Asp.net dropdownlist selecteditems错误

我有3个下拉列表和1个按钮..当页面加载时，它会转到最后一个其他部分…如果我选择任何一个下拉列表并单击btn..它显示为空，即使我在数据库中有数据。。。我想我在选择的部分犯了一些错误…你能给我一些建议吗

if (employee_name_ddl.SelectedItem.Text != "Select" & employee_paricularpro_ddl.SelectedItem.Text == "Select" & employee_status_ddl.SelectedItem.Text == "All Status")
            {
                ............
            }
            else if (employee_name_ddl.SelectedItem.Text == "Select" & employee_paricularpro_ddl.SelectedItem.Text != "Select" & employee_status_ddl.SelectedItem.Text == "All Status")
            {
                ............................
            }
            else if (employee_name_ddl.SelectedItem.Text == "Select" & employee_paricularpro_ddl.SelectedItem.Text == "Select" & employee_status_ddl.SelectedItem.Text != "All Status")
            {
               ............................
            }
            else if (employee_name_ddl.SelectedItem.Text != "Select" & employee_paricularpro_ddl.SelectedItem.Text != "Select" & employee_status_ddl.SelectedItem.Text == "All Status")
            {
                ...............................
            }
            else if (employee_name_ddl.SelectedItem.Text != "Select" & employee_paricularpro_ddl.SelectedItem.Text == "Select" & employee_status_ddl.SelectedItem.Text != "All Status")
            {
               ........................
            }
            else if (employee_name_ddl.SelectedItem.Text == "Select" & employee_paricularpro_ddl.SelectedItem.Text != "Select" & employee_status_ddl.SelectedItem.Text != "All Status")
            {
               ...................
            }
            else if (employee_name_ddl.SelectedItem.Text != "Select" & employee_paricularpro_ddl.SelectedItem.Text != "Select" & employee_status_ddl.SelectedItem.Text != "All Status")
            {
       .........................
            }
            else
            {
                ....................
            }

的默认值为null，因此，当未选择任何DropDownList时，您可能会在文本属性的某个位置获得NullReferenceException

您还使用了按位比较&而不是正确的逻辑&&

下面是一个示例来说明&和&&的区别：

---

嗨@Tim Schmelter，谢谢你回复我…我应该使用ifemployee_name_ddl.SelectedItem！=空&员工姓名ddl.SelectedItem==null@user1285783当前位置我认为这是不言自明的。否，您应该使用ifemployee\u name\u ddl.SelectedItem！=null&&employee\u name\u ddl.SelectedItem.Text==选择带有注释的第一种方法//无例外如果我使用&&it将同时检查这两个条件？

if(employee_name_ddl.SelectedItem != null && employee_name_ddl.SelectedItem.Text == "Select")
{
    // no exception whether or not employee_name_ddl.SelectedItem is null
}

if(employee_name_ddl.SelectedItem != null & employee_name_ddl.SelectedItem.Text == "Select")
{
    // exception when employee_name_ddl.SelectedItem is null,
    // because the second condition will be evaluated anyway
}