Category Archives: Uncategorized

How to avoid boxing value types in F# equality comparisons

This week I realized, and promptly reported to the Visual Fsharp compiler, that F#’s operator (=) boxes user-defined value types. That’s right, any value type you might want to use from a third-party library, or any value type you define for performance reasons, well, don’t compare them using (=) in F#, currently. Unless performance is of no concern to you, but then why would you use value types in the first place?

type AnyType =
    // whatever

AnyType() = AnyType() // allocates on the GC heap!?!

I don’t want to get into the how the problem happens as that would just be guesswork, I don’t work on the F# compiler. I just know it is there, and based on the feedback so far, it’s not going to be a quick fix.

So what can we do if we want to use value types in F#? Can’t use Equals, we’d need to cast to IEquatable(T) which is in itself a boxing operation, and Object.Equals boxes and casts both operands. One working alternative (thanks latkin) is to define op_Equality and use the new F# 4.0 NonStructuralModule, but that disables structural comparisons altogether. A workaround around that problem is to alias the equality operators in NonStructuralModule themselves, but then one has to remember to use the aliased operators, and I don’t trust anyone to remember anything, starting with myself.

Based on a later suggestions I received (thanks manofstick), this is what I would personally do:

#nowarn "86"

module FastEquals =
    let inline eq<'a when 'a :> System.IEquatable<'a>> (x:'a) (y:'a) = x.Equals y    
    let inline (=) x y = eq x y
    let inline (<>) x y = not (eq x y)
    let inline (=@) x y = Microsoft.FSharp.Core.Operators.(=) x y
    let inline (<>@) x y = Microsoft.FSharp.Core.Operators.(<>) x y

First, we are redefining the (=) operator in terms of IEquatable(T).Equals. This does the correct thing, not just for value types, but for classes, records and discriminated unions. There’s no overhead, as the method compiles to a single x86 instruction for all basic numerics types, and yet we still get structural equality for most F# types. F# complains with warning #86, hence, the #nowarn. Shut up F#, we’re fixing you up.

But of course not all types implement IEquatable(T), and the beauty of this is that it won’t compile in this case! So you can confidently use (=) everywhere, and trust the compiler that it won’t let you misuse it. Idiomatic, correct and fast. That’s what we want. I feel good now.

I’m not entirely sure if it’s a good idea to expose the default F# operator, but just in case, I did it by renaming it “=@”, that should make it obvious. The obvious case for using this operator would be structural collection comparison, but be aware that, again, this boxes all elements if they’re user-defined value types! Again, not behavior we’d like to accidentally run into. I’m considering getting rid of the default (=) altogether and just redefining structural equality manually for these collections as needed, perhaps using overloaded static members.

Unfortunately, you won’t able to prevent third-party F# code from using the default operator, starting with everything in FSharp.Core: List, Array, etc. It then becomes a matter of avoiding the functions that do box value types. For example:

List.contains value // boxes value types
List.exists (fun v -> v = value) // fast if (=) is redefined as suggested above

Of course, as the compiler warning suggests and I’ve been advised on the fsharp compiler thread, this might not be a good thing to do: I’m unsure as to why but since I don’t use F# in anything large-scale yet, I may be missing some insight into unintended consequences of this. You might want to use another operator for non-boxing comparisons, but I really value being able to use (=) idiomatically and not worry about performance pitfalls.

So far, I do think redefining the operator as I’ve suggested is actually more consistent and less surprising than what the default does (which suffers from other bugs and inconsistencies). What do I mean by that?

More consistent: By default, (=) gives you structural equality on everything except classes, where the behavior is reference equality. This is inconsistent and surprising in F#. By redefining (=) as suggested, where the default would silently use reference equality, the override will cause a compilation error: and that’s probably a good thing. If you really intend to use reference equality (which is too often a bogus and lazy way to compare), you can write Object.ReferenceEquals and it becomes obvious.

I’m only now starting to use this approach and I’ll update this post with my findings as I go along. I’m open to suggestions, refutations, let me know what you think!


Re: 8 Most common mistakes C# developers make

A recent blog article by Pawel Bejger lists some common mistakes made by C# developers. As noted by several in the comments, some of this information can be misleading or incorrect. Let’s review the problematic points:

String concatenation instead of StringBuilder

The author argues that using string concatenation is inefficient compared to using StringBuilder. While this is true in the general case, including the example presented, this must be taken with a grain of salt. For instance, if you just want to concatenate a list of strings as in the example presented, the easiest and fastest way is String.Concat(), or String.Join() if you need to insert something in between each pair of strings.

