MySql - 连接必须有效并且打开

Question

以下代码在cmd上调用ExecuteNonQuery（）的行上抛出以下异常。

System.InvalidOperationException: Connection must be valid and open.
   at MySql.Data.MySqlClient.ExceptionInterceptor.Throw(Exception exception)
   at MySql.Data.MySqlClient.MySqlCommand.Throw(Exception ex)
   at MySql.Data.MySqlClient.MySqlCommand.CheckState()
   at MySql.Data.MySqlClient.MySqlCommand.ExecuteReader(CommandBehavior behavior)
   at MySql.Data.MySqlClient.MySqlCommand.ExecuteNonQuery()
   at Oasis.AlertManager.DB.MySQL.Insert(MySqlCommand cmd) in c:\BuildAgent\work\8b0cd51e92588e6d\Oasis\Oasis.AlertManager.DB\MySQL.cs:line 471

public int Insert(MySqlCommand cmd)
{
    int rows;
    using (masterConnection = new MySqlConnection(masterAlertConnectionStr))
    {
        masterConnection.Open();
        cmd.Connection = masterConnection;
        rows = cmd.ExecuteNonQuery();
    }
    return rows;
}

我很困惑，因为在masterConnection上调用了Open（）。谁看过这个吗？它可能与命令有关，还是没有在某处设置的超时？