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Backups can fail. Fortunately, this is a rare occurrence and is usually accompanied by some additional information that will help you to diagnose the problem and correct it before trying to perform another backup. I won t go into all the possible reasons for a failed backup, but I will show you where to get additional information to help you find the source of the problem. During the course of a manual backup, the backup on the computer I was using failed. A message balloon appeared with a message saying that the backup failed because of an error reading data from my hard drive, as shown in Figure 7-25.

There s still a lot more you can do with this example. One improvement would be to implement some form of task queuing. This approach prevents the possibility that the user might start a huge number of threads running simultaneously, ensuring that none can get enough system resources to finish their work (a problem known as thread starvation). Instead, you allow only a set number of threads to work at once. Once you reach the limit, you add any additional requests to a queue and execute them only when a thread becomes available. To manage this work, you need to replace the simple collection of worker threads (from the previous example) with a dedicated class that wraps the queuing and thread management work. This model requires too much code to show it all here, but you can see the complete code in the downloadable examples for this chapter.

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Note You can also use the ThreadPool class from the System.Threading namespace for a simple implementation of thread queuing that uses threads from the CLR s pool. However, the ThreadPool doesn t give you much flexibility for example, you can t stop tasks, report progress, control how many tasks execute at once, and change priorities. However, the ThreadPool implementation is still better than the example you ll consider in this section in one respect. Because it reuses threads for more than one task, you avoid the overhead of creating new threads.

The basic idea is that your form uses a new TaskManager class. The TaskManager class derives from Component, so it can be added to a form at design time. This makes it easy to hook up event handlers. public class TaskManager : System.ComponentModel.Component { ... } The TaskManager allows you to choose how many tasks can be performed at a time through a MaxThreads property: private int maxThreads = 2; public int MaxThreads { get { return maxThreads; } set { maxThreads = value; } } The TaskManager class wraps three ThreadWrapper collections. These collections reflect tasks that are queued, currently underway, and completed: // Track ongoing workers. List<ThreadWrapper> workers = new List<ThreadWrapper>(); // Track queued requests. List<ThreadWrapper> workersQueued = new List<ThreadWrapper>(); // Task completed requests. List<ThreadWrapper> workersCompleted = new List<ThreadWrapper>();

Summary

Figure 7-25. A message balloon advising of a failed backup The same error message appeared in the Backup Status dialog box, as shown in Figure 7-26. It is useful to view the backup status messages, as you may not see the message balloon. It will appear and disappear again in only a few seconds, so if you happen to be away from your computer at the time, you could miss it.

To add a new task to the queue, the client simply calls EnqueueTask(). This method doesn t start the work instead, it adds it to the collection of queued requests. public void EnqueueTask(ThreadWrapper task) { lock (workersQueued) { workersQueued.Add(task); } } The magic happens in the AllocateWork() method, which runs continuously on a lowpriority thread. The TaskManager doesn t actually create this thread and start allocating work until the client calls StartAllocatingWork(). private Thread allocateWork; private bool working = false; private Control invokeContext; public void StartAllocatingWork(Control invokeContext) { if (!working) { this.invokeContext = invokeContext; allocateWork = new Thread(new ThreadStart(AllocateWork)); allocateWork.Priority = ThreadPriority.BelowNormal; allocateWork.IsBackground = true; working = true; allocateWork.Start(); } } There s another important detail here. When the client calls AllocateWork(), it passes in a reference to the current form. The TaskManager uses this to call Control.Invoke() before raising any events. That way, the events are always raised on the user interface thread, and the client application is completely insulated from the threading complexities. The AllocateWork() method has the bulk of the work. It walks through the three collections of ThreadWrapper objects. It performs its work in a continuous loop, sleeping for a few seconds after each pass to allow other threads to do some work. private void AllocateWork() { while (true) { // (Allocate work, check for completed items, and report progress here.) Thread.Sleep(TimeSpan.FromSeconds(5)); } } The AllocateWork() method performs three tasks in its loop. First it removes completed tasks and fires the appropriate completion events.

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