In addition, compile-time concatenations are automatically translated by the compiler into the appropriate calls to String.Concat(). Therefore, the use of StringBuilder should be reserved to building complex strings at runtime; don’t go blindly replacing every string concatenation code with StringBuilders.

Casting with “(T)” instead of “as (T)”

The author argues that if there’s the slightest probability a cast could fail, “as (T)” should be used instead of the regular cast. This is a very common myth and is misleading advice. First, let’s review what each cast does:

  • “(T)” works with both value and reference types. It casts the object to T, and throws an InvalidCastException if the cast isn’t valid.
  • “as (T)” works only with reference types. It returns the object casted to T if it succeeds, and null if the cast isn’t valid.

Each cast expresses a different intent. “(T)” means that you fully expect this cast to succeed, and that if it doesn’t, this is an error in the code. This is the simple and general case. “as (T)” means that you fully expect this cast NOT to succeed at least some of the time, that this is a normal occurrence, and that you will take care of manually handling it via a null check after.

The real mistake I often see is “as (T)” not followed by a null check. The developer fully expects the cast to succeed so doesn’t bother to write the null check, but the day something goes awry, no exception is thrown on the invalid cast, no null check is performed, and a hard to track bug has found its way into the code base. So my advice is to always use the regular cast “(T)” unless you intend to check yourself for the invalid cast via “as (T)” and a null check.

Using “foreach” instead of “for” for anything else than collections

The author claims that using “for” to iterate “anything that is not a collection (so through e.g. an array)””, the “for” loop is much more efficient than the “foreach” loop.

As with any claims regarding performance, this is easily verified. In this case, the verification proves the author wrong.

As the page he links to claims (and which is very outdated), the generated IL for a “foreach” loop is slightly larger than that for a “for” loop. This is no indication of its performance however, as the speed of code doesn’t correlate with its size, and IL isn’t actually executed but compiled just-in-time to assembly code, where further optimizations may happen. A very simple test performing the sum of an array of ints shows that the machine code generated for a foreach loop is in fact shorter, and the execution time also shorter, than for the for loop. Results on my machine (x86 release mode with optimisations, no debugger attached):

SumForEach: 482 ms

SumFor: 503ms

As you can see, this is in fact a very small difference, so I would advise that in general you use the loop that is the most idiomatic or readable, and not worry too much about the performance difference. But if you’re writing performance-sensitive code, don’t rely on someone else’s benchmark and test things out yourself. You may find that it’s not because  code is visually more compact or more C-like that it performs any better.

Darkstone’s MTF file format, part 2

Following my first attempt at unraveling Darkstone’s MTF file format, I suddenly found a ton of information on the format on the web. Well, so to speak. It is vaguely described at Xentax Game File Format Central, and there’s even a working open-source extractor: Dragon UnPACKer. The relevant code is in drv_default.dpr. It’s apparently Pascal, a language I had never looked at before, but that didn’t present a problem as it’s highly readable.

Remember I said some entries had invalid sizes? Well, they did not: the specified size was simply the uncompressed size, and I assumed no file compression. I had noticed that most files in DATA.MTF began with AF BE or AE BE even though they were of completely different formats, but it had not sprung to my mind that this was because they were compressed. Anyway, had I guessed, I couldn’t have figured out the technique to decompress them; even with the description at Xentax plus working source code in front of me, writing my own implementation was touchy.

My extractor now does at least as well as Dragon UnPACKer, so I’m pretty confident about having nailed the file format. Rather than try to explain it in words, I’ll refer you to my code on pastebin, which has enough comments that it should be clear even you don’t know C#.

Now that I at least extract all files properly, the internal file formats are starting to make more sense.

O3D: a file format for static meshes, can be opened at least with makeo3d.exe. The “Flyff” 3d converter (o3d2obj.exe) appears not to work with these.

DAT: mainly ASCII strings separated by long strings of 0s or other repeated numbers. I still have no real clue as to what they mean, but at least they’re partially legible now.

CDF: references O3D files by their names, separated by long strings of 0s. Maybe level descriptions?

