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Sunday, July 22, 2007

Compression Classes Enhance I/O in .NET 2.0

The System.IO.Compression namespace introduces a series of classes that provide compression and decompression capabilities. The classes exhibit similar behaviors and characteristics, but the compression classes operate against streams. To my knowledge, the .NET Framework 2.0 will include two algorithms for compression/decompression: Deflate and GZip.

Sample code to compress a file

The following sample code reads the contents of a file, compresses it, and then writes it to an output file. It should clearly demonstrate how easy it is to add compression capabilities into your applications:

FileStream inputFile = null;
GZipStream compressedZipStream = null;
try
{
   // Open and read the contents of the file
   inputFile = new FileStream("c:\\mylog.txt", FileMode.Open,
                              FileAccess.Read, FileShare.Read);
   byte[] buffer = new byte[inputFile.Length];
   int count = inputFile.Read(buffer, 0, buffer.Length);
   inputFile.Close();
 
   // Compress the file contents by writing them to a memory stream
   MemoryStream memoryBuffer = new MemoryStream();
   compressedZipStream = new GZipStream(memoryBuffer,
   CompressionMode.Compress, true);
   compressedZipStream.Write(buffer, 0, buffer.Length);
   compressedZipStream.Close();
   Console.WriteLine("Original Size: {0}, Compressed Size: {1}",
                     buffer.Length, memoryBuffer.Length);
 
   // Pause so we can see the output
   Console.ReadLine();
}
finally
{
   if (inputFile != null) inputFile.Close();
   if (compressedZipStream!= null) compressedZipStream.Close();
}



I created a file named mylog.txt. I then typed in the well known test phrase "The quick brown fox jumped over the slow lazy dog." into the file about thirty times. I use files for the examples, but you could just as easily use any other sort of stream.


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Figure 1. File Compression

Note: The examples here are all with the GZipStream class, which you could just as easily swap out for the DeflateStream class.

Sample code to decompress a file

The following sample code reads the contents of a file, decompresses it, and then writes it to the console. It relies on the file created in the prior sample:

FileStream inputFile = null;
GZipStream compressedZipStream = null;
 
try
{
   // Determine the uncompressed size of the file;
   // we can't rely on the compressed size because
   // it's not the true size
   inputFile = new FileStream("c:\\mylogcompressed.txt",
                              FileMode.Open, FileAccess.Read,
                              FileShare.Read);
   compressedZipStream = new GZipStream(inputFile,
                                        CompressionMode.Decompress);
   int offset = 0;
   int totalBytes = 0;
   byte[] smallBuffer = new byte[100];
   while (true)
   {
      int bytesRead = compressedZipStream.Read(smallBuffer, 0, 100);
      if (bytesRead == 0)
      {
         break;
      }
      offset += bytesRead;
      totalBytes += bytesRead;
   }
   compressedZipStream.Close();
 
   // Open and read the contents of the file now that
   // we know the uncompressed size
   inputFile = new FileStream("c:\\mylogcompressed.txt",
                              FileMode.Open, FileAccess.Read,
                              FileShare.Read);
 
   // Decompress the file contents
   compressedZipStream = new GZipStream(inputFile,
                                        CompressionMode.Decompress);
   byte[] buffer = new byte[totalBytes];
   compressedZipStream.Read(buffer, 0, totalBytes);
 
   // Display the decompression information
   Console.WriteLine(System.Text.Encoding.UTF7.GetString(buffer));
   Console.WriteLine("Compressed Size: {0}, Decompressed Size: {1}",
                     inputFile.Length, buffer.Length);
   compressedZipStream.Close();
 
   // Pause so we can see the output
   Console.ReadLine();
}
finally
{
   if (inputFile != null) inputFile.Close();
   if (compressedZipStream != null) compressedZipStream.Close();
}
 
Decompression is more complex because of the way the Read method works. Because the file is compressed, there is no way to know how much data to read. You can approach this in a number of ways. For this sample code, I added code up front to determine the size of the file first. This results in the file being read twice, which is not ideal.



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Figure 2. File Decompression

Compression and Decompression Classes

You have received a sneak preview of the compression and decompression classes that are coming in the .NET Framework 2.0, due out later in the year. The simplicity of the examples demonstrates how easy it will be for you to use the functionality within your applications.

 

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