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Working with Archived files

By , 31 Oct 2004
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Introduction

Some time ago, I was sent to work with an old mainframe system. There I became familiar with a source repository where a single file on the system contains several files from the source code. Although this seems very familiar to many people (through TAR or ZIP files), I wanted the ability to work with the files within without copying them to a temporary location. This article is the result of this quest.

The Archive Class

I would be very unpractical to create such a class just to store files. It would work just like many other compression libraries out there – except for no compression at all. Plus, it would impose many limitations to store just files and no directories to organize them.

I created a simple file system based on what has been described as WinFS (Windows Longhorn FileSystem based on SQL). A single file contains a table with every file or folder inside the Archive. It's a simple record, so there's nothing more than a name, number of parent folder, entry points for files, or indexes for folders. I chose to record no dates or times. I also chose to use UNIX-style path separator to avoid escaped backslashes or verbatim strings.

In the end, it's a very simple class containing many of the functionality provided by File and Directory classes in the System.IO namespace, including methods like OpenRead and OpenText.

Why would I use it?

At first sight, many would consider a waste of time to use an Archive. And maybe you're right. The usage of every piece of technology depends on its need within the project. Maybe it's not your case.

At first, I designed an Archive to contain source codes for all of my projects. When it was near completion, I realized it wasn't such a great idea but found other uses for it such as small databases (specially to store serialized objects) and/or files I don't want users picking at. You can even find others I haven't thought of, let us know.

Limitations

As far as I can see, there are very few limits to expose:

  • An Int32 is used to index file blocks, so the archive size limit is 1,5Kb * 2,147,483,647 (= 3,221,225,470 Kb, theoretically). This is also the biggest a single file can get.
  • Directories differ from files by using negative start indexes, thus limiting an Archive to 2,147,483,648 directories. Currently, a stored index is used to retrieve the index for a new directory and this is never reset. (Should I revise it?)
  • Maybe others that I can't remember right now.

Expanded Universe

This is the goal of open-source, isn't it? So, the Archive class is made easy to understand, maintain and modify. In a few minutes reading the code, you'll be able to expand the class personalizing it to your own needs. Here are some examples that can be easily accomplished:

  • Replace the "Archive:" header for a more complex version detailing the use or meaning of the files within;
  • Simple scramble cryptography – OK, to use a crypto-stream, the file would require a temporary stream, but some simple cryptography (i.e., using XOR) is easy;
  • Additional file information like creation and modification times (which I chose not to include);
  • A new property to define which character is to be used for path separation;
  • etc.

Some of the modifications might even be useful to other users, so I suggest if you modify the Archive class, post a brief of the modifications you made here too.

Points of Interest

The code has been widely tested, except for files (as can be seen in the Main procedure), although the code used was tested in a previous version. So, there should be no problems with it. If there are any problems, let me know.

Also, the Archive class uses widely .NET Generic collections, limiting its use to .NET Framework 2.0. If anyone is interested in porting it to prior versions, I can post the results here.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Author

Leonardo Pessoa
Software Developer
Brazil Brazil
I'm that strange type, who likes to code (mostly Objective-C and Javascript nowadays) and hates to use a database (I'd rather code one, instead!). I'm mainly interested in programming languages, compilers, interpreters and the like.
 
Although I don't place any form of restriction upon using the codes I provide, I would appreciate to be mentioned as it's author in any projects using them.
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Comments and Discussions

 
QuestionAppending files to the archive Pinmemberallanhaugsted19-Sep-05 10:23 
AnswerRe: Appending files to the archive PinmemberHarkos23-Sep-05 1:59 
GeneralRe: Appending files to the archive Pinmemberscottcurrier10-Dec-05 12:37 
AnswerRe: Appending files to the archive PinmemberHarkos12-Dec-05 0:07 
GeneralMultiple streams using NTFS PinmemberSteven Campbell1-Nov-04 8:15 
GeneralRe: Multiple streams using NTFS PinmemberHarkos2-Nov-04 8:36 
QuestionMisnamed article and why do this? PinmemberDale Thompson1-Nov-04 3:34 
AnswerRe: Misnamed article and why do this? PinmemberHarkos1-Nov-04 6:42 
AnswerRe: Misnamed article and why do this? Pinmemberjdraper38-Aug-05 6:37 

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