How to fix Open XML SDK created spreadsheet for iOS devices

Note that this is only for Open XML SDK 2.0. I haven’t tested for SDK 2.5 (or later, but there’s no “later” version at the point of this writing).

The fix

You have to manipulate the [Content_Types].xml file. This XML file is zipped together in the spreadsheet file, and is at the root of the file. If you don’t know what I’m talking about, you’re probably reading the wrong article.

Unfortunately, this can’t be done within Open XML SDK. As I understand it, the SDK runs internally on System.IO.Packaging namespace objects.

Now a package is a zip file with structure. But a zip file is not necessarily a package. (go maths logic!) See Microsoft reference (bottom of page).

As I understand it, you can’t manipulate a package’s [Content_Types].xml with any of the methods or classes from the System.IO.Packaging namespace. This XML file is supposed to be tamper-proof.

So if you can’t change it with the Packaging stuff, then you’ll have to manipulate it as a pure zip file. And as at .NET Framework 3.5 (because I’m using Open XML SDK 2.0 as reference), there’s no in-built zipping mechanism for the kind of zipping algorithm you need (the Gzip and Deflate isn’t exactly the tool).

This means you need an outside zip library. I leave it to you to find your favourite library. Assuming you found one, here’s why you need to change the [Content_Types].xml file.

The one generated by Excel looks something like this:

<?xml version="1.0" encoding="UTF-8" standalone="true"?>
<Types xmlns="http://schemas.openxmlformats.org/package/2006/content-types">
<Default ContentType="application/vnd.openxmlformats-package.relationships+xml" Extension="rels"/>
<Default ContentType="application/xml" Extension="xml"/>
<Override ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet.main+xml" PartName="/xl/workbook.xml"/>
<Override ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.worksheet+xml" PartName="/xl/worksheets/sheet.xml"/>
<Override ContentType="application/vnd.openxmlformats-officedocument.theme+xml" PartName="/xl/theme/theme.xml"/>
<Override ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.styles+xml" PartName="/xl/styles.xml"/>
<Override ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.sharedStrings+xml" PartName="/xl/sharedStrings.xml"/>
<Override ContentType="application/vnd.openxmlformats-package.core-properties+xml" PartName="/docProps/core.xml"/>
</Types>

The one generated by Open XML SDK 2.0 looks something like:

<?xml version="1.0" encoding="UTF-8"?>
<Types xmlns="http://schemas.openxmlformats.org/package/2006/content-types">
<Default ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet.main+xml" Extension="xml"/>
<Default ContentType="application/vnd.openxmlformats-package.relationships+xml" Extension="rels"/>
<Override ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.worksheet+xml" PartName="/xl/worksheets/sheet.xml"/>
<Override ContentType="application/vnd.openxmlformats-officedocument.theme+xml" PartName="/xl/theme/theme.xml"/>
<Override ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.styles+xml" PartName="/xl/styles.xml"/>
<Override ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.sharedStrings+xml" PartName="/xl/sharedStrings.xml"/>
<Override ContentType="application/vnd.openxmlformats-package.core-properties+xml" PartName="/docProps/core.xml"/>
</Types>

Spot the difference time!

The workbook part is missing. Specifically, look for the XML tag with the ContentType attribute equal to “application/vnd.openxmlformats-officedocument.spreadsheetml.sheet.main+xml”

Open XML SDK, for whatever reason, doesn’t explicitly set the PartName=”/xl/workbook.xml” part. This is why iOS devices can’t see the spreadsheet file.

I don’t know whether the fix will work on Android devices or Windows phones. But a developer emailed me that iOS 6.0 devices seem to still be able to read the spreadsheet files by Open XML SDK, but iOS 7.0 just gives up.

Microsoft, please, you created the Open XML specs. Other companies are following the specs and the one useful tool you made to help generate Open XML documents fail to follow the specs.

How to found a company

Just do it.

But jokes aside, the bigger question is not how. It’s why.

[Disclaimer: any information you read here pertains to Singapore laws and regulations. Your country's laws and regulations might differ.]

I have people emailing me “Wow Vincent, you’re running your own business! That’s totally awesome. I wish I could do something like you did.”

Or “I don’t really have a product to sell.”

Or “I have this small thing that I’m working on, but it’s too early to tell.”

Or “I have a family to take care of.”

Or “I’m scared.”

All perfectly valid reasons. And at some point in time, all of them were applicable to me. And I’m still working for myself now.

