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GZipStream compresses data. It saves data efficiently—such as in compressed log files. We develop a utility method in the C# language that uses the System.IO.Compression namespace. It creates GZIP files. It writes them to the disk.

Example. Here we will use the .NET Framework's built-in methods. It provides these new methods because compression and decompression using GZIP is an extremely common problem in any programming language. GZIP is widely used on the Internet.

Note: The GZIP compression algorithm is used on many web pages to boost performance. This method shows the use of GZIP on a string.

C# program that compresses

using System.IO;
using System.IO.Compression;
using System.Text;

class Program
    static void Main()
	    // 1.
	    // Starting file is 26,747 bytes.
	    string anyString = File.ReadAllText("TextFile1.txt");

	    // 2.
	    // Output file is 7,388 bytes.
	    CompressStringToFile("new.gz", anyString);
	    // Could not compress.

    public static void CompressStringToFile(string fileName, string value)
	// A.
	// Write string to temporary file.
	string temp = Path.GetTempFileName();
	File.WriteAllText(temp, value);

	// B.
	// Read file into byte array buffer.
	byte[] b;
	using (FileStream f = new FileStream(temp, FileMode.Open))
	    b = new byte[f.Length];
	    f.Read(b, 0, (int)f.Length);

	// C.
	// Use GZipStream to write compressed bytes to target file.
	using (FileStream f2 = new FileStream(fileName, FileMode.Create))
	using (GZipStream gz = new GZipStream(f2, CompressionMode.Compress, false))
	    gz.Write(b, 0, b.Length);

    Starting file is 26,747 bytes.
    Output file is 7,388 bytes.

The method compresses strings to disk. GZipStream requires bytes and this example uses a byte array. We can use the File static methods to write to temporary files. You see several useful framework methods.

Note: Remember to add System.IO.Compression at the top. You can find more information about byte arrays here.

Byte Array: Memory Usage, Read All Bytes

In part A, we combine the Path.GetTempFileName() method and File.WriteAllText. This writes the string to a temporary file. In part B, we use FileStream here to read in bytes from the file.

File Handling

Using: The using statement provides effective system resource disposal. More examples are available.


Finally: In part C, we use a new GZipStream to write the compressed data to the disk. We must combine it with a FileStream.

Reliable. This is a tested and reliable method that I have used in deployed applications with no problems. It however can be slow so if you are working on a program with extremely strict performance requirements, don't use this code.

Note: This article used to have an example that was incorrect. It had an encoding problem. I thank Jon for his bug report.

Summary. We saw an example of using GZipStream to compress to a file with an underlying FileStream. More complex methods are available for compression and this example alone does not meet most requirements.

Tip: For the best compression ratio, going outside the .NET Framework and embedding 7-Zip is often helpful.