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Programming language: C#
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Tags: Compression    

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Introducing the DotNetZip Library and Tools

Maintainers wanted – ping @haf to talk

DotNetZip is the name of an open-source project that delivers a .NET library for handling ZIP files, and some associated tools.

  • The library allows .NET programmers to build applications that read, create and modify ZIP files.

  • The tools are .NET programs that rely on the library, and can be used by anyone on any Windows machine to build or extract ZIP files.

How to build?

bundle && rake

CI: https://ci.appveyor.com/project/haf/dotnetzip-semverd

Why DotNetZip?

The Microsoft .NET Framework base class library lacks a good set of built-in classes for creating and reading ZIP files, and Windows itself lacks full-powered built-in ZIP tools. DotNetZip fills those needs.

There are other ZIP libraries available, but some of them have licenses that may be unfriendly, some of them are too hard to use or not powerful enough, and some of them are too expensive (not free). DotNetZip provides one more alternative. The goals for this alternative:

  • easy to adopt - low cost (Free), and a friendly license
  • fully-featured
  • good performance - in speed and compression
  • easy to use.

DotNetZip background

Many people seem to think, incorrectly, that the classes in the System.IO.Compression namespace, like GZipStream or DeflateStream, can create or read zip files. Not true.

The System.IO.Compression namespace, available starting with .NET v2.0 for the desktop Framework and v3.5 for the Compact Framework, includes base class libraries supporting compression within streams - both the Deflate and Gzip formats are supported. But these classes are not directly useful for creating compressed ZIP archives. GZIP is not ZIP. Deflate is not ZIP. The GZipStream in System.IO.Compression is able to read and write GZIP streams, but that is not the same as reading or writing a zip file. Also, these classes deliver poor compression in practice, especially with binary data, or previously-compressed data.

Yes, it is possible to read and write zip files, using the classes in the .NET Framework.

  • You can do it with the System.IO.Packaging.ZipPackage class, added in .NET v3.0. Actually this class lets you create a package file, which is a zipfile with a particular internal structure. It includes a manifest and some other things. But the interface is odd and confusing if all you want is a regular-old ZIP file. Also, the classes in this namespace do not provide control for things like passwords, comments, AES encryption, ZIP64, Unicode, and so on.

  • You can also create and read zip files with the J# runtime. This also has its drawbacks. First, J# is going out of support, or may be out of support now. Second, the J# runtime is huge, and you have to swallow the whole thing, even if all you want is zip file capability. Also, the J# runtime is based on the java.util.zip classes from Java v1.4, dating from 1998. The runtime hasn't been updated in years and still includes bugs in zip file handling. It lacks support for AES, for ZIP64, and Unicode. It is not accessible from COM. Finally, the zip classes in the J# runtime are decidedly un-dotnet. There's no support for events, or enumerators to let you do things like For Each in VB, or foreach in C#. The interface is clunky. It does not feel like a .NET class library, because it isn't a .NET class library. So for all those reasons, J# isn't ideal.

  • You can also rely on P/Invoke to the shell32.dll, and the ShellClass. This works in a limited fashion. The drawbacks here: it isn't documented. It isn't a very intuitive or powerful programming interface. There are no events, so embedding it into a Winforms app with a progress bar would be difficult. Again it lacks an easy way to use or access many ZIP features, like encryption or ZIP64 or self-extracting archives. Also, the shell32.dll is designed for use within Windows Explorer, and presumes a user interface. In fact in some cases, calling into this DLL to perform a ZIP extraction can display a dialog box, so it may not be suitable for use within server or "headless" applications.

There are other libraries out there than do zip files for .NET. But there are compromises with each one. Some are commercial and expensive. Some are slow. Some are complicated to use. Some of these options lack features. Some of them have more than one of these drawbacks.

DotNetZip provides another option. It's a very simple-to-use class library that provides good ZIP file support. Using this library, you can write .NET applications that read and write zip-format files, including files with passwords, Unicode filenames, ZIP64, AES encryption, and comments. The library also supports self-extracting archives. It is well documented and provides good performance.

Though DotNetZip is implemented in .NET and was originally intended to provide a managed-code library for ZIP files, you can now use it library from any COM environment, including Javascript, VBScript, VB6, VBA, PHP, Perl, and others. Using DotNetZip, you could generate an AES-encrypted zip file from within the code of a macro running in MS-Excel, for example.