Now for the big news. I founded a company! Yay! (ok I co-founded a company, but the circumstantial details are more complex than I’d like it to be. I might talk about it in an email. Sign up on the right side bar [or wherever it might be in case the position is changed]).

So about the how thing. It’s not hard. It’s just frustratingly tedious. Mostly because I don’t know what forms to fill in, in what sequence, and “Is there anything else I need to do because I don’t wanna get sued and go to jail!”.

Also, because I’ve never done it before. Most people will feel scared if they’re doing something they’ve never done before. So they find it frustrating and tedious and moan and groan and go drink a bottle of whiskey and go to bed.

You go to ACRA, come up with a company name, decide on the shares allocated to each share holder (probably decide beforehand and not on the spot), put in share holder details, company official member details, pay ACRA for the registration and you’re done!

There’s also quite a bit of legalese and fine print. Since I don’t want to get sued and go to jail, I read them. You might want to go on the wild side and just skip them. To each his own.

So it’s not so much the how. Find a strong enough reason for why, and you’ll just do it.

By the way, I founded a company! The company name is One North Soft. I have a secret project in the works. Tell you more in future.

Regular polygon equation (solved)

So I’ve finally solved this. You can read about the background and context for the question on:

This is the Wolfram Alpha friendly command:
polarplot [ cos(Pi/7)/cos( | (t mod (2Pi/7)) - (2Pi/(2*7)) | ) , {t,0,2Pi}]

That will generate a regular polygon with 7 sides, with a circumradius of 1 unit. Substitute all the 7′s with the number of sides you want and voila! And the general equation is thus:

cos(Pi/N) / cos( ABS( (t mod (2Pi/N)) – (2Pi/(2*N)) ) )

where t is in [0,2Pi], and N is the number of sides

So how did I get the equation?

The formula for the apothem is cos(Pi/N). The apothem is the shortest distance from the centre to the side. With that, I bring your attention to this illustration. (this is a regular polygon with 4 sides. “It’s a square!”. Yes, I know)

Apothem and regular polygon equation explanation

The length “A” is the apothem, and t is the angle running in the equation we stated above. “apm” is the angle the apothem (for this particular segment of t) makes with the positive X axis.

And L is what we want to find.

We will define the first segment as the segment immediately after the segment whose apothem lies on the positive X axis. (so the apothem illustrated above is for the first segment) That will uniquely identify our segments.

Now convince yourself that the angle apm is in multiples of 2PI/N radians.

To find L, we need to find the angle s (I’m running out of colours…). And angle s = t – apm.

So s = t – 2PI/2N

“But that’s not exactly right!” you say. And you’re right. Because that didn’t take care of the multiples of 2PI/N radians thing.

To get the working t angle we’re using, it should be

“working t angle” = t modulus 2PI/N

Convince yourself that’s true. Substitute N with 4 or 5 or 100.

“But that’s not exactly right!” you say. And you’re right.

Because s = “working t angle” – 2PI/2N
can be negative (suppose the red line L is on the right side of A). That’s why we have

s = ABS( (t modulus 2PI/N) – 2PI/2N )

Why do we need to find s again? Because we want to find L. And L can be found with this equation:

A/L = cos(s)

Revise your trigonometry rules. Cosine of the acute angle is equal to adjacent side divide by the hypotenuse.

So L = A/cos(s)
= cos(PI/N) / cos(ABS( (t modulus 2PI/N) – 2PI/2N ))

So why do we need to find L again? In polar coordinates, you only need the angle and the radius (or length from origin) to uniquely determine a point. Since we have the angle, we just need the radius (or length).

That’s why the polar plotting from Wolfram Alpha works.

You can probably convert that from a polar coordinate point equation representation to a Cartesian point equation representation, but I’m done for now.

Considerations for storing Excel cell value in code

You may hate Excel, but you may find a discussion of how Excel stores cell values interesting.

So I have a spreadsheet library. The biggest concern at the initial stage was how to store all the spreadsheet data efficiently. I hear people talking about millions of cells, so I’m scared. If my program stores a spreadsheet cell using 10 bytes (for example), a million cells would take up 10 million bytes in memory.

Let’s start by looking at all the different types of information you can type into a spreadsheet cell. You have:

  • booleans: TRUE or FALSE
  • numbers
  • text
  • rich text (different styled text within the entire text itself)
  • dates and times

For us programmers, “numbers” can be separated into floating point or integer types. An Excel user won’t see a difference.

