Tuesday, September 30, 2014

Should you avoid foreach loops?

A common suggestion which I've come across many times in the Unity forums and elsewhere is to avoidforeach loops and use for or while loops instead. The reasoning behind this seems sound at first sight. Foreach is really just syntactic sugar, because the compiler will preprocess code such as this:
foreach (SomeType s in someList)
...into something like the the following:
using (SomeType.Enumerator enumerator = this.someList.GetEnumerator())
    while (enumerator.MoveNext())
        SomeType s = (SomeType)enumerator.Current;
In other words, each use of foreach creates an enumerator object - an instance of theSystem.Collections.IEnumerator interface - behind the scenes. But does it create this object on the stack or on the heap? That turns out to be an excellent question, because both are actually possible! Most importantly, almost all of the collection types in the System.Collections.Generic namespace (List<T>, Dictionary<K, V>, LinkedList<T>, etc.) are smart enough to return a struct from from their implementation of GetEnumerator()). This includes the version of the collections that ships with Mono 2.6.5 (as used by Unity).
[EDIT] Matthew Hanlon pointed my attention to an unfortunate (yet also very interesting) discrepancy between Microsoft's current C# compiler and the older Mono/C# compiler that Unity uses 'under the hood' to compile your scripts on-the-fly. You probably know that you can use Microsoft Visual Studio to develop and even compile Unity/Mono compatible code. You just drop the respective assembly into the 'Assets' folder. All code is then executed in a Unity/Mono runtime environment. However, results can still differ depending on who compiled the code! foreach loops are just such a case, as I've only now figured out. While both compilers recognize whether a collection's GetEnumerator() returns a struct or a class, the Mono/C# has a bug which 'boxes' (see below, on boxing) a struct-enumerator to create a reference type.
So should you avoid foreach loops?
  • Don't use them in C# code that you allow Unity to compile for you.
  • Do use them to iterate over the standard generic collections (List<T> etc.) in C# code that you compile yourself with a recent compiler. Visual Studio as well as the free .NET Framework SDK are fine, and I assume (but haven't verified) that the one that comes with the latest versions of Mono and MonoDevelop is fine as well.
What about foreach-loops to iterate over other kinds of collections when you use an external compiler? Unfortunately, there's is no general answer. Use the techniques discussed in the second blog post to find out for yourself which collections are safe for foreach[/EDIT]

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