DotNetZip works with applications running on PCs with Windows. There is a version of this library available for the .NET Compact Framework, too.

I have not tested DotNetZip for use with Mono, but I've heard reports that people use the binary releases with Mono successfully, without change.


This software is open source. It is released under the Microsoft Public License of October 2006. The use of the "Microsoft Public License" does not mean it is licensed by Microsoft. See the License.txt file for details.

DotNetZip is derived in part from ZLIB, the C-language library by Mark Adler and Jean-loup Gailly . See the License.ZLIB.txt file included in the DotNetZip download for details.

What is DotNetZip? and How is it packaged?

DotNetZip is primarily a managed library for dealing with ZIP files.

It is packaged as a DLL that your application must reference: Ionic.Zip.dll. In the "developer's kit" package, there is documentation, code examples, and debug versions of the DLL.

The ZIP library depends on a set of supporting classes for doing compression and decompression; these are exposed in other namespaces.

The classes in the ZIP library reside in these namespaces:

namespace interesting classes

Ionic.Zip ZipFile, ZipEntry, ZipOutputStream, and ZipInputStream.

Ionic.Zlib DeflateStream, GZipStream, ZlibStream

Ionic.BZip2 BZip2InputStream, BZip2OutputStream

Ionic.Crc CRC32

If you want only ZLIB (raw compression and decompression, RFC 1950, 1951, and 1952), the ZLIB classes are packaged independently, in Ionic.Zlib.dll. Likewise, if you want to do BZIP2 compression, outside the scope of a zip file, you can use the Ionic.BZip2.dll assembly.

If you want ZIP, or both ZIP and ZLIB, then your application should depend soly on Ionic.Zip.dll; this assembly includes a superset of the classes in Ionic.Zlib.dll and Ionic.BZip2.dll.

For each DLL, there is a version for the regular .NET Framework and another for the Compact Framework.

DotNetZip also includes command-line and GUI tools for manipulating zip files; these can be helpful to developers when building applications that create or manipulate zip files. They also can be helpful as end-user tools.

There are other downloads for DotNetZip - the source package, the runtime-only package (DLLs and no helpfile or tools), the documentation-only package, etc.

Using the Zip Class Library: The Basics

The examples here provide just the basics.

There are many other examples available: some are included in the source package, some in the class reference documentation in the help file, and others on the web. Those examples provide many illustrate how to read and write zip files, taking advantage of all the various features of zip files exposed by the library. For a full set of examples, your best bet is to see the documentation. Here's a basic primer:

The main type you will use to fiddle with zip files is the ZipFile class. Full name: Ionic.Zip.ZipFile. You use this to create, read, or update zip files. There is also a ZipOutputStream class, which offers a Stream metaphor, for those who want it. You should choose one or the other for your application.

The simplest way to create a ZIP file in C# looks like this:

  using(ZipFile zip= new ZipFile())

Or in VB.NET, like this:

 Using zip As ZipFile = New ZipFile
 End Using

The using clause is important; don't leave it out.

The simplest way to Extract all the entries from a zipfile looks like this:

  using (ZipFile zip = ZipFile.Read(NameOfExistingZipFile))

But you could also do something like this:

  using (ZipFile zip = ZipFile.Read(NameOfExistingZipFile))
    foreach (ZipEntry e in zip)

Or in VB, extraction would be like this: Using zip As ZipFile = ZipFile.Read(NameOfExistingZipFile) zip.ExtractAll End Using

Or this: Using zip As ZipFile = ZipFile.Read(NameOfExistingZipFile) Dim e As ZipEntry For Each e In zip e.Extract Next End Using

That covers the basics.

Notice that a using clause is always employed. DOn't forget this. Don't leave it off. If you don't understand what it is, don't just skip it. It's important.

There are a number of other options for using the class library. For example, you can read zip archives from streams, or you can create (write) zip archives to streams, or you can extract into streams. You can apply passwords for weak encryption. You can specify a code page for the filenames and metadata of entries in an archive. You can rename entries in archives, and you can add or remove entries from archives. You can set up save and read progress events. You can do LINQ queries on the Entries collection. Check the documentation for complete information, or use Visual Studio's intellisense to explore some of the properties and methods on the ZipFile class.

