Can Visual Studio target earlier C# syntax in addition to earlier .NET framework versions?

Question

The easy part:

Targeting the .NET 2.0 framework in a Visual Studio 2010 project using the dropdown.

The hard part:

Is it possible to target a specific syntax version - for example var s = "hello world" is valid syntactic sugar in VS2008 and above, but would not compile in VS2005. Can VS2010 be configured to flag this at compile time?

Bowdlerize

Why would you want to do this? Can't you use VS2005 directly? – Metts

Is there a reason to downgrade the language version? I thought C# is 100% backward compatible. – Leonerd

@Axel: Not 100%. There are some subtle changes which can sometimes affect behaviour. Besides, there's another reason to do this: if some of your developers are still using VS2005, it's useful if those using VS2008 don't write code which compiles fine for them, but not their colleagues... – Heartstrings

@Jon This is exactly the reason. I'm running top of the line VS2010 Premium for most projects, but occasionally have to provide changes to projects normally maintained by parties on VS2005 or VS2008. It's embarrassing to introduce a breaking change that won't even allow files to compile. – Bowdlerize

Answer 1

This can be done by specifying the language version in the project settings. To set the language version to C# 2.0 do the following

• Right Click on the project and select "Properties"
• Go to the "Build" Tab
• Click the "Advanced" Button
• Change the "Language Version" drop down to "ISO-2"

Here are the other selections and their meanings in Visual Studio 2010.

• ISO-1: C# 1.0 / Visual Studio RTM and 2003
• ISO-2: C# 2.0 / Visual Studio 2005
• C# 3.0: C# 3.0 / Visual Studio 2008
• default: C# 4.0 / Visual Studio 2010

Ide

Answer 2

Yes. From the IDE, set:

Project Properties -> Build -> Advanced -> Language Version : ISO-2

Fairlead

Answer 3

Yes, you can do this, as others have said - but it's not a perfect simulation of the C# 1 compiler, as I discovered while I was giving a presentation. It will spot "big" changes in syntax, but not some subtle changes in behaviour. For example, in C# 1 there's no method group conversion, so you couldn't do this:

delegate void Foo(string x);
void Bar(string y) {}
...
Foo foo = Bar;

... but simply setting the C# 4 compiler to target C# 1 doesn't pick this up.

Heartstrings

The reason why, IIRC, is because the language version is a parser only artifact. It is designed for syntax changes only and doesn't dig into semantic differences between the language versions. Since there is no C# 4.0 syntax involved here (at least as my half awake self can see) it won't produce any errors by design. But yes, definitely not perfect – Ide

@JaredPar: Yup. I'm in no way suggesting this should be changed - just something to be aware of. Will edit to make that clearer :) – Heartstrings

@JaredPar: I don't believe it's just the parser. My first attempt at showing the flaw used delegate variance (not generic variance, just the delegate variance introduced in C# 2). It spotted that: "Test.cs(11,19): error CS0410: No overload for 'Bar' has the correct parameter and return types" – Heartstrings

@Jon, Hmm, I know the VB version of this switch is parser only and was designed after the C# feature. Perhaps they added a bit more than just at the parser level. Now I'm curious ... – Ide

@Jared: Odd... I've just looked back at an email sent to Eric (from exactly this situation) and he also says it's a switch just for the parser. It definitely seems to have an effect in this case though. Drop me a mail if you want my test code. – Heartstrings

@Jared: One bit of further investigation: I wonder whether this is a special case where the compiler is already detecting a possibly backward-incompatible change (where a delegate initializer expression means one thing in C# 1 and something else in C# 2) in order to give a warning. Given that that does need the language version, maybe that whole area of code uses the version, even if most of the rest of the compiler doesn't. – Heartstrings

@Jon, checked and the compiler does make use of the switch in places other than the parser. When you think about it a bit it actually has to for certain contextual keywords. Take var for instance. It doesn't know if this is actually a C# 3.0 construct until it tries to bind the type var because if it does bind then it's legal C# 2.0 code. – Ide

@JaredPar: Yes, that's a very good example. Without knowing much about compiler theory, I guess it could parse that as a different token type depending on the language version, and then act accordingly later on. I do envy you being able to just check like that though :) – Heartstrings