Introduction

A while ago I needed to design a file format for my application. So after giving it a bit of thought I created what at the time seemed like the perfect file format for my app... All was going well until I needed to sore additional information in the file. Oops, this I had not planned for. So I had to sit down and again design a new format, this time allowing for additional data at a later stage. This file format works and is flexible but it is still not flexible enough.

I am sure something similar has happened to you. Some time or another you must have found a file format a bit limiting. Would it not be nice if there was a powerful and flexible way of saving data. Why not go for powerful, flexible and easy (ok, ok.. relatively easy ;-)?

Well, there is. Structured Storage.
A Structured Storage file is also called a DocFile or an OLE compound file. I'll use these terms interchangeably throughout this article.

DocFiles have many features that make them a valuable tool. For example

Load & Save only portions of the file. Ie No need to load the whole thing to get hold of some block of data. ie Incremental access.
DocFiles are supported internally by Windows, there are ways to store info about the DocFile, within the DocFile itself. To see this in action, view the properties of a WinWord or Excel File in explorer. The info on the summary and statistics pages is read from the DocFile.

Microsoft has said that Structured Storage will be the default file format in future editions of windows. The MS-Office products are already using structured storage. So learning it now will give you a head start.

So what are DocFiles?

The best way to think of a DocFiles is as having a file system within a file. What that means is that a DocFile can contain "directories" and "files".

For example a DocFile say Example.ole could contain

Each "file" is separate from the other. You just open a "file" and work with it, without opening or affecting any of the other "files" in the DocFile.

Some Terminology

Storage Equivalent of a directory, within the DocFile

Stream Equivalent of a file in a DocFile. A stream exists in a storage just as a file exists in a directory.

Storage	Equivalent of a directory, within the DocFile
Stream	Equivalent of a file in a DocFile. A stream exists in a storage just as a file exists in a directory.