Anyway… this is probably going nowhere as I don’t have infinite time nor much experience with reverse-engineering game files, but at least I get to enjoy the music knowing I extracted it myself! Ha.



Darkstone’s MTF file format

One of my guilty pleasures as a programmer is writing extractors for weird/old/undocumented file formats. I’m not very good at it, but I try. My latest victim was the “.MTF” format used by Delphine Software’s 1998 Darkstone, the first RPG I ever owned:

All the data files except for movies are in these obscure binary archives:

  • dsp001.MTF
  • dsp004.MTF

I searched Google in vain for any information on the format. No, this is not the same thing as Microsoft Tape Format. The only thing I found to help was a utility called dsxtract.exe, which extracted all the mp2 music files from MUSIC.MTF. It didn’t run on Windows 7 x64 (and of course the source code is nowhere to be found), but DosBox did the trick.

Now let’s take a look at MUSIC.MTF in a hex editor:

Ugh, not even an ASCII header. We can see that starting at byte 9, there’s an ASCII string, so the first 8 bytes are probably integers.

First things first: integers are usually little-endian. This means that if you blindly paste the highlighted 4 bytes in calc.exe and make it convert from hex to decimal, you’ll get 419430400, a number that has no obvious meaning. The trick is to invert the order of bytes: 00000019 is 25 in decimal. Does “25” make any sense?

Well when I ran dsxtract.exe, it produced 25 mp2 tracks. So it would appear that this first integer is the number of entries in the file.

So let’s look at the next integer. D is 13 in decimal. Hum, ok?

What about that next string. “MUSIC\22.MP2”. That looks like a path name. And it’s 12 characters long. Hum, almost 13… wait! The next byte is 00, the null character, so this is a 13-byte null-terminated string, and the integer before it was its length! Every string at the beginning of this file has 12 characters followed by 00, and each one is prefixed with the number 13. To confirm this hypothesis, I also looked at DATA.MTF which had paths of various lengths, and each was prefixed with the number of characters + 1. So, there we go.

Between “MUSIC\22.MP2” and the next string, there are 8 bytes:

75 02 00 00 93 FC 15 00

This is most likely two integers: 629 and 1440915. They might somehow indicate where this file is located within the archive. Let’s see what we have at offset 629:

FF FD 90 04 53 33 11 11 11 11 11 11 11 11 11 24

Well, I know nothing of the MP2 file format, but if this is the start of “MUSIC\22.MP2” then it probably looks similar to any other MP2 file. Let’s look at one of the extracted files in a hex editor:

FF FD 90 04 55 22 11 11 11 11 11 11 11 11 11 24

Sweet! Now let’s look at the file size of 22.MP2: 1 440 915 bytes. That’s exactly the second number following the path name, so that would give its size.

Note that I make this look very easy; in fact I spent about 3 hours to find this. Anyway.

At this point we can write the spec down for Darkstone’s MTF file format:

4 bytes – numFiles: integer; Number of files in the archive

This is followed by [numFiles] data entries. Each is structured like so:

4 bytes – pathLength: integer; Length in bytes of the next string.

[pathLength] bytes – null-terminated ASCII string: path of the entry.

4 bytes – offset: integer; absolute offset of the data in the archive.

4 bytes – size: integer; size of the data in the archive

Then follows all the data.

I then wrote a little C# application that read an entry, fetched the corresponding data, put it in a file of the same name, and proceeded to the next entry until they were all done. That almost worked. It would throw exceptions while reading DATA.MTF, because some of the specified sizes there are invalid and result in reading past the end of the file. Ugh.

So I had to resort to a more involved approach. Instead of processing one entry at a time, I start by reading all entries (path, offset, size), making a list of that, sort it by offset, and then go over that list to fetch the corresponding data. For each entry, I check if the size makes sense; if it doesn’t, I use the next entry’s offset to calculate the “real” size. Note that the real size could actually be smaller (there are unused bytes), but I suppose that’s the best I can do.

