Finding different answer in Visual studio 2010 and 2012

Question

Here is the sample code.

var values = new List<string>() { "Stack", "Over", "Go" };

var funcs = new List<Func<string>>();

foreach(var v in values) 

  funcs.Add( ()=>v );
foreach(var f in funcs) 

  Console.WriteLine(f());

When I ran this code with visual studio 2010 I got the output as: Go Go Go

But when I tried the same code in visual studio 2012 the output was: Stack Over Go

Why is it behaving differently?

Can you make sure the code you provide matches your stated output. It seems somewhat unlikely that the above code would ever generate either "stupid stupid stupid" or "Bob is stupid" — Jon Egerton, Feb 25 '13 at 11:33
**WHAT??** How even possible output of this code could `stupid stupid stupid` or `Bob is stupid` ? — Soner Gönül, Feb 25 '13 at 11:33
I suspect it would generate `Go Go Go` and `Stack Over Go`, but post the code which is generating the output you have mentioned. — LukeHennerley, Feb 25 '13 at 11:34
http://stackoverflow.com/questions/14907987/access-to-foreach-variable-in-closure/14917351#14917351 — ta.speot.is, Feb 25 '13 at 11:34
and http://blogs.msdn.com/b/ericlippert/archive/2009/11/12/closing-over-the-loop-variable-considered-harmful.aspx — ta.speot.is, Feb 25 '13 at 11:34
Ohhhh,Iam really sorry guys It gives GO GO Go in visual studio 2010. — Rajeev Ranjan, Feb 25 '13 at 11:39
Ok please take values = new List{"Bob","is","stupid"} and sorry for mistake due to copy paste......:( — Rajeev Ranjan, Feb 25 '13 at 11:42

Daniel Hilgarth · Accepted Answer · 2013-02-25T11:39:45.757

9

The reason is that the compiler was changed to create a temporary copy of variables captured in a closure.

Take the following code in VS 2012:

foreach(var v in values) 
    funcs.Add(() => v);

The code the VS 2012 compiler generates from this is equivalent to code generated by the VS 2010 compiler for this code:

foreach(var v in values) 
{
    var tmp = v;
    funcs.Add(() => tmp);
}

The reason for this change was the fact that many developers found the behaviour of the VS 2010 compiler unexpected.

edited Feb 25 '13 at 11:39

answered Feb 25 '13 at 11:33

Daniel Hilgarth

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+1. See also: Eric Lippert's blog post on this topic: http://blogs.msdn.com/b/ericlippert/archive/2009/11/12/closing-over-the-loop-variable-considered-harmful.aspx – Ergwun Feb 25 '13 at 11:36
It's so nice to stop making loop copies of variables everywhere - always felt so dumb having to do it all the time. – Jon Egerton Feb 25 '13 at 11:36
@Ergwun: Thanks - I was just searching for that link to add something official. – Daniel Hilgarth Feb 25 '13 at 11:38
Well if we're going for official then the [page that tells you the differences between C# in Visual Studio 2010 and C# in Visual Studio 2012](http://msdn.microsoft.com/en-us/library/vstudio/hh678682.aspx) would be a good resource to look at/Google for. – ta.speot.is Feb 25 '13 at 11:39
@ta.speot.is: As Eric was a developer on the C# team, that's official enough for me. Furthermore, it is much more understandable than the overview. – Daniel Hilgarth Feb 25 '13 at 11:40
The point of my comment was to imply that if you Googled for *c# in visual studio 2012 differences* and could be bothered looking so far as the second link, the first item on the page explains exactly this behaviour. i.e. extreme laziness on someone's behalf... – ta.speot.is Feb 25 '13 at 11:43
1

@ta.speot.is: I am not really sure how to take your comment. If you are implying that I am lazy - that's laughable. If you are implying that the OP is lazy - that's debatable, but also not really fair. As a beginner, he most likely will be overwhelmed with the search results and the overview page. – Daniel Hilgarth Feb 25 '13 at 11:56
OP's question could be answered by the second link of a Google search. Yes, he is lazy. – ta.speot.is Feb 25 '13 at 11:57

Finding different answer in Visual studio 2010 and 2012

1 Answers1