So how does Excel actually store those values? I’m going to focus only on Open XML because I’m not interested in BIFF files…

  • booleans: TRUE stored as text “1″ and FALSE stored as text “0″
  • numbers: stored as text
  • text: duh
  • rich text: stored in a separate shared strings list, with the index to that list stored as text here.
  • dates and times: stored as number that’s in text form

You will see everything is basically stored as text. That’s because the underlying XML files are text files. There’s a property (XML attribute) that differentiates the data, such as boolean, number, string, inline string, shared string.

So why are dates stored as a number? It’s easier to do date calculations with 41449 than “24 June 2013″. So how is this number obtained? See here.

So if you’ve been looking closely enough, Excel’s optimisation tactic is to store everything as numeric text as far as possible. So I want to follow that.

Before doing so however, I went to read what other people are doing AKA open source spreadsheet libraries. In code, they use an object to store the cell value. As in System.Object, the mother of all data types in .NET.

So you have an integer? Dump it into the object variable. Floating point? Dump into object. String of characters? Dump.

How do you read it out? Boxing and unboxing. You remember it’s a floating point value and cast it back from an object to a double variable type.

So what did I do? I have a double variable and a string variable, and I store the cell value in one or the other based on the input.

The “all in object” way has variable (no pun intended) memory size, based on the contents. Sort of. I’m not an expert in this.

My way has a fixed memory size for double’s. Each double takes up 8 bytes (for sure?). A string variable takes up variable size, but because the optimisation tactic is to store data as a number, I can assign the data to the double variable and set the string variable to null. This means the string variable size is sort of fixed too.

So this is what I do. If it’s a number, I store it in the double variable and set the string variable to null. If it’s text, I convert it to a number by using shared strings (out of scope for discussion here) and store the index into the double variable and set the string variable to null. The only cases where the string variable is actually used is if I store the text there, or if I want to store the actual number there (because “1.23456789″ may not be stored exactly as that in a double variable. Go read on how floating points are implemented for details), which are rare.

According to Jon Skeet, strings take up 20 + (n/2)*4 bytes (where n is the number of characters). But a null string takes up 8 bytes (it’s either 4 or 8 bytes. I’ll assume the worse scenario).

This means for the most part, each cell has a double variable that takes up 8 bytes and a null string that takes up 8 bytes. A cell value of 10 or 3.14 or 12345678.9 takes up 16 bytes regardless.

Since 16 bytes is less than 20 + (n/2)*4 bytes, I save more memory in most cases. I also have less boxing and unboxing operations, which make things go faster.

File upload size limit in IIS

Yay file uploads. As if letting the users to type in stuff into the web application giving me SQL injection nightmares weren’t enough, now I have to let users upload files.

Peachy.

So during my investigations into the limits of file uploading, I found that I couldn’t upload a file more than 30MB on my test server. It failed faster than Superman could jump a building in a single bound, and with just as much sound.

In short, here are my findings. The default file size limit set in IIS (6 and below? Read on for more details) is 4MB. In IIS7 (on Windows Server 2008), the file size limit is 30MB (technically it’s 28.61MB because it’s 30000000 bytes but who’s keeping track. Hey you read on!).

So how do you change the limits? In the web.config file. We’re doing ASP.NET applications.

<httpRuntime executionTimeout="3600" maxRequestLength="20480" />

That will give you a timeout period of 1 hour (3600 seconds) and a file size limit of 20MB (20480 KB. Yes, that attribute is in kilobytes).

For IIS7, we do this:

<security>
    <requestFiltering>
        <requestLimits maxAllowedContentLength="134217728" />
    </requestFiltering>
</security>

That gives you a 128MB limit (128 * 1024 * 1024). Yes it’s in number of bytes.

So why was I doing file uploads? Documents from university staff or students. The most important of which is the final doctoral thesis.

I asked how large can that thesis be, assuming it’s in PDF form. I got an answer where a 40MB limit seems too small. Really?

I had trouble auto-generating an Excel file of 40MB just to test the server limits. Do you know how large 40MB is?

If it’s a video or sound file, then yes I can believe it. I have video files of over 100MB, some over 200MB. But a PDF? With mostly text?

Go check out the Open XML specs from ECMA. The largest document is about 28MB. It’s over 5000 pages. I doubt any thesis can match that number of pages.