在实体对象中访问导航属性里的属性值出现异常“There is already an open DataReader associated with this Command which must be

上面图片中的代码在执行到第二行的时候会报上述错误，解决方法：在数据库的连接字符串上添加MultipleActiveResultSets=true

<connectionStrings>
    <add name="ContosoUniversity"
         connectionString="Server=.;Database=ContosoUniversity;Trusted_Connection=True;MultipleActiveResultSets=true" />
  </connectionStrings>

SQL条件!=null查不出数据

不要使用!=nulll，使用 is not null判断

WHERE
(
    order_amount is not null 
    and order_amount != '' 
    and received_amount is not null 
    and received_amount != ''
 )