A place for spare thoughts


Implicit closures in C# lambdas, part 2

Filed under: c# — Ivan Danilov @ 17:59

Wow. I’ve just found that not all I said in first part is true (I’ve already marked wrong place so readers would not be confused). Shame on me.

My mistake was to rely on ReSharper’s warnings totally. So, in the ending of last post I had changed code to make copy of variable for closure like that:

        public static void DoSomething(object a, object b, Observer observer, Receiver r)
            var receiverCopy = r;
            EventHandler action1 = (s, e) => receiverCopy.Method(a);
            EventHandler action2 = (s, e) => r.Method(b);
            observer.X += action1;
            observer.X += action2;

And assumed that it is enough to convince compiler to capture variables for two lambdas in two different compiler-generated classes – based on the fact that ReSharper warning is gone. It turned out (by experiencing very real memory leak in unit-test first and then looking at generated IL to understand) absence of warning means that ReSharper is just unable to find such implicit closure, not that implicit closure is gone. Compiler is still generates single class for all these closures. And at this time it is probably an optimization from compiler’s side – namely, try to decrease number of types in the assembly by generating one less.

What I ended up with is hand-writing explicit class very similar to that in previous post. This solved problem with memory leak, and looking at IL proved that it helped. But code is now ugly. And worse – I don’t know how to spot such problems easily without thorough analysis of the code.

UPD: Pretty much the same thing in another blog post.

UPD2: As Eric answered at SO, this behavior was implemented because it was easier (for compiler developers) and part about lambda rewriting was already pretty complicated at the time. And, hopefully with Roslyn we may have better.

Also, good alternative was offered as well.



  1. […] 17/08/2012 at 17:59 | #1 Implicit closures in C# lambdas, part 2 « A place for spare thoughts […]

    Pingback by Implicit closures in C# lambdas « A place for spare thoughts — 17/08/2012 @ 17:59

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