Another type you will use is ZipEntry. This represents a single entry - either a file or a directory - within a ZipFile. To add an entry to a zip file, you call one of the AddEntry (or AddFile) methods on the ZipFile class. You never directly instantiate a ZipEntry type. The AddEntry/AddFile returns a ZipEntry type; you can then modify the properties of the entry within the zip file, using that object.

For example, the following code adds a file as an entry into a ZipFile, then renames the entry within the zip file:

  using(ZipFile zip= new ZipFile())
    ZipEntry e = zip.AddFile(filename);
    e.FileName = "RenamedFile.txt";

Extracting a zip file that was created in this way will produce a file called "RenamedFile.txt", regardless of the name of the file originally added to the ZipFile.

As an alternative to using ZipFile type to create a zip file, you can use the ZipOutputStream type to create zip files . To do so, wrap it around a stream, and write to it.

  using (var fs = File.Create(filename))
    using(var s = new ZipOutputStream(fs))
      byte[] buffer = Encoding.ASCII.GetBytes("This is the content for entry #1.");
      s.Write(buffer, 0, buffer.Length);

Unlike the ZipFile class, the ZipOutputStream class can only create zip files. It cannot read or update zip files.

If you want to read zip files using a streaming metaphor, you can use ZipInputStream. Think of ZipInputStream and ZipOutputStream as alternatives to using ZipFile to manipulate zip files. The former is for reading zip files; the latter is for writing them.

About Directory Paths

One important note: the ZipFile.AddXxx methods add the file or directory you specify, including the directory. In other words, logic like this: ZipFile zip = new ZipFile(); zip.AddFile("c:\a\b\c\Hello.doc"); zip.Save();

...will produce a zip archive that contains a single entry, or file, and that file is stored with the relative directory information. When you extract that file from the zip, either using this Zip library or winzip or the built-in zip support in Windows, or some other package, all those directories will be created, and the file will be written into that directory hierarchy. At extraction time, if you were to extract that file into a directory like c:\documents, then resulting file would be named c:\documents\a\b\c\Hello.doc .

This is by design.

If you don't want that directory information in your archive, then you need to use the overload of the AddFile() method that allows you to explicitly specify the directory used for the entry within the archive:

zip.AddFile("c:\\a\\b\\c\\Hello.doc", "files");

This will create an archive with an entry called "files\Hello.doc", which contains the contents of the on-disk file located at c:\a\b\c\Hello.doc .

If you extract that file into a directory e:\documents, then the resulting file will be called e:\documents\files\Hello.doc .

If you want no directory at all, specify "" (the empty string). Specifying null (Nothing in VB) will include all the directory hierarchy in the filename, as in the orginal case.

Pre-requisites to run Applications that use DotNetZip

To run desktop applications that depend on DotNetZip: .NET Framework 4.0 or later

In more detail: The Zip Class Library

The Zip class library is packaged as Ionic.Zip.DLL for the regular .NET Framework and Ionic.Zip.CF.dll for the Compact Framework. The Zip library allows applications to create, read, and update zip files.

This library uses the DeflateStream class to compress file data, and extends it to support reading and writing of the metadata - the header, CRC, and other optional data - defined or required by the zip format spec.

The key object in the class library is the ZipFile class. Some of the important methods on it:

  - AddItem - adds a file or a directory to a zip archive
  - AddDirectory - adds a directory to a zip archive
  - AddFile - adds a file to a zip archive
  - AddFiles - adds a set of files to a zip archive
  - Extract - extract a single element from a zip file
  - Read - static methods to read in an existing zipfile, for
           later extraction
  - Save - save a zipfile to disk

There is also a supporting class, called ZipEntry. Applications can enumerate the entries in a ZipFile, via ZipEntry. There are other supporting classes as well. Typically, 80% of apps will use just the ZipFile class, and will not need to directly interact with these other classes. But they are there if you need them.

If you want to create or read zip files, the Ionic.Zip.DLL assembly is the one you want.

When building applications that do zip stuff, you need to add a reference to the Ionic.Zip.dll in Visual Studio, or specify Ionic.Zip.dll with the /R flag on the CSC.exe or VB.exe compiler line.

In more detail: The Zlib Class Library

The Zlib class library is packaged as Ionic.Zlib.DLL for the regular .NET Framework and Ionic.Zlib.CF.dll for the Compact Framework. The ZLIB library does compression and decompression according to IETF RFC's 1950 (ZLIB), 1951 (Deflate), and 1952 (GZIP).

