菜鸟求助一点问题，未将对象引用设置到对象的实例
调试代码时行27说有错误，未将对象引用设置到对象的实例，求大神指点行 25:
行 26: SqlConnection conn = new SqlConnection();//创建连接对象
行 27: conn.ConnectionString = ConfigurationManager.ConnectionStrings["NorthwindConnnectString"].ToString();
行 28: conn.Open();
行 29: SqlCommand cmd = new SqlCommand();
源代码
SqlConnection conn = new SqlConnection();//创建连接对象
conn.ConnectionString = ConfigurationManager.ConnectionStrings["NorthwinfConnectionString "].ToString();
conn.Open();
SqlCommand cmd = new SqlCommand();
cmd.Connection = conn;
cmd.CommandText = "INSERT INTO myTable VALUES('" + "张三" + ',' + "男')";//数据库执行语句
//通过返回的数值，判断数据库命令是否执行
int val = cmd.ExecuteNonQuery();
if (val > 0)
{
Response.Write("数据已经接加载成功");
}
//判断数据库是否连接
if (conn.State == ConnectionState.Open)
conn.Close();
web。config也只有这个
该怎么配置
<connectionStrings>
<add name="DefaultConnection" providerName="System.Data.SqlClient" connectionString="Data Source=(LocalDb)\v11.0;Initial Catalog=aspnet-connnect-20130511145815;Integrated Security=SSPI;AttachDBFilename=|DataDirectory|\aspnet-connnect-20130511145815.mdf"/>
</connectionStrings> --------------------编程问答-------------------- <connectionStrings>
<add name="DefaultConnection" providerName="System.Data.SqlClient" connectionString="Data Source=(LocalDb)\v11.0;Initial Catalog=aspnet-connnect-20130511145815;Integrated Security=SSPI;AttachDBFilename=|DataDirectory|\aspnet-connnect-20130511145815.mdf"/>
</connectionStrings>
红色部分改成NorthwindConnnectString
<connectionStrings>
<add name="NorthwindConnnectString" providerName="System.Data.SqlClient" connectionString="Data Source=(LocalDb)\v11.0;Initial Catalog=aspnet-connnect-20130511145815;Integrated Security=SSPI;AttachDBFilename=|DataDirectory|\aspnet-connnect-20130511145815.mdf"/>
</connectionStrings> --------------------编程问答-------------------- 还是不行，一样的毛病
百度了一下，说要在web.config的<connectionstrings></connectionStrings>这个里面添加一个才行，但是不知道怎么添加 --------------------编程问答-------------------- conn.ConnectionString = ConfigurationManager.ConnectionStrings["DefaultConnection"].ToString();
补充：.NET技术 , ASP.NET
