Streams

In order to provide similar programmatic interfaces to the broad range of I/O

devices with which a programmer has to contend, a stream-based architecture

was developed in .NET. I/O devices can be anything from printers to

hard disks to network interfaces.

Not all devices support the same functions. For example, it is possible

to read only the second half of a 1-Mb file, but not possible to download

only the second half of a Web page. Therefore, not all streams support the

same methods.

Properties such as

canRead()

,

canSeek()

, and

canWrite()

indicate the

capabilities of the stream when applied to a particular device           

The most important stream in the context of this book is the

network-

Stream

, but another important stream is

fileStream

, which is used extensively

throughout this book to demonstrate file transfers over networks.

Streams can be used in two ways: asynchronously or synchronously.

When using a stream synchronously, upon calling a method, the thread will

halt until the operation is complete or fails. When using a stream asynchronously,

the thread will return from the method call immediately, and whenever

the operation is complete, a method will be called to signify the

completion of the operation, or some other event, such as I/O failure.

It is not user friendly to have a program “hang” when it is waiting for an

operation to complete. Therefore, synchronous method calls must be used

in a separate thread.

Through the use of threads and synchronous method calls, computers

achieve the illusion of being able to do several things at once. In reality,

most computers have only one central processing unit (CPU), and the illusion

is achieved by quickly switching between tasks every few milliseconds.

The following application illustrates the two techniques. The code in

this book will tend toward using synchronous streams, but it is important

to be able to recognize and understand asynchronous streams.

This code requires a little explanation. First, the magic number, 4096, is

simply a performance characteristic because it is quicker to transfer data

from disks in 4-Kb chunks than 1 byte at a time.

The final parameter in the

FileStream

constructor indicates whether

the operation is to be completed asynchronously or synchronously.

The most important thing to note is that there is no reference to

tbResults;

this implies that some other function must handle the data once

the read is complete. The

AsyncCallback

constructor refers to another function,

which is also referenced in the

BeginRead

method, so this must be it.

As you can see from the code, the

fs_StateChanged

function has not yet

been implemented. This function is called whenever the file is finished

reading.

Note:

Synchronous use of

FileStream

is more efficient when the file size is

less than 64 Kb and the file is located on the local machine.                                           

This code doesn’t perform any file handling; instead, it creates a new

thread, whose entry point is the

syncRead

function. When this thread runs,

it does so in parallel with any other code that is running at the same time,

which includes the background operating system (OS) “housekeeping”

(Windows message handling) functions.

If the code above were replaced by a simple call to

syncRead()

, the program

would still operate; however, if the file happened to be several

gigabytes in size, the user would quickly perceive the application to be

“hung.” A hung application is notably nonresponsive and may turn white

when dragged behind another application. What is actually happening is

that the main thread of application is taking 100% processor time and

does not give the OS time to handle simple tasks such as redrawing the

user interface.

In certain time-critical applications, it may be necessary to take 100%

processor time, but any application with a user interface should remain

responsive at all times.

The next task is to implement the

syncRead

function:                                                                                                             

In the above code, you will notice that the

FileStream

constructor is

enclosed in a

try/catch

block. This allows the program to recover gracefully

from problems such as a missing file or an unreadable disk. In realworld

applications, any operation that relies on the existence of files or network

resources should be contained within a

try/catch

block. This allows

programs to continue execution, even if something unexpected happens. In

most examples throughout this book,

try/catch

blocks are not used in

order to keep the examples concise and readable.

Three namespaces must be included in the code as follows:

Note:

The most concise way to read text files (under 1 Gb) is:

(new StreamReader(filename)).ReadToEnd();

Hibernate و Standby کردن کامپیوتر

' Hibernate
        Application.SetSuspendState(PowerState.Hibernate, True, True)

        '// Standby
        Application.SetSuspendState(PowerState.Suspend, True, True)

شفاف کردن پشت زمینه ی کنترلها

control.BackColor = Color.FromArgb(trancparency, control.BackColor)

بستن یک پروسه در حال اجرا

System.Diagnostics.Process.Start("TaskKill", "/f /im Notepad.exe /t")

بدست آوردن مسیر دایرکتوری جاری برنامه

MessageBox.Show(Application.StartupPath)

نكته : درفايل ترتيبي امكان استفاده از ساختاروجود ندارد اين مورد درفايل تصادفي توضيح داده مي شود.

امادراين يادداشت  به بررسي فايل ترتيبي مي پردازيم وفايل تصادفي رابه يادداشتي ديگرموكول مي كنيم.
حال مي خواهيم ببينيم چگونه مي توان يك فايل ترتيبي ايجاد كرد اطلاعاتي درداخل آن نوشت آطلاعات آنرا خواند وهمچنين چگونه مي توان آن فايل را حذف كرد.
ايجادكردن ويابازكردن يك فايل ترتيبي:
براي ايجاد كردن ويا بازكردن يك فايل ترتيبي ازدستور FileOpen استفاده مي كنيم.
اين دستور سه آرگومان دريافت مي كند:
1- درآرگومان اول بايد يك شماره به فايل خوداختصاص دهيد وازاين به بعد با استفاده ازآن شماره مي توان اطلاعات درون فايل را

 خواند يا نوشت وهمچنين كارهاي ديگررا نيز با استفاده از آن شماره انجام

ادامه مطلب کلید کنید

نوع :  PDF    تعداد صفحه :81    حجم:520KB   دريافت فايل

بر گرفته از مطالب:وحيد نصيري

Microsoft DAO 3.51  Object Libraryتیک دارمی کنیم.

بعد دو متغیر ازنوع تعریف می کنیم . Recordsetوdatabaseتعریف می کنیم .

(ادرس بانک اطلاعاتی)Set b = DBEngine.OpenDatabase

نکته:بااستفاده ازدستور app.pathمی توان آدرس برنامه را بدست آوردوبا چسباندن ان به نام بانک اطلاعاتی در رایانه های دیگرهم بدون تعغیر ادرس استفاده کرد.

(Set g = b.OpenRecordset("name table", dbOpenDynaset

dbOpenDynaset:باعث می شود جدول قابل تعغیر با شد

برای ارتبا ط ان با هر کنترلی ازخاصیت ()fieldsستفاده می کنیم مثلا

(  شماره فیلد یا "نام فیلد")text1=g.fields

وبرای ذخیره کردن   

g.addnew
g.fields=text
g.update

وبقیه خواص هایش مانند کنترل  ado  است