System.Configuration.ConfigurationSettings.AppSettings取值有关问题？多谢

System.Configuration.ConfigurationSettings.AppSettings取值问题？高手请进，谢谢
System.Configuration.ConfigurationSettings.AppSettings
取值
config文件配置如下
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
   
  <appSettings>
  　　<add key="strCon" value="data source=CHINA-799D1397F;database=tempData;uid=sa"></add>
　</appSettings>
   
  <system.web>
catch捕捉到的错误
@"只允许元素 (c:\inetpub\wwwroot\subjectPRO\web.config line 4)"

[解决办法]
web.config 格式问题