See http://www.ietf.org/rfc/rfc1950.txt http://www.ietf.org/rfc/rfc1951.txt and http://www.ietf.org/rfc/rfc1952.txt

The key classes are:

ZlibCodec - a class for Zlib (RFC1950/1951/1952) encoding and decoding. This low-level class does deflation and inflation on buffers.

DeflateStream - patterned after the DeflateStream in System.IO.Compression, this class supports compression levels and other options.

GZipStream - patterned after the GZipStream in System.IO.Compression, this class supports compression levels and other options.

ZlibStream - similar to the GZipStream in System.IO.Compression, this class generates or consumes raw ZLIB streams.

If you want to simply compress (deflate) raw block or stream data, this library is the thing you want.

When building applications that do zlib things, you need to add a reference to the Ionic.Zlib.dll in Visual Studio, or specify Ionic.Zlib.dll with the /R flag on the CSC.exe or VB.exe compiler line.

NB: If your application does both Zlib and Zip stuff, you need only add a reference to Ionic.Zip.dll. Ionic.Zip.dll includes all the capability in Ionic.Zlib.dll. Ionic.Zip.dll is a superset.

In more detail: The BZip2 Class Library

The BZip2 class library is packaged as Ionic.BZip2.DLL for the regular .NET Framework and Ionic.BZip2.CF.dll for the Compact Framework. The BZip2 library does compression according to the bzip2 format created by Julian Seward. See http://en.wikipedia.org/wiki/Bzip2

NB: If your application does a combination of BZip2, Zlib and Zip stuff, you need only add a reference to Ionic.Zip.dll. Ionic.Zip.dll includes all the capability in Ionic.Zlib.dll and Ionic.BZip2.dll. Ionic.Zip.dll is a superset.

If you try to link to more than one of these, you will get compiler warnings about "duplicate types".

Namespace changes for DotNetZip

The namespace for the DotNetZip classes is Ionic.Zip. Classes are like: Ionic.Zip.ZipFile Ionic.Zip.ZipEntry Ionic.Zip.ZipException etc

(check the .chm file for the full list)

For the versions prior to v1.7, the namespace DotNetZip was Ionic.Utils.Zip. The classes were like so: Ionic.Utils.Zip.ZipFile Ionic.Utils.Zip.ZipEntry etc

If you have code that depends on an older version of the library, with classes in the Ionic.Utils.Zip namespace), a simple namespace replacement will allow your code to compile against the new version of the library.

In addition to the Zip capability, DotNetZip includes capability (new for v1.7). For Zlib, the classes are like this: Ionic.Zlib.DeflateStream Ionic.Zlib.ZlibStream Ionic.Zlib.ZlibCodec ...

(again, check the .chm file for the full list)

For v1.9.1.6, the CRC class moved from the Ionic.Zlib namespace to the Ionic.Crc namespace.


Originally, this library was designed to depend upon the built-in System.IO.Compression.DeflateStream class for the compression. This proved to be less than satisfactory because the built-in compression library did not support compression levels and also was not available on .NET CF 2.0.

As of v1.7, the library includes a managed code version of zlib, the library that produces RFC1950 and RFC1951 compressed streams. Within that version of zlib, there is also a DeflateStream class which is similar to the built-in System.IO.Compression.DeflateStream, but more flexible, and often more effective as well.

The Documentation

There is a single set of developer reference documentation for all of the DotNetZip library features, including Zip and Zlib stuff. It is packaged in two ways: As a .chm file, and as a Help Viewer 1.0 resource. The latter is the new format suitable for viewing within VS2010.

If you only use the Zlib stuff, then you should focus on the doc in the Ionic.Zlib namespace. Likewise BZip2. If you are building apps for mobile devices running the Compact Framework, then ignore the pieces that deal with SaveSelfExtractor() and AES.

Consult the help file for more specifics here.

In some cases, upon opening the .chm file for DotNetZip, the help items tree loads, but the contents are empty. You may see an error: "This program cannot display the webpage." or, "Address is invalid." If this happens, it's likely that you've encountered a problem with Windows protection of files downloaded from less trusted locations. To work around this, within Windows Explorer, right-click on the CHM file, select properties, and Unblock it, using the button in lower part of properties window.

