 pop = new Accounting.pop.add_category_1099();
                    pop.WindowStartupLocation = WindowStartupLocation.CenterScreen;
                    if (pop.ShowDialog() == true)
                    {
                        BindingCom();
                        vcategory1099Combox.SelectedValue = pop.returnID;
                    }
                }
            }

        }


为什么在SelectionChanged事件，怎么都设不上combox的值呢？（各种方法都不使）
以上代码大该意思当用户选择了 addNew 就弹出页新增，关闭后combox刷新并选择新增那条数据。 WPF

补充：.NET技术 ,  C#