dropdownlist-selecteditems错误

Question

我有3个下拉列表和1个按钮..，当页面加载时，它转到最后的其他部分...如果我选择任何一个下拉列表并单击btn ..即使我在数据库中有数据也显示为空...我认为我在选择的部分犯了一些错误......你能不能提出一些想法....

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
            {
                ....................
            }

Answer 1

ListControl.SelectedItem的默认设置为null，因此，如果未选择任何NullReferenceException，您可能会在Text属性上获得DropDownList。

您还使用按位比较&而不是正确的逻辑&&。

以下示例显示&和&&：

的区别

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
}

What is the diffrence between & and && operators in C#