The help is also packaged in a format that you can integrate into Visual Studio 2008, or Visual Studio 2010. VS2008 requires MS Help 2.0, while VS2010 requires a different, newer format, sometimes called MS Help 3, and sometimes (confusingly) called "MS Help Viewer 1.0 format".

The DotNetZip "devkit" download includes help in all these formats.

The Zip Format

The zip format is described by PKWare, at http://www.pkware.com/business_and_developers/developer/popups/appnote.txt

Every valid zipfile conforms to this specification. For example, the spec says that for each compressed file contained in the zip archive, the zipfile contains a byte array of compressed data. (The byte array is something the DeflateStream class can produce directly.) But the zipfile also contains header and "directory" information - you might call this "metadata". In other words, the zipfile must contain a list of all the compressed files in the archive. The zipfile also contains CRC checksums, and can also contain comments, and other optional attributes for each file. These are things the DeflateStream class - either the one included in the .NET Framework Class Library, or the one embedded in this library - does not read or write.

Managing the metadata in a zip file is most of what DotNetZip does.

Which DLL to use?

The binary releases of DotNetZip include multiple distinct DLLs or assemblies. Which one should you use?

The likely answer is: use Ionic.Zip.dll.

That's the mainstream library, the full library, and it includes all the capability. If you have particular requirements, like you want a smaller library, or you want to exclude the Self-Extracting stuff, or you only want the ZLIB capability, then you may want to choose a different assembly.

Here's a summary of the options.

Usage scenario Reference this DLL

reading or writing Zip files Ionic.Zip.dll

raw block or stream compression, ZLIB, GZIP, Ionic.Zlib.dll or DEFLATE

raw block or stream compression, BZIP2 Ionic.BZip2.dll

both raw compression as well as reading Ionic.Zip.dll or writing Zip files

reading or writing Zip files, using desktop Ionic.Zip.Reduced.dll .NET framework but never creating a self-extracting archive

Never reference both Ionic.Zlib.dll and Ionic.Zip.dll, or both Ionic.BZip2.dll and Ionic.Zip.dll in the same application. If your application does both Zlib and Zip stuff, you need only add a reference to Ionic.Zip.dll. Ionic.Zip.dll includes all the capability in Ionic.Zlib.dll and Ionic.BZip2.dll You always need to reference only a single Ionic DLL, regardless whether you use Zlib or BZip2 or Zip or some combination.

Self-Extracting Archive support

The Self-Extracting Archive (SFX) support in the library allows you to create a self-extracting zip archive. An SFX is both a standard EXE file and a ZIP file. The exe contains boilerplate program logic to unzip the embedded zip file. When the user executes the SFX runs, the boilerplate application logic just reads the zip content and then unzips itself. You can open an SFX in WinZip and other zip tools, as well, if you want to view it.

Running the SFX (unpacking from the SFX) requires the .NET Framework installed on the machine, but does not require the DotNetZip library.

There are two versions of the SFX - one that presents a GUI form, and another that runs as a console (command line) application.

NB: Creation of SFX is not supported in the Compact Framework version of the library.

Also, there is no way, currently, to produce an SFX file that can run on the .NET Compact Framework.

The Reduced ZIP library

The full DotNetZip library is currently about 400k in size. The SFX (Self-Extracting Archive) support is responsible for more than half the total size of the library. Some deployments may wish to omit the SFX support in order to get a smaller DLL. For that you can rely on the Ionic.Zip.Reduced.dll. It provides everything the normal library does, except the SaveSelfExtractor() method on the ZipFile class.

For size comparisons...these approximate figures are for v1.9.1.6 of the library:

Desktop Framework:
  assembly              ~size   comment
  Ionic.Zlib.dll         100k   {Deflate,GZip,Zlib}Stream and ZlibCodec

  Ionic.BZip2.dll         57k   BZip2{Input,Output}Stream

  Ionic.Zip.dll          460k   includes ZLIB and BZIP2 compression,
                                SFX, selector logic, WinZIP AES encryption,
                                and the ComHelper class

  Ionic.Zip.Reduced.dll  250k   includes everything in the main ZIP
                                library except SFX. (ability to save
                                Self-extracting archives)


There is no official support for this library. I try to make a good effort to answer questions and monitor the work items raised on the project portal at:


About Intellectual Property

I am no lawyer, but before using this library in your app, it may be worth reviewing the various licenses.

