2010-11-22

More MonoTouch Gotchas

In my previous post I took a break about discussing my adventure developing for the iPhone with .NET and discussed some issues I've had blogging with Blogger. In this post I get back to the main of subject and describe an odd issue I've encountered with MonoTouch.

The issue is due to the way that MonoTouch compiles to native iPhone binaries. It does not support JIT compilation, which a "normal" .NET environment does support. In most cases this is fine, but sometimes the compiler cannot fully detect the types that are needed and therefore doesn't compile them into the binary. Then, during runtime, when it detects it needs a type that was not compiled it crashes.

The code below is a pretty simple example of this issue:
/// <summary>Initializes a new instance of the <see cref="Shape"/> class.</summary>
/// <param name="points">The points.</param>
public Shape(Vector2d[] points)
     : this(points.Select(point => new Vector2((float)point.X, (float)point.Y)).ToArray())
{
}

This is a very simple alternate constructor for my Shape class. It is a convenience constructor to allow passing in Vector2d values (these are double based vectors) instead of Vector2 values (these are float based). The LINQ expression just enumerates over the array and converts the values.

So, why doesn't this work? Well, the error I get is something like:
Attempting to JIT compile method
'System.Linq.Enumerable/PredicateOf`1<OpenTK.Vector2d>:.cctor
()' while running with --aot-only.

I believe what this is saying is that the constructor for System.Linq.Enumerable/PredicateOf`1<OpenTK.Vector2d> is not found, and since JIT isn't supported it can't compile it at runtime. The reason it isn't compiled initially is because the compiler doesn't recognize that the LINQ expression will ultimately need this constructor. The way around this is it reference the needed type explicitly in code. Unfortunately, I didn't figure out a way to do that. Fortunately, I found a different workaround, although it does include a little bit of unnecessary overhead:

/// <summary>Initializes a new instance of the <see cref="Shape"/> class.</summary>
/// <param name="points">The points.</param>
public Shape(Vector2d[] points)
     : this(new List<Vector2d>(points).Select(point => new Vector2((float)point.X, (float)point.Y)).ToArray())
{
}

In this case, the LINQ expression is over the List<Vector2d>, which works fine. I believe the reason this works is because List<Vector2d> explicitly implements System.Linq.Enumerable/PredicateOf`1<OpenTK.Vector2d>, where the array does not.

I need to play around with this a bit more to see how I can fix it without the List wrapper. The problem is this only shows up when I deploy to the iPhone, so the build/run/test cycle is a bit slower. For now my workaround works ok, so improving this is not a high priority.

Next time: Phunky Physics.

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