Visual Basic.NET How to Program (PDF)

Chapter 17 Files and Streams 827
93 MessageBoxButtons.OK, MessageBoxIcon.Error)
94 End If
95
96 txtAccount.Clear() ' clear text box
97 End Sub ' DeleteRecord
98
99 End Class ' FrmDeleteDialog
Fig. 17.23 FrmDeleteDialog class enables users to remove records from files in
transaction-processor case study (part 3 of 3).
SUMMARY
All data items processed by a computer ultimately are reduced to combinations of zeros and ones.
The smallest data items that computers support are called bits and can assume either the value 0
or the value 1.
Digits, letters and special symbols are referred to as characters. The set of all characters used to
write programs and represent data items on a particular computer is called that computer’s character
set. Every character in a computer’s character set is represented as a pattern of 1s and 0s
(characters in Visual Basic are Unicode characters, which are composed of 2 bytes).
A field is a group of characters (or bytes) that conveys some meaning.
A record is a group of related fields.
At least one field in a record is chosen as a record key, which identifies that record as belonging
to a particular person or entity and distinguishes that record from all other records in the file.
Files are used for long-term retention of large amounts of data and can store those data even after
the program that created the data terminates.
A file is a group of related records.
Data maintained in files is often called persistent data.
Class File enables programs to obtain information about a file.
Class Directory enables programs to obtain information about a directory.
Class FileStream provides method Seek for repositioning the file-position pointer (the byte
number of the next byte in the file to be read or written) to any position in the file.
The most common type of file organization is a sequential file, in which records typically are
stored in order by the record-key field.
When a file is opened, an object is created, and a stream is associated with the object.