This worked well. Extracting these archives reveals the many file formats used by Darkstone (a lot more fun in store!):

  • MP2 – well-known sound format, used for all music and speech
  • WAV – well-known sound format, used for most sounds
  • DAT – could be anything; used by a few relatively small files: “SND.DAT”, “LANGUAGE.DAT”, etc.
  • AND – looks related to 3D models, I don’t know.
  • B3D – maybe this? I don’t know.
  • BRM – no clue
  • CLD – very repetitive (FC 01 FC 01 FC C0 FC 00 01 FC 01 FC C0 FC 01 FC 01 FC C0…), but I don’t know.
  • MBR – obscure binary format, I haven’t got a clue
  • MDL – idem
  • SKA – idem
  • O3D – seems used for meshes, maybe it’s Objective-3D, about which no one knows apparently.


Here’s the full listing if anyone wants to use it. This must be compiled with the /UNSAFE compiler option:

Icewind Dale 2 Ultimate Installation Guide

Many people wonder if old Infinity Engine games like Icewind Dale 2, Planescape: Torment and Baldur’s Gate run well under Windows 7/Vista. The answer is yes: these games are not that old, and in many cases they will work fine out of the box like any other game. However, should you have any issues, this guide is here to help. I will show you how to install, configure, patch and mod the game for an optimal gameplay experience. In this guide, we’ll focus on Icewind Dale 2, but a lot of this information also applies to other Infinity Engine games.

Step 1) Installation

If there’s only one thing you should remember from this section, it is do not install to C:\Program Files! Windows Vista and 7 have new security restrictions that the developers of such old games did not expect, so some things may break.

Icewind Dale 2 ships on DVD, 2 CDs, or can be downloaded at I will only illustrate the DVD installation here, but they are all very similar. For the CD version, insert CD 1 first, and then CD 2 when the setup asks for it, and finally CD 1 again when the setup asks for it at the end of the installation. You will use CD 2 to play the game.

After selecting Install, click your way to this screen. Select “Full” and click Next.

The default Destination Folder will start with C:\Program Files. Make sure to change this! This also applies if you are installing the game from Here I install the game to my D drive. If you don’t have another drive, you could install, for example, under C:\Games\Icewind Dale II. As long as you’re not installing to C:\Program Files, you’re fine.

After that, click Next twice again and the game will install.

There is no need to re-install DirectX. Windows Vista and Windows 7 automatically keep your version of DirectX up-to-date.

At the end of the Setup, check the box to run the config utility. Alternatively, the config utility can be found in the Start Menu as illustrated below, and simply in your game’s installation folder. In my case it is at D:\Program Files (x86)\Black Isle\Icewind Dale II\Config.exe.

Step 2) Configuration

However you do it, here is what you should see when the Configuration Utility starts:

The three sliders under General, Graphics and Audio will be at the “Average” setting. Any computer running Windows Vista or 7 should be fast enough to run the game at its highest settings, so set all of these to “High End” as illustrated above.

Now select the Game tab.

Check “Display Quest Experience”, and, optionally, increase the Cache Size to at least 300 MB. If you have plenty of memory, feel free to set that even higher, but it doesn’t make a huge difference.

Make sure all your options are exactly like in this screenshot (“Display Movie Subtitles” is personal preference, but I like it), and click OK.

Step 3) Patching

This step should not be necessary if you are using the GOG version, as it should already be the latest version (2.01). You can double-check this by starting the game and going to the options page.

Note that the game may take a long while to start the first few times. I don’t know what causes this. Just wait patiently, after 10-30 seconds it should start.

This screenshot shows my Icewind Dale 2 in-game Options page right after installing from DVD. The version is earlier than 2.01, therefore we must install the patch.

Fortunately, there is only one patch to install, and that is the Official Icewind Dale 2 Patch v2.01 (file name is IWD2Patch201.exe). At the time of this writing, you can find it at

Anyway, simply run the executable, click through the wizard’s screen and that is that.

Step 4) Essential Mods

Icewind Dale 2 is already a highly polished game, so feel free to skip this step entirely. That said, I suggest installing the Icewind Dale 2 Tweak Pack, currently at . Very good installation instructions are provided in the Readme. I always use the following tweaks:

  • Unlimited Ammo Stacks
  • Unlimited Jewelry and Gem Stacks
  • Unlimited Potion Stacks
  • Unlimited Scroll Stacks
  • Bottomless Bags of Holding
  • Weapon Animation Tweaks
  • Force All Dialogue to Pause
  • 100% Learn Spells
  • Stores Sell Larger Stacks of Items

These mostly remove a lot of tedious inventory management, so you can focus more on the actually fun parts of the game.

