C#更改配置文件问题

程序中配置文件代码如下：
<?xml version="1.0" encoding="utf-8" ?>
<configuration>

<connectionStrings>
<!--<add name="constr" connectionString="server=60.90.174.174;database=msg;uid=message;pwd=ms_user" providerName="MySql.Data.MySqlClient"/>-->
<add name="constr" connectionString="server=.;database=DB;uid=sa" providerName="System.Data.SqlClient"/>
</connectionStrings>

<appSettings>
<!--<add key="constr" value="server=60.190.174.174;database=msg;uid=message;pwd=ms_user"/>-->
<add key="constr" value="server=.;database=DB;uid=sa"/>
</appSettings>
</configuration>

抽象工厂中代码如下：
public IData CreateData(string constr)
{
switch (constr)
{

配置文件修改成如下代码：
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<connectionStrings>
<!--<add name="mysql" connectionString="server=60.90.174.174;database=msg;uid=message;pwd=ms_user" providerName="MySql.Data.MySqlClient"/>-->
<add name="sqlserver" connectionString="Data Source=.;Initial Catalog=DB;Integrated Security=True" providerName="System.Data.SqlClient;"/>
</connectionStrings>

<appSettings>
<!--<add key="constr" value="mysql"/>-->
<add key="constr" value="sqlserver"/>
</appSettings>
</configuration>

工厂修改成如下代码：

public IData CreateData(string constr)
{
switch (constr)
{
case "mysql":