Difference in LINQ query results in .NET 3.5 and 4.5

Question

I executed the following code with C# 3.5 and 4.0. The results are entirely different.

    static void Main()
    {       
        int[] numbers = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
        List<IEnumerable<int>> results = new List<IEnumerable<int>>();

        foreach (var num in numbers)
        {
            results.Add(numbers.Where(item => item > num));
        }

        foreach (var r in results)
        {
            Console.WriteLine("{0}", r.Count());
        }        
    }

With Microsoft (R) Visual C# 2008 Compiler version 3.5.30729.5420 the output is 0 0 0 0 0 0 0 0 0 0.

But with Microsoft (R) Visual C# Compiler version 4.0.30319.17929 the output is 9 8 7 6 5 4 3 2 1 0.

I have a faint idea that this is because of deferred execution or lazy evaluation but haven't clearly understood how it is responsible for different outputs here.

Correction: Sorry it was .NET 3.5 and 4.5 and also added compiler versions Please explain.

I get the same output in 3.5 and 4.0, { 9 8 7 ... } are you sure you're not doing something different? — James Gaunt, Sep 05 '13 at 15:11
@JoeEnos It has nothing to do with the version of .NET, it is related to the version of C# being used. — Servy, Sep 05 '13 at 15:13
@Servy Gotcha, thanks. Hard to keep those distinctions clear sometimes. — Joe Enos, Sep 05 '13 at 15:14
@JoeEnos I made a mistake with .NET version but in the question I mentioned the version of compiler right — mssrivatsa, Sep 05 '13 at 15:15
@mssrivatsa No you didn't. It was changed in C# 5.0, not 4.0. There also is no such thing as C# 3.5. — Servy, Sep 05 '13 at 15:15
@Servy No I had the different output with C# 4.0 compiler Microsoft (R) Visual C# Compiler version 4.0.30319.17929 — mssrivatsa, Sep 05 '13 at 15:16
@mssrivatsa Then you are mistaken. It was changed in C# 5.0. The 4.0 compiler wouldn't produce that result. — Servy, Sep 05 '13 at 15:20
@Servy: Though there is no *language* C# 3.5, there is a *compiler release* that is version 3.5. The compiler version numbers match the version of the framework they shipped with, not the language they analyze. Yes, this is unnecessarily confusing, and I heartily wish that my former colleagues would get their versioning story straight. This is a perennial problem across Microsoft. — Eric Lippert, Sep 05 '13 at 15:24
@Servy Yes it did. Here is the screenshot http://jmp.sh/b/VQQmAsiUIoEy9jByahPY — mssrivatsa, Sep 05 '13 at 15:26
@mssrivatsa Well, perhaps Eric can clarify the versioning system further. You're compiling against the C# 5.0 language specs. Apparently the version of the C# compiler that you are using is capable of compiling against the C# 5.0 language. This is a feature that has always had a lot of attention here, because people post this exacted problem several times a day back when most new C# programmers were using C# 3-4. — Servy, Sep 05 '13 at 15:31
@Servy I agree with you but i was talking in terms of compiler and you were in terms of language versions — mssrivatsa, Sep 05 '13 at 15:36

score 7 · Answer 1 · edited Sep 11 '13 at 16:20

7

Since c# 5, the loop variable in foreach is compiled such that it exists within the loop scope rather than outside it.

This means that when you close over the loop variable, you get different results.

Here's what Eric Lippert had to say about the original problem.

edited Sep 11 '13 at 16:20

Eric Lippert

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answered Sep 05 '13 at 15:10

spender

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6

It's since C# 5.0, not .NET 4.0. – Servy Sep 05 '13 at 15:11
1

This doesn't answers the question. OP asks in c# 4.0. but your 're taking about c# 5.0 – Sriram Sakthivel Sep 05 '13 at 15:12
1

OP asks about .net4. This is the only possibility for the observed results. – spender Sep 05 '13 at 15:13
For compatibility use var t = num; in foreach loop. – mswietlicki Sep 05 '13 at 15:13
Much better. earlier question had c#4.0. now edited – Sriram Sakthivel Sep 05 '13 at 15:14
@Habib I know that very well so that am confused how OP got that result and with this answer too. Now am clear. Confusion is question was wrong. – Sriram Sakthivel Sep 05 '13 at 15:16
@SriramSakthivel Installing 4.5 changes the behavior of .NET 4.0 due to it updating the BCL without changing the version numbers. – Scott Chamberlain Sep 05 '13 at 15:16
@ScottChamberlain Yes, IIRC that will change the behavior of .net 3.5 too. – Sriram Sakthivel Sep 05 '13 at 15:18
1

@SriramSakthivel just tested it, you are correct. – Scott Chamberlain Sep 05 '13 at 15:19

score 6 · Answer 2 · edited Sep 11 '13 at 16:20

6

You have accessed a variable inside a closure, therefore the results will differ on different versions of the compiler.

In C# 5.0, the variable is redefined in each iteration of the loop, whereas in previous C# versions it was only defined once.

For more info, see Eric Lippert's great blog post

More notably, the opening paragraph:

UPDATE: We are taking the breaking change. In C# 5, the loop variable of a foreach will be logically inside the loop, and therefore closures will close over a fresh copy of the variable each time. The "for" loop will not be changed. We return you now to our original article.

edited Sep 11 '13 at 16:20

Eric Lippert

647,829
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1,238
2,067

answered Sep 05 '13 at 15:11

Mathew Thompson

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1

The scope of the variable has *nothing* to do with the version of .NET. It has everything to do with the version of C#. It was changed in C# 5.0. – Servy Sep 05 '13 at 15:12
Have you read eric's blog `We are taking the breaking change. In C# 5` – Sriram Sakthivel Sep 05 '13 at 15:13
@Servy Yeah I clarified that, thanks. – Mathew Thompson Sep 05 '13 at 15:13
Yes, in VS2012 compiling with .NET 3.5, it's work perfectly. – Alessandro D'Andria Sep 05 '13 at 15:13
2

@AlessandroD'Andria That's because you're using the C# 5.0 compiler. The version of .NET used is irrelevant. – Servy Sep 05 '13 at 15:18
@Servy Exactly what I wanted to point out. – Alessandro D'Andria Sep 05 '13 at 15:20

Difference in LINQ query results in .NET 3.5 and 4.5

2 Answers2