Why is there a difference in behaviour of an application built in VS 2010 v.s. VS 2012?

Question

I was checking whether installing .NET 4.5 on our build machines changes the output IL image generated by VS 2010.

Since I know the behaviour of foreach has changed in .

Answer 1

Note: I removed much of my original response. It was answering the wrong question. A better response follows.

Ah, now I see what you're asking: "How does Visual Studio 2010 know to compile to C# 4 instead of C# 5 after .NET 4.5 is installed, even Visual Studio 2010 and Visual Studio 2012 use the same csc.exe and pass the same options to it?"

@mletterle But .NET 4.5 is an in place upgrade to .NET 4. So really there is only .NET 4 present on my machine. The only possibility is that the IDE has stashed away a hidden copy of .NET 4 compiler that I cannot see.

I'm not sure where you heard that or why you assumed that. .NET 4.5 is NOT an in-place upgrade. It is a different version of the tool. There will be differences. This is one of them.

Update 1:

Looks like we were using a different definition of "in-place" upgrade. My usage of "in-place" is "an upgrade that should have no discernible differences between versions." The definition given in the article you linked to uses it in a different way: "in place" in their usage is "uses the same CLR, but adds new libraries."

Since C# 5 is different than C# 4, that change is NOT "in place" in the usage I'm familiar with.

As a result, the difference is not the CLR you're targeting, but the language version you're using - the CLR is an "in place" upgrade (both the 4.0 CLR), but the language is not (C# 4 in VS2010, C#5 in VS2012.)

Update 2:

Within the .csproj file (which is actually an msbuild file managed by Visual Studio), there is an attribute that specifies the target framework. Projects made with Visual Studio 2012 have this by default:

v4.5

Whereas projects in Visual Studio 2010 that target Version 4 look like:

v4.0

This tells Visual Studio to set up the environment when building for one or the other target framework. While it looks like csc.exe is being invoked directly from a command prompt, it really isn't: the msbuild project is actually what's being processed, and it's happening in "Visual Studio"'s custom process environment.

I can only assume the specifics of what's happening, but likely after the upgrade, having the "TargetFrameworkVersion" attribute set to v4.0 returns the environment to v4.0 during the compilation of a project targeting v4.0. On the other hand, by invoking csc.exe from the command line without the environment set up by msbuild, it uses the "defaults" for its version (which is now defaulting to C# 5) giving you the new C# 5 behavior even though you're using the VS 2010 command prompt. When you invoke the build via MSBuild, though, it knows how to return to the original C# 4 environment for the duration of the build (since MSBuild is part of the .NET tool chain, too.)