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Zip and UnZip files in C# using J# libraries
by Mohammed Habeeb
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Recently I had to build a private online photo album application. I planned to provide the users the option to upload images as a ZIP file. I needed this option to be implemented in ASP.NET 1.1, as my hosting service provider didn't have .NET 2.0 support. Online search lead me to a blog with this wonderful idea of using the ZIP option in J# and it really attracted me. I did want to try it out and the results were pretty cool. Even though this sample application has been written in .NET 1.0, .NET Framework 2.0 has libraries to work with compressing and uncompressing of files. I will soon come up with an article to illustrate the use of .NET 2.0 libraries for compress and uncompressing functionalities. In this article, I will explain the usage of ZIP functionality in J# from C# code. The code in this application has been designed to reuse in a copy paste fashion and not as a library.


This application consumes J# classes internally. For this we must first refer to the J# .NET library. Physically it resides as a file named vjslib.dll. If you are not very sure how to refer to a library in your project please follow the below steps:

Right click your project in Server Explorer and click on "Add Reference" -> Select the .NET tab -> Scroll down and select "vjslib" -> Click OK and you are there. Now you can refer the Java library classes within your application. In fact this was the first time I am trying to refer to the J# classes and personally it was a moment I would never forget in my programming life.  It made me thrilled about the usage of the whole power and options of the Java language within my C# programs (if need arises).

Import the following namespaces for ease of coding.

Listing 1 - Directives

using java.util;

The namespace contains the classes and methods to implement the compress and uncompress functionalities within our code. The main classes used from the above namespaces are

·         ZipFile

·         ZipEntry

·         ZipOutputSteam

·         Enumeration

Programmatically, a ZipFile object can be considered equivalent to a physical ZIP file. A ZipFile can contain multiple ZipEntry objects apart from the actual content of the zipped files. In fact each ZipEntry object is the metadata about a ZIP file. The ZipOutputStream class represents a writable stream pointing to a ZIP file. This stream can be used to write ZipEntry objects and content to the ZIP file. Enumeration enables iteration through each element in a collection.

Below is a code listing to create a ZIP file.

Listing 2 – Create a ZIP file

private void Zip(string zipFileName, string[]sourceFile)
  FileOutputStream filOpStrm = new FileOutputStream(zipFileName);
  ZipOutputStream zipOpStrm = new ZipOutputStream(filOpStrm);
  FileInputStream filIpStrm = null;
  foreach (string strFilName in sourceFile)
    filIpStrm = new FileInputStream(strFilName);
    ZipEntry ze = new ZipEntry(Path.GetFileName(strFilName));
    sbyte[]buffer = new sbyte[1024];
    int len = 0;
    while ((len = >= 0)
      zipOpStrm.write(buffer, 0, len);

The above Zip() method accepts two parameters,

zipFileName - ZIP file name including the path and

sourceFile - string array of file names that are to be zipped.

The FileOutputStream class is capable of writing content to a file. Its constructor accepts the path of the file to which we wish to write. FileOutputStream object is then supplied to an instance of ZipOutputStream class as a parameter. The ZipOutputStream class represents a writable stream to a ZIP file.

The foreach loops through each file to be zipped, creates corresponding zip entries and adds each to the final ZIP file. Taking a deeper look into the code, a FileInputStream object is created for each file to be zipped. The FileInputStream object is capable of reading from a file as a stream. Then a ZipEntry object is created for each file to be zipped. The constructor of the ZipEntry class accepts the name of the file. Path.GetFileName() returns the file name and extension of the specified path string.

The newly created ZipEntry object is added to the ZipOutputStream object using its putNextEntry() method. In fact, a ZipEntry merely represents metadata of a file entry. You still need to add the actual contents into the ZIP file. Therefore you need to transfer data from source FileInputStream to destination FileOutputStream. This is exactly what the while loop does in the above piece of code. It reads content from the source file and writes it into the output ZIP file. Finally, the closeEntry() method of the ZipOutputStream class is called and this causes the physical creation of the ZIP file. All the other streams created are also closed.