The specification for the zip format, which PKWARE owns, includes a paragraph that reads:

PKWARE is committed to the interoperability and advancement of the .ZIP format. PKWARE offers a free license for certain technological aspects described above under certain restrictions and conditions. However, the use or implementation in a product of certain technological aspects set forth in the current APPNOTE, including those with regard to strong encryption or patching, requires a license from PKWARE. Please contact PKWARE with regard to acquiring a license.

Contact pkware at: [email protected]

This library does not do strong encryption as described by PKWare, nor does it do patching. But again... I am no lawyer.

This library uses a ZLIB implementation that is based on a conversion of the jzlib project http://www.jcraft.com/jzlib/. The license and disclaimer required by the jzlib source license is referenced in the relevant source files of DotNetZip, specifically in the sources for the Zlib module.

This library uses a BZip2 implementation that is based on a conversion of the bzip2 implementation in the Apache Commons compression library. The Apache license is referenced in the relevant source files of DotNetZip, specifically in the sources for the BZip2 module.


There are a few limitations to this library:

It does not do strong encryption.

The GUI tool for creating zips is functional but basic. This isn't a limitation of the library per se.

...and, I'm sure, many others

But it is a good basic library for reading and writing zipfiles in .NET applications.

Building the Library

This section is mostly interesting to developers who will work on or view the source code of DotNetZip, to extend or re-purpose it. If you only plan to use DotNetZip in applications of your own, you probably don't need to concern yourself with the information that follows.

Pre-requisites to build DotNetZip

.NET Framework 4.0 SDK or later -or- Visual Studio 2010 or later


ILMerge - a tool from Microsoft that combines multiple managed assemblies into a single DLL or image. It is in similar in some respects to the lib tool in C toolkits.

You can get it here: http://www.microsoft.com/downloads/details.aspx?familyid=22914587-b4ad-4eae-87cf-b14ae6a939b0&displaylang=en

Building DotNetZip with the .NET SDK

To build the library using the .NET Framework SDK v4.0,

  1. extract the contents of the source zip into a new directory.

  2. be sure the .NET 4.0 SDK and runtime directories are on your path.

  3. Modify the .csproj files in Zip and ZLIB and BZip2 to eliminate mention of the Ionic.pfx and Ionic.snk files.

The various DLLs (Zip Partial, ZLIB, etc.) are signed with my private key. You will want to remove the mention of the private key in the project files. I cannot distribute my private key, so don't ask me! That would be silly. So you have to modify the project in order to build without the key.

  1. open a CMD prompt and CD to the DotNetZip directory.

  2. msbuild

Be sure you are using the .NET 3.5 version of MSBuild. This builds the "Debug" version of the library. To build the "Release" version, do this:

msbuild /t:Release

  1. to clean and rebuild, do msbuild /t:clean msbuild

  2. There are two setup directories, which contain the projects necessary to build the MSI file. Unfortunately msbuild does not include support for building setup projects (vdproj). You need Visual Studio to build the setup directories.

I am in the process of converting these from .vdproj to .wixproj, so they can be built from the command line using msbuild. .

Building DotNetZip with Visual Studio

To build DotNetZip using Visual Studio 2010,

  1. Open the DotNetZip.sln file in VS2010.

  2. If necessary, Remove the dependencies on Ionic.pfx and Ionic.snk.

(References to these will have already been removed from the zipped source distributions, but if you get your source from the TFS server, then you will have to remove references to the keyfiles manually)

The various DLLs (Zip, ZLIB, etc.) are signed with my (Dino Chiesa's) private key. I do not distribute that key for anyone else's use. If you build the DotNetZip library from source, You will want to remove the mention of the private key in the project files. I will not distribute my private key, that would be silly. So don't ask me!

  1. Press F6 to build everything.

The Project Structure and Build approach

The function here is grouped into three basic sets: Zip, ZLIB/Deflate/GZIP, and BZip2. The Zip group is a superset of the ZLIB and BZIP2 groups.

Each group of functionality is packaged into various assemblies, one assembly per "platform". The platforms supported are: .NET (Desktop).

There is also a special "Zip Reduced" library, available only on the Desktop platform; it is a reduced-function version of the regular Desktop Framework zip library. It provides an option of using a smaller library for those zip-handling applications that don't produce Self-extracting archives.

