dropdownlist-selecteditems错误

Question

我有3个dropdownlist和1个按钮..当页面加载时，它会转到最后一个部分...如果我选择任何一个下拉列表并单击btn..its显示空即使我有数据数据库...我想我将selectedItem部分做了一些错误...ü可以PLZ提出一些想法这个....

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

为ListControl.SelectedItem默认为null，因此你可能会在一个NullReferenceExceptionText某处没有选择任何DropDownList。

您还正在使用按位比较&而不是正确的逻辑&&。

下面是一个例子来说明的&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#