Detect the C# version at compile time

Question

Is it possible to detect if the current version of C# is 6 or above with a preprocessor directive, so at compile time?

I want to do something like this:

var myVar = ...;
string name;

#if VERSION_6_OR_MORE
    name = nameof(myVar);
#else
    name = "myVar";
#endif

I use Visual Studio 2015 and C# 6, so I can use nameof(). Someone else wanting to compile this code however may be using an older version, where nameof() isn't present.

I want to use a preprocessor directive so I can keep the nameof() in C# 6, but someone else who doesn't use that version can compile it as well.

Answer 1

You can explicitly specify language version and define conditional constant in your project file:

<LangVersion>6</LangVersion>
<DefineConstants>DEBUG;TRACE</DefineConstants>
<DefineConstants Condition="'$(LangVersion)' == '6'">DEBUG;TRACE;LANG_VERSION_6</DefineConstants>

Answer 2

The nameof() operator is meant to reduce maintenance: using it you don't have identifiers hidden in strings, so when you rename a variable, parameter or member, the place where you use it (for example an ArgumentNullException(nameof(paramName))) will be updated when you refactor the paramName name.

Now using your approach, you're effectively doubling your maintenance surface instead of reducing it (as you now have both the string variant and the nameof() version, both of which having to be maintained), negating the use of nameof() altogether.

So if you want to support older C# versions, stick to features that work in those versions: use identifiers in strings instead.

Answer 3

I am not aware of a pre-existing directive. But, here's what you can do:

1. Define two build configurations: Say CSharp6 and CSharp5 (in the configuration manager).
2. Select C6 from the configuration manger, and then go to the project properties. And define a symbol say CSHARP6.
3. Change configurations and go to CSharp5
4. Open project properties again and define a new symbol: CSHARP5

Then your directive conditional would look like:

#if CSHARP6
    name = nameof(myVar);
#else
    name = "myVar";
#endif

Delegate the selection of the configuration profile to the building agent, or as part of an instruction manual for compilation of the project.

Answer 4

The C# compiler doesn't let you define symbols with a numerical value. Any symbol is either defined or undefined, which means it's just a boolean.

Neither #define//define nor #if support any syntax other than boolean.

All C# versions so far are backwards-compatible with previous versions (except for some nuances like iterator variable semantics in foreach loops), so if you want to write code compatible with older versions, just don't use the newer features.

You could, define a symbol externally after detecting the available compiler version in your build script, but it's not worth the hassle IMO, as the lower-version C# code will be more legible and maintainable than duplicate blocks scattered around your code.

Answer 5

Not sure about constants like that, but...

Something like typeof(int).Assembly.ImageRuntimeVersion could help out at runtime.

Additionally: You can create constants:

<DefineConstants Condition=" '$(LanguageVersion)' == 'v6.0' ">USING_CS6</DefineConstants>
<DefineConstants Condition=" '$(LanguageVersion)' != 'v6.0' ">NOT_USING_CS6</DefineConstants>

• @see Detect target framework version at compile time
• @see https://msdn.microsoft.com/en-us/library/vslangproj80.csharpprojectconfigurationproperties3.languageversion.aspx

I have not tested this though.