code 128 c# The console application that uses the ClassCat class in Visual C#

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Listing 1.5 The console application that uses the ClassCat class
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using System; namespace Cat { class ClassMain { [STAThread] static void Main(string[] args) { ClassCat theCat; Contains a reference ClassCat.DidSomething notify; to ClassCat notify = new ClassCat.DidSomething(AddLine); theCat = new ClassCat("Tiger",notify); theCat.DoCatStuff(250); Creates an instance of ClassCat }
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static private void AddLine(string message) { Console.Write(message); } } }
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Is invoked when an action occurs
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Our cat will perform many actions. In order for the ClassMain class to know that the cat has performed an action, we must supply it with a delegate. The DidSomething delegate that s passed in to the constructor is invoked by the instance of the cat class whenever it accomplishes some task. The instance of the DidSomething delegate that s passed in is associated with the AddLine method. This method accepts a string as its only parameter. It then writes the contents of that string to the console. When we create our cat we pass in the instance of the DidSomething delegate along with the cat s name. After we ve created Tiger we tell it to do 250 iterations. This occurs on the main thread of the application. Once DoCatStuff completes, the application terminates. The following is a sample of the output produced by the program:
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"Zzz" "Meow" "Zzz" "Zzz" "Zzz" "Play with toy" "Wander around" "Zzz" "Zzz" "Take a bath" "Zzz" "Zzz" "Zzz" "Zzz" "Zzz" "Zzz" "Zzz" "Zzz" "Play with toy" "Zzz"
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We ve explored a simple example of how a thread resembles a cat. In the next section we take a look at processes from the Task Manager perspective. 8
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C H A PT E R 1
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PROCESS AND THREAD BASICS
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Task Manager To see examples of processes, you need look no further than the Windows Task Manager, shown in figure 1.3.
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Windows Task Manager lists the processes that are currently executing.
Processes are assigned a priority that is used in scheduling its threads. In figure 1.3 the column Base Pri contains the priority of the process. A process itself does not execute. Instead the threads contained within a process execute. Their execution is controlled in part by their priority. The OS combines each thread s priority with that of the process containing them to determine the order in which the threads should execute. Three of the most common values for base priority High, Normal, and Low are listed in figure 1.3. The columns Mem Usage, Handles, USER Objects, and GDI Objects are examples of memory and resources that a process uses. These resources include things like file handles and Graphical Device Interface (GDI) objects. A file handle is used to interact with a file system file while a GDI object is used to display graphical output, such as circles and lines, on the screen. Processes allow the actions of one thread in a process to be isolated from all other processes. The goal of this isolation is to increase the overall stability of the system. If a thread in a process encounters an error, the effects of that error should be limited to that process.
BACKGROUND
MULTITASKING
When computers ran only one program at a time, there was no need to be concerned with multitasking. Not that long ago a computer executed only one process a single task at a time. In the days of DOS the computer started up to a command prompt. From that prompt you typed the name of the program to execute. This single tasking made it very difficult to interact with multiple programs. Typically users were forced to exit one program, saving their work, and start another. For many it is unimaginable that a computer could run only a single program at once, such as a word processor or spreadsheet. Today users routinely execute a relatively large number of processes at the same time. A typical user may be surfing the Web, chatting using an instant messaging program, listening to an MP3, and checking email simultaneously. When an OS supports execution of multiple concurrent processes it is said to be multitasking. There are two common forms of multitasking: preemptive and cooperative, which we ll explore next.
Cooperative multitasking Cooperative multitasking is based on the assumption that all processes in a system will share the computer fairly. Each process is expected to yield control back to the system at a frequent interval. Windows 3.x was a cooperative multitasking system. The problem with cooperative multitasking is that not all software developers followed the rules. A program that didn t return control to the system, or did so infrequently, could make the entire system unusable. That s why Windows 3.x would occasionally freeze up, becoming unresponsive. This occurred because the entire OS shared a common thread processing messages. When Windows 3.x started a new application, that application was invoked from the main thread. The OS would pass control to the application with the understanding it would be returned quickly. If the application failed to return control to the OS in a timely fashion, all other applications, as well as the OS, could no longer execute instructions. Development of applications for Window 3.x was more difficult than newer versions because of the requirements of cooperative multitasking. The developer was required to process Windows messages on a frequent basis, requiring that checks to the message loop be performed regularly. To perform long-running operations, such as looping 100 times, required performing a small unit of work, and then posting a message back to yourself indicating what you should do next. This required that all work be broken up into small units, something that isn t always feasible. Let s review the way that current Windows applications function. The main thread executes a loop called a message pump. This loop checks a message queue to see if there s work to do. If so, it performs the work. The click event, which occurs when a user clicks a control such as a button, enters work into the message queue indicating which method should be executed in response to the user s click. This method is known as an event handler. While the loop is executing an event handler, it cannot process additional messages.
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