MSDN Magazine - October 2007 - (Page 125)

Task Scheduler 2.0 KENNY KERR ask Scheduler received a complete overhaul in Windows Vista®. Although there are some similarities, the new Task Scheduler (dubbed Task Scheduler 2.0) is far more powerful than the original, which has been around since Windows® 98. It is no longer just a simple tool for end users, but a powerful platform for designing and managing complex background operations—so much so that it can remove the need for Windows service development in many cases. Imagine your project needs to check for updates automatically. You might think to write a Windows service that runs in the background and checks for updates every few days. Instead of a service that has to run all the time, you can design a scheduled task that only runs every few days, checks for updates, and then stops. Even better, you can ensure that it runs only when a user is logged in so that resources aren’t needlessly consumed when no one is around to perform an update. It is evident that Microsoft developers consider the new Task Scheduler ready for prime time by the simple fact that it is used by many parts of Windows Vista to manage background tasks. This has the added benefit of reducing the number of new services that would inevitably have been created to handle all those tasks. It also simplifies Windows administration and diagnostics since it is far simpler to interrogate a task to see what it’s doing than it is to figure out what a black-box Window service is up to. For tasks that form part of Windows itself, the Task Scheduler also acts as a task host, consolidating many task-specific processes into one and thereby further reducing the resources that are required to host those operations. In this column I am going to explore the main concepts and building blocks that make up the Task Scheduler so you can get up to speed as quickly as possible and start benefiting from this great new service right away. T recognize tasks that are written directly to the file system and will likely refuse to run tasks that have been tampered with. Since Task Scheduler is such an integral part of Windows Vista, it is no longer possible for administrators to simply disable it. This is good news for developers because it means you can safely rely on it to perform your background operations and abstract the task storage. By convention, tasks are grouped by company, product, and component name. For example, the Windows Disk Defragmenter stores its tasks in the \Microsoft\Windows\Defrag subfolder. Each task is stored as an XML document defined by the Task Scheduler schema.You can then create tasks directly with an XML Since Task Scheduler is such parser like XmlLite (see my ® an integral part of Windows previous MSDN Magazine article at msdn.microsoft.com/ Vista, it is no longer possible msdnmag/issues/07/04/Xml/), or use the extensive COM disable it. interfaces provided by the Task Scheduler API. The COM interfaces also allow you to provide and query the XML definitions for tasks directly. In that way you get the benefit of using XML to author and query tasks while being shielded from their actual storage. Task Scheduler is also accessible remotely. for administrators to simply Using the Task Service The ITaskService interface is the gateway into the Task Scheduler API. All of the definitions you will need are located within the taskschd.h header file. Of course, you will need the latest Windows SDK that includes support for Windows Vista. Start by creating a local, in-process instance as follows: CComPtr service; HR(service.CoCreateInstance(__uuidof(TaskScheduler))); Task Service and Storage The story begins with the Task Scheduler service, the service responsible for the actual scheduling of tasks. Since Task Scheduler uses the file system to store task information, it is possible to enumerate and prepare tasks without the help of the service, but you won’t get very far without it. Indeed, future versions of the Task Scheduler may well change the storage format and location, so relying on this is not advised. In fact, it is critical that you not rely on accessing the storage directly as the Task Scheduler may not (I use the HR macro in these samples to clearly identify where methods return an HRESULT that needs to be checked; you can replace this with appropriate error handling—whether that is throwing an exception or returning the HRESULT yourself.) Next, call the Connect method to establish a connection to the task scheduler on the computer of your choice. The Connect method is defined as follows: HRESULT Connect( VARIANT computer, VARIANT user, VARIANT domain, VARIANT password); The use of the VARIANT type is unfortunate, but easy enough october2007 125 http://msdn.microsoft.com/msdnmag/issues/07/04/xml http://msdn.microsoft.com/msdnmag/issues/07/04/xml

Table of Contents Feed for the Digital Edition of MSDN Magazine - October 2007

Cover Contents Toolbox CLR Inside Out Basic Instincts Data Points Cutting Edge Pooled Threads WPF Threads Parallel Linq Parallel Performance Mobile Apps Test Run Foundations Windows with C++ Netting C++ .NET Matters { End Bracket } Net Nuptials

MSDN Magazine - October 2007

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