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Compress/Decompress a String in C#

December 26, 2012 Comments off

The feature I was working on today involved saving some temporary data into an HTTP session cookie. The data was an object serialized into JSON. Writing and reading cookies is easy enough but, after testing my implementation a bit, I found out that my code was intermittently failing to save the cookie in my browser. After some deeper investigation, I found that the reason my cookie wasn’t being saved (sometimes) was because the JSON I was attempting to store in the cookie was exceeding the 4K limit of a cookie. The object I was serializing to JSON was a List of other classes so I could not prevent users from exceeding the limit without imposing arbitrary restrictions on the usage of the feature. I also wanted to keep the data on the client’s machine, as opposed to a DB or other storage medium, because the data really did seem to belong in the user’s session. The only option I could think of to solve my problem was to compress the JSON string.

Unfortunately, when I Google’d “C# compress string” (or something like that), the results weren’t very helpful. Most of the code I came across was either for .NET 2.0 or was doing File IO and byte array manipulation with pointers. Many examples didn’t even make proper usage of using() blocks to properly close Streams. What I needed was .NET 4.0 code to Compress and Decompress strings to strings, preferably without needing to do any byte pointer magic. Fortunately, I eventually came across this some-what helpful Stack Overflow Q&A which was close enough to what I was trying to do. The OP was looking for a .NET 2.0 solution and his compressed medium was a byte[] instead of a string but, with a few simple modifications, I was able to adapt the code to do exactly what I needed.

using System;
using System.IO;
using System.IO.Compression;
using System.Text;

...

public static string Compress(string s)
{
    var bytes = Encoding.Unicode.GetBytes(s);
    using (var msi = new MemoryStream(bytes))
    using (var mso = new MemoryStream())
    {
        using (var gs = new GZipStream(mso, CompressionMode.Compress))
        {
            msi.CopyTo(gs);
        }
        return Convert.ToBase64String(mso.ToArray());
    }
}

public static string Decompress(string s)
{
    var bytes = Convert.FromBase64String(s);
    using (var msi = new MemoryStream(bytes))
    using (var mso = new MemoryStream())
    {
        using (var gs = new GZipStream(msi, CompressionMode.Decompress))
        {
            gs.CopyTo(mso);
        }
        return Encoding.Unicode.GetString(mso.ToArray());
    }
}

It is simple, elegant, and best-of-all… it works! Luckily for me, JSON compresses extremely well. The previous data which was failing to save was about 4.5 K but I was able to reduce that down to only 1.5 K after compression! The only drawback I have noticed so far of storing compressed data in a cookie is that it is not human readable. You could spin that as a good thing because users will be less likely to want to see/modify obfuscated cookie values, but as a web-developer, I usually like knowing exactly what is being stored on my computer by the websites I visit. An (unintentional) positive side-effect of compressing the cookie is that it helps minimize the amount of data being sent back-and-forth from the users and our servers. 4K might not seem like a lot of data, but when it is being sent in every request from the user’s browser, it can add up. Either way, this simple code helped me solve this problem… moving on!