Next we'll see see how to reverse the process and unzip a file.

Listing 3 – Extract a ZIP file

private void Extract(string zipFileName, string destinationPath)
  ZipFile zipfile = new ZipFile(zipFileName);
  List < ZipEntry > zipFiles = GetZippedFiles(zipfile);
  foreach (ZipEntry zipFile in zipFiles)
    if (!zipFile.isDirectory())
      InputStream s = zipfile.getInputStream(zipFile);
        Directory.CreateDirectory(destinationPath + "\\" +
        FileOutputStream dest = new FileOutputStream(Path.Combine
          (destinationPath + "\\" + Path.GetDirectoryName(zipFile.getName()),
          int len = 0;
          sbyte[]buffer = new sbyte[7168];
          while ((len = >  = 0)
            dest.write(buffer, 0, len);

The ExtractZipFile() method accepts two parameters; ZIP file name (including path) to be extracted and destination path where the files are to be extracted. It then creates a ZipFile object and retrieves entries in the ZIP file using GetZipFiles() method. This method will be discussed afterwards in this article. The foreach loop iterates through all the entries in the ZIP file and in each iteration the entry is extracted to the specified folder. The code in the foreach loop executes only if the entry is not a folder. This condition is verified using the isDirectory() method of the ZipEntry object. Each entry is read into an InputStream using getInputStream() method of ZipFile object. This InputStream acts as the source stream. The destination stream is a FileOutputStream object which is created based on the specified destination folder. Here we use the getName() method of ZipEntry object to get the file name (including path) of the entry. During the extraction, the original folder structure is maintained. In the while loop that follows, contents from the source InputStream are written to the destination FileOutputStream. The source stream is read to a buffer using the read() method. It reads 7Kb in a sequence into a temporary buffer and the write() method of destination FileOutputStream writes the content to the stream from the buffer, using the write method. In the finally block that follows, the destination FileOutputStream is closed and the content is physically written to disk.

Listing 4 – Get the contents of a ZIP file

private List < ZipEntry > GetZipFiles(ZipFile zipfil)
  List < ZipEntry > lstZip = new List < ZipEntry > ();
  Enumeration zipEnum = zipfil.entries();
  while (zipEnum.hasMoreElements())
    ZipEntry zip = (ZipEntry)zipEnum.nextElement();
  return lstZip;

The GetZipFiles() method returns a generic List of ZipEntry objects taking a ZipFile object as argument. The method creates a generic collection of ZipEntry type. Now comes the use of an interesting feature in the Java language: the use of Enumeration. Note that it's not the enum type that we have in C#. An object that implements the Enumeration interface generates a series of elements, one at a time. Successive calls to the nextElement() method return successive elements of the series. The hasMoreElements() method returns a boolean value indicating if the Enumerator contains more elements. Here, the entries() method of the ZipFile class returns an Enumeration of ZipEntry objects. The code then iterates through the Enumeration and populates the List. Finally, the populated List is returned.  Something similar exists in many .NET collections that implement the IEnumerable and IEnumerator interfaces.

Apart from the above listed code, the downloadable source code for the sample application contains some extra code to handle the UI part of the application, ie., entries made to the ListBox and handling the progress bar. I haven't included them in this article because I didn't want to lose the focus of the main objective of the article. The code is comprehensive, but the UI controls can be handled in better ways, keeping performance and usability in mind. One good option might be to keep our zip functionality separate from the UI thread, so that interactivity is maintained.

Points of Interest

One interesting fact we note while using the Java library is the difference in naming conventions, especially the naming of methods. For C# coders, methods give the feel of variable names because of their casing. Also the way the classes are named is also a bit different from the C# norm.

Even though we can't use the features of java beyond an extend because the runtime decides the main advantages of a platform; we can still take advantage of the java libraries. This gives an upper hand to C# over java. I wish Microsoft will continue to support J#.


In this article, you have learned how to ZIP and UNZIP files using the J# library in C# with the help of a sample application.

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