In a previous guise, DotNetZip relied on the ILMerge tool to combine distinct DLLs into a single package. This is no longer the case.

Because the ZIP projects include the ZLIB and BZIP2 function, the appropriate source modules for the ZLIB and Bzip2 are "linked" into each of the ZIP projects (Desktop).

Regarding the missing Ionic.pfx and Ionic.snk files

The binary DLLs shipped in the codeplex project are signed by me, Dino Chiesa. This provides a "strong name" for the assembly, which itself provides some assurance as to the integrity of the library, and also allows it to be run within restricted sites, like apps running inside web hosters.

For more on strong names, see this article: http://msdn.microsoft.com/en-gb/magazine/cc163583.aspx

Signing is done automatically at build time in the Visual Studio project or in the msbuild build. There is a .pfx file that holds the crypto stuff for signing the assembly, and that pfx file is itself protected by a password. There is also an Ionic.snk file which is referenced by the project, but which I do not distribute.

People opening the project ask me: what's the password to this .pfx file? Where's the .snk file?

Here's the problem; those files contain my private key. if I give everyone the password to the PFX file or the .snk file, then anyone can go and build a modified Ionic.Zip.dll, and sign it with my key, and apply any version number they like. This means there could be multiple distinct assemblies with the same signature. This is obviously not good.

Since I don't release the ability to sign DLLs with my key, the DLL signed with my key is guaranteed to be produced by me only, which is in fact the exact intent of code signing in .NET.

If anyone wants to modify the project and re-compile it, they have a couple options:

  • sign the assembly themselves, using their own key.
  • produce a modified, unsigned assembly

In either case it is not the same as the assembly I am shipping, therefore it should not be signed with the same key.

All clear?

As for those options above, here is some more detail:

  1. If you want a strong-named assembly, then create your own PFX file and .snk file and modify the appropriate projects to use those new files.

  2. If you don't need a strong-named assembly, then remove all the signing from the various projects.

In either case, you will need to modify the "Zip", BZip, and the "Zlib" projects.

Building the Documentation

The documentation files are built using the Sandcastle Helpfile Builder tool, also available on CodePlex at http://www.codeplex.com/SHFB . It is built from in-code xml documentation, augmented with some additional out-of-band html documentation.

If you want to build the help files yourself, you will need to have Sandcastle from May 2008 (or later, I guess), and SHFB, from February

  1. Both are free tools available from http://codeplex.com . I think you can get a package download of both of these by installing v1.9.3.0 of SHFB .

The helpfile projects are:

HtmlHelp1.shfbproj - to build the .chm file MSHelp2.shfbproj - to build the MS Help 2.0 content HelpViewer.shfbproj - to build the MS Help Viewer 1.0 content

(The MSHelp2 project is broken at the moment.)

To build the documentation in any of these formats, first build the "zip Full DLL" project in the source (Ionic.Zip.dll), then run:

msbuild HtmlHelp1.shfbproj


msbuild HelpViewer.shfbproj

The Help Viewer 1.0 content can be viewed in the help viewer that is integrated into VS 2010, or in an alternative viewer, such as H3Viewer.exe. See http://mshcmigrate.helpmvp.com/viewer .


The source solution also includes a number of example applications showing how to use the DotNetZip library and all its features - creating ZIPs, using Unicode, passwords, comments, streams, and so on. Most of these will be built when you build the solution. Some of them do not - you will need to build them independently.


There are two source projects in the VS Solution that contain Unit Tests: one for the zlib library, one for the bzip2 library, and another for the Zip library. If you develop any new tests for DotNetZip, I'd be glad to look at them.


This library is mostly original code.

There is a GPL-licensed library called SharpZipLib that writes zip files, it can be found at http://www.sharpdevelop.net/OpenSource/SharpZipLib/Default.aspx

This library is not based on SharpZipLib.

I think there may be a Zip library shipped as part of the Mono project. This library is also not based on that.

Now that the Java class library is open source, there is at least one open-source Java implementation for zip. This implementation is not based on a port of Sun's JDK code.

There is a zlib.net project from ComponentAce.com. This library is not based on that code.

This library is all new code, written by me, with these exceptions:

  • the CRC32 class - see above for credit.
  • the zlib library - see above for credit.
  • the bzip2 compressor - see above for credit.

*Note that all licence references and agreements mentioned in the DotNetZip.Semverd README section above are relevant to that project's source code only.