Visual Basic 1002005 [A Developers Notebook] [Electronic resources] نسخه متنی

5.5. Compress and Decompress Data

Even with the ever-increasing capacity of
hard drives and the falling price of computer memory, it still pays
to save space. In .NET 2.0, a new
System.IO.Compression namespace makes it easy for
a VB 2005 programmer to compress data as she writes it to a stream,
and decompress data as she reads it from a stream.

Note: Need to save space before you store data in a file or
database? . NET 2.0 makes compression and decompression
easy.

5.5.1. How do I do that?

The new System.IO.Compression namespace introduces
two new stream classes: GZipStream and
DeflateStream, which, as you'd
guess, are used to compress and decompress streams of data.

The
algorithms used by these classes are lossless,
which means that when you compress and decompress your data, you
won't lose any information.

To use compression, you need to understand that a compression stream
wraps another stream. For example, if you want
to write some compressed data to a file, you first create a
FileStream for the file. Then, you wrap the
FileStream with the GZipStream
or DeflateStream.
Here's how it works:

Dim fsWrite As New FileStream(FileName, FileMode.Create)
Dim CompressStream As New GZipStream(fsWrite, CompressionMode.Compress)

Now, if you want to write data to the file, you use the
GZipStream. The GZipStream
compresses that data, and then writes the compressed data to the
wrapped FileStream, which then writes it to the
underlying file. If you skip this process and write directly to the
FileStream, you'll end up writing
uncompressed data instead.

Like all streams, the GZipStream only allows you
to write raw bytes. If you want to write strings or other data types,
you need to create a StreamWriter. The
StreamWriter accepts basic .NET data types (like
strings and integers) and converts them to bytes.
Here's an example:

Dim Writer As New StreamWriter(CompressStream)
' Put a compressed line of text into the file.
Writer.Write("This is some text")

Finally, once you're finished, make sure you flush
the GZipStream so that all the data ends up in the
file:

Writer.Flush( )
CompressStream.Flush( )
fsWrite.Close( )

The process of decompression works in a similar way. In this case,
you create a FileStream for the file you want to
read, and then create a GZipStream that
decompresses the data. You then read the data using the
GZipStream, as shown here:

fsRead = New FileStream(FileName, FileMode.Open)
Dim DecompressStream As New GZipStream(fsRead, CompressionMode.Decompress)

Example 5-5 shows an end-to-end example that writes
some compressed data to a file, displays the amount of space saved,
and then decompresses the data.

Example 5-5. Compress and decompress a sample file

Imports System.IO
Module FileCompression
Public Sub Main( )
' Read original file.
Dim SourceFile As String
SourceFile = My.Computer.FileSystem.CurrentDirectory & "\test.txt"
Dim fsRead As New FileStream(SourceFile, FileMode.Open)
Dim FileBytes(fsRead.Length - 1) As Byte
fsRead.Read(FileBytes, 0, FileBytes.Length)
fsRead.Close( )
' Write to a new compressed file.
Dim TargetFile As String
TargetFile = My.Computer.FileSystem.CurrentDirectory & "\test.bin"
Dim fsWrite As New FileStream(TargetFile, FileMode.Create)
Dim CompressStream As New GZipStream(fsWrite, CompressionMode.Compress)
CompressStream.Write(FileBytes, 0, FileBytes.Length)
CompressStream.Flush( )
CompressStream.Close( )
fsWrite.Close( )
Console.WriteLine("File compressed from " & _
New FileInfo(SourceFile).Length & " bytes to " & _
New FileInfo(TargetFile).Length & " bytes.")
Console.WriteLine("Press Enter to decompress.")
Console.ReadLine( )
fsRead = New FileStream(TargetFile, FileMode.Open)
Dim DecompressStream As New GZipStream(fsRead, CompressionMode.Decompress)
Dim Reader As New StreamReader(CType(DecompressStream, Stream))
Console.WriteLine(Reader.ReadToEnd( ))
Reader.Close( )
fsRead.Close( )
End Sub
End Module

5.5.2. What about...

...unzipping
.zip files? Unfortunately, the .NET 2.0
compression streams can't deal with ZIP files, file
archives that are commonly used to shrink batches of files (often
before storing them for the long term or attaching them to an email
message). If you need this specific ability, you'll
probably be interested in the freely downloadable

#ziplib (available at http://www.icsharpcode.net/OpenSource/SharpZipLib).

5.5.3. Where can I learn more?

For more information about the GZipStream and
DeflateStream algorithms, look them up in the MSDN
Help. You can also look up the
"compression" index entry for a
Windows application example that uses these classes.