连接字符串放到配置文件中，会出现未将对象引用设置到对象的实例

             //这步主要是将连接字符串放到配置文件中时，用这表达
            string connStr = ConfigurationManager.ConnectionStrings["ConnStr"].ConnectionString;运行到这，会出现未将对象引用设置到对象的实例
            using (SqlConnection conn = new SqlConnection(connStr))
            {
                conn.Open();
                using (SqlCommand cmd = conn.CreateCommand())
                {
                     = "delect from IP_table";
                    cmd.ExecuteNonQuery();
                }
            }

    配置文件设置：
<configuration>
  <connectionStrings>
    <add name="ConnStr" connectionString="Server =.\SQLEXPRESS;AttachDBFilename
             =C:\USERS\ADMINISTRATOR\DESKTOP\C\手机IP查询\手机IP查询\手机IP查询\DATABASE1.MDF;
            Integrated Security =true;User instance = true"/>
  </connectionStrings>
</configuration>