Minimal ZIP file – part II

This article is part of a series.

1. Creating a valid ZIP-file
2. Use .NET’s DeflateStream 👈
3. No dependencies (coding the DEFLATE algorithm)
4. Calculating the CRC-32

Introduction

In part 1 a valid ZIP-file was created using uncompressed data. To be more useful, data should be compressed.

The most simple way is the so-called DEFLATE algorithm¹. DEFLATE is supported back to PKZIP 2.x.

Compressing the data

Using .NET’s DeflateStream it is easy to compress the data. I consider this a little bit of cheating, but it is a good starting point². See also the next part to fix this.

It takes only a few modifications to our previous program to compress the data to a MemoryStream³ and write it to the ZIP-file:

var filedata = "BANANABANANABANANABANANABANANA"u8.ToArray();
// ...

// Compress the data
using var outputStream = new MemoryStream();
using (var deflateStream = new DeflateStream(outputStream, CompressionLevel.Optimal))
{
    deflateStream.Write(filedata);
}
var data = outputStream.ToArray();

Other than that, only a few modifications are needed when writing the headers:

• uncompressed size vs. compressed size
• compression method none vs. DEFLATE

Source code

The modified code:

Zip2.cs

Next up

In the next part the dependency on System.IO.Compression.DeflateStream will be removed.