无法在脱机的数据库上调用sp\u detach\u db

我可以在sqlmanager中运行此命令来分离数据库

ALTER DATABASE mydb SET OFFLINE WITH ROLLBACK IMMEDIATE

GO

dbo.sp_detach_db @dbname = N'mydb',@keepfulltextindexfile = N'false'

当我使用相同的连接时,通过ado.net运行相同的commadn失败,并出现错误:

The database 'mydb' can not be opened because it is offline 

(错误从德语翻译过来。)

ADO.NET代码是

            SqlCommand cmdOffline = new SqlCommand(@"ALTER DATABASE mydb SET OFFLINE WITH ROLLBACK IMMEDIATE");
            cmdOffline.Connection = prepareMasterDBConnection;
            cmdOffline.ExecuteNonQuery();


            SqlCommand cmdDetach = new SqlCommand(@"dbo.sp_detach_db @dbname = N'mydb',@keepfulltextindexfile = N'false'");
            cmdDetach.Connection = prepareMasterDBConnection;
            cmdDetach.ExecuteNonQuery();

连接设置为master-db并打开。第一个命令成功地执行了。

从ADO和SQL管理器调用代码有什么区别?