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copied!<p>Visual Studio uses an <a href="http://blogs.msdn.com/b/ed_maurer/archive/2008/06/11/a-tale-of-two-compilers.aspx" rel="nofollow noreferrer">in-process compiler</a>, so it knows which version of C# it's using.</p> <p>As you noted, csc.exe from the command line, on the other hand, uses whatever C# version it's made to compile, so in your case it'll be C# 5.0. Since it's an in-place upgrade (in terms of installation directory), it might break code that relied on the <code>foreach</code> binding being the same across the whole loop (odd, but possible).</p> <hr> <blockquote> <p>NOTE: Old answer for the wrong question: the OP knows this and was testing it from the command line.</p> </blockquote> <p>The blog post you link to already answers your question. I think this question is related to <a href="https://stackoverflow.com/q/12112881">this one</a>.</p> <p>It's the compiler that changed, so this:</p> <pre><code>foreach (int value in values) { // ... } </code></pre> <p>used to generate something along the following code:</p> <pre><code>{ int value; for (/* iteration */) { value = /* get from enumerator */; // ... } } </code></pre> <p>while the new C# compiler now generates the equivalent of moving the variable to inside the loop:</p> <pre><code>for (/* iteration */) { int value = /* get from enumerator */; // ... } </code></pre> <p>This makes a great difference, since closures within the <code>// ...</code> will capture a new <code>value</code> binding in each cycle, instead of sharing the same <code>value</code> binding that used to be declared outside the loop.</p> <p>The catch is, if you want your code to work correctly for both older and newer compilers, you must declare your own variable inside the <code>foreach</code> loop:</p> <pre><code>foreach (int value in values) { int newValue = value; // ... } </code></pre> <hr> <p>The current C# 4.0 specification in Visual Studio 2010 says:</p> <blockquote> <p>(...) A foreach statement of the form</p> <pre><code>foreach (V v in x) embedded-statement </code></pre> <p>is then expanded to:</p> <pre><code>{ E e = ((C)(x)).GetEnumerator(); try { V v; while (e.MoveNext()) { v = (V)(T)e.Current; embedded-statement } } finally { … // Dispose e } } </code></pre> </blockquote> <p>The C# 5.0 specification in Visual Studio 2012 says:</p> <blockquote> <p>(...) A foreach statement of the form</p> <pre><code>foreach (V v in x) embedded-statement </code></pre> <p>is then expanded to:</p> <pre><code>{ E e = ((C)(x)).GetEnumerator(); try { while (e.MoveNext()) { V v = (V)(T)e.Current; embedded-statement } } finally { … // Dispose e } } </code></pre> </blockquote>

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