Another popular mod is the Widescreen Mod, which will be discussed in the next section.

Step 5) Fix the resolution

Icewind Dale 2 is designed to run at 800×600, which is a 4:3 resolution. Chances are you are using a widescreen monitor, which the Infinity Engine doesn’t adapt well to. Here are two possible results:

There are basically two ways to deal with these problems: either fix the aspect ratio scaling, or install the widescreen mod.

5.1) Fix the aspect ratio scaling

If your image is stretched like in the “INCORRECT” screenshot, one option is to fix that in your video card’s driver. I cannot provide help for all drivers because these change all the time, but for what it’s worth, here’s how it works with the current NVIDIA Control Panel:

  1. Right-click the desktop and select NVIDIA Control Panel
  2. Under “Display”, select “Set desktop size and position”
  3. Choose the “width/height ratio” option
  4. Choose to do the scaling on the GPU rather than the Display
  5. Check the box below to override the scaling mode defined by games and programs.

Here’s a screenshot (mine is in French, but the options should be in the same place)

AMD drivers should have similar options. Please do not contact me for help with your video card’s drivers, instead go to their respective forums:

5.2) Install the Widescreen mod

Whether or not your image is stretched, you might want to take advantage of your entire screen rather than fiddle with drivers and then still have black bars. Well, that is possible with the Widescreen Mod, available at . Installation instructions are provided there, so I will not repeat them. I suggest using the lowest available resolution that respects your monitor’s aspect ratio. Higher resolutions are of course possible, but will make everything look even smaller.

  • If your monitor is 16:9 (your highest resolution is 1280×720, 1600×900 or 1920×1080, for example), try 1280×720.
  • If your monitor is 16:10 (your highest resolution is 1440×900, 1680×1050 or 1920×1200, for example), try 1280×800.

Either way, here’s what the result looks like, for example at 1280×720:

Using the widescreen mod has several drawbacks:

  • The UI does not scale or stretch, so it ends up docked to the left with a weird black box to the right.
  • Also, everything looks quite a bit smaller than in 800×600. The higher the resolution, the worse it gets.
  • There’s also a bug in the Fell Wood (seen in the above screenshot!), where if you are running any resolution other than 800×600, the game will freeze after the battle with treants.

Therefore, while the widescreen mod allows you to play the game at a more widescreen-friendly resolution, it is by no means perfect. Whether you select this or the first option is up to you. Personally, I do not use the Widescreen Mod for IWD2.

Step 6) Fix visual artifacts and lag issues

If you haven’t done so already, start the game and see if it’s playable. Chances are everything works smoothly and you are set for a nice game of Icewind Dale 2!

However, several players report significant lag, especially when several spells animations are on-screen. Also, the fog of war may appear blocky and weird. You may try to set the game to 16-bit color mod and fiddle with the BLT Modes in the Configuration Utility (as illustrated in step 2), but even if it works, it doesn’t look as good as 32-bit color.

Here’s the real fix for all these issues. It might seem a bit complicated, but I will guide you step-by-step.

First, open up your browser and search for the Microsoft Application Compatibility Toolkit.

Click the first link. This should take you to Microsoft’s Download Center, where we want the ApplicationCompatibilityToolkitSetup.exe :

Install the toolkit by running the executable and just selecting Next all the way through, nothing special needs to be done there.

Once it’s installed, start the Compatibility Administrator (32-bit), which will be in your Start Menu under Microsoft Application Compatibility Toolkit:

Make sure a new database is selected as in the screenshot, and click Fix. Enter the name of the program and browse to its location (in this case IWD2.exe):

The next page will be “Compatibility Modes”; we don’t need anything there, so click Next again.

Under “Compatiblity Fixes”, check ForceDirectDrawEmulation. We don’t need anything else on the next 2 pages, so simply click next and then finish.

You have now created a fix. Click Save, give it some sensible name and save it anywhere you’d like. (I save it in the IWD2 install folder, but any other place is good.)

With the database saved, right-click it and select Install. You should see the following dialog:

Step 7) ???

If you still have technical issues with Icewind Dale 2 after following this guide, or if you have any comments, you can contact me and other helpful people at the Sorcerer’s Place Forums (, so register and come say hello! My nickname is Dr_Asik, you can send me a message directly, but you’ll probably get a quicker answer by creating a new thread under the appropriate section.