Conditional Compilation Depending on the Framework Version in C#

Is it possible to conditionally compile to .NET Framework version?

There are no builtin precompiler constants that you can use. But, it is easy enough create your own build configurations in VS with each configuration having its own set of defined constants and of course a target framework version. A lot of people do this to conditionally compile based 32 or 64 bit differences.

Conditional compilation depending on the framework version in C#

I don't think there are any predefined 'preprocessor' symbols. However you can achieve what you want like this:

  1. Create different configurations of your project, one for every version of CLR you want to support.

  2. Choose a symbol like VERSION2, VERSION3 etc. per CLR version.

  3. In every configuration, define the one symbol associated with it and undefine all others.

  4. Use these symbols in conditional compilation blocks.

Is conditional compilation bad practice when targeting .NET Standard and .NET Framework?

Well, you probably shouldn't multi-target in the first place, unless you're writing a library that will absolutely (not might, but absolutely!) be used to support legacy projects. Today, there is no reason not to write .Net5 code, unless you're writing UWP/Xamarin apps.

However, if you are multi-targetting, you need to write code that compiles under all your targets. That involves both #if and MsBuild targetted items, to conditionally include entire source code files.

Conditional compilation and framework targets

One of the best ways to accomplish this is to create different build configurations in your project:

<PropertyGroup Condition="  '$(Framework)' == 'NET20' ">
<DefineConstants>NET20</DefineConstants>
<OutputPath>bin\$(Configuration)\$(Framework)</OutputPath>
</PropertyGroup>

<PropertyGroup Condition=" '$(Framework)' == 'NET35' ">
<DefineConstants>NET35</DefineConstants>
<OutputPath>bin\$(Configuration)\$(Framework)</OutputPath>
</PropertyGroup>

And in one of your default configurations:

<Framework Condition=" '$(Framework)' == '' ">NET35</Framework>

Which would set the default if it wasn't defined anywhere else. In the above case the OutputPath will give you a separate assembly each time you build each version.

Then create a AfterBuild target to compile your different versions:

<Target Name="AfterBuild">
<MSBuild Condition=" '$(Framework)' != 'NET20'"
Projects="$(MSBuildProjectFile)"
Properties="Framework=NET20"
RunEachTargetSeparately="true" />
</Target>

This example will recompile the entire project with the Framework variable set to NET20 after the first build (compiling both and assuming that the first build was the default NET35 from above). Each compile will have the conditional define values set correctly.

In this manner you can even exclude certain files in the project file if you want w/o having to #ifdef the files:

<Compile Include="SomeNet20SpecificClass.cs" Condition=" '$(Framework)' == 'NET20' " />

or even references

<Reference Include="Some.Assembly" Condition="" '$(Framework)' == 'NET20' " >
<HintPath>..\Lib\$(Framework)\Some.Assembly.dll</HintPath>
</Reference>

Detect target framework version at compile time

The linked SO question with 'create N different configurations' is certainly one option, but when I had a need for this I just added conditional DefineConstants elements, so in my Debug|x86 (for instance) after the existing DefineConstants for DEBUG;TRACE, I added these 2, checking the value in TFV that was set in the first PropertyGroup of the csproj file.

<DefineConstants Condition=" '$(TargetFrameworkVersion)' == 'v4.0' ">RUNNING_ON_4</DefineConstants>
<DefineConstants Condition=" '$(TargetFrameworkVersion)' != 'v4.0' ">NOT_RUNNING_ON_4</DefineConstants>

You don't need both, obviously, but it's just there to give examples of both eq and ne behavior - #else and #elif work fine too :)

class Program
{
static void Main(string[] args)
{
#if RUNNING_ON_4
Console.WriteLine("RUNNING_ON_4 was set");
#endif
#if NOT_RUNNING_ON_4
Console.WriteLine("NOT_RUNNING_ON_4 was set");
#endif
}
}

I could then switch between targeting 3.5 and 4.0 and it would do the right thing.

Conditional compilation for .NET 4

The compiler isn't aware of any particular .NET Framework version. All it sees is the reference assemblies. Nor is there any guarantee that your program will run with the .NET version that you target. It is quite possible to run with the .NET 4.0 CLR even if you built for 2.0.

Use Environment.Version instead.

Conditional compilation depending on compiler version

No, from what I know they didn't change anything :-)

You could perhaps do a little magic inside the csproj to define the constants... but it is complex...

There is no property that specifies directly the version of the CSC... There is a property ($(LangVersion)) that specifies the version of the language standard required... but it is normally set to default so "the maximum the compiler supports"...

Or you could look at the path of the CSC compiler... It is stored in the CscToolPath or, if that is empty, in the MsBuildToolsPath. From there perhaps you can discern the version of the CSC.

Implement interface depending on framework version

You could use a compiler directive, e.g.

#if NET45
//specific code for .NET 4.5
#endif

Then create two projects, one for each version, using the Project Linker to maintain a single codebase. In the .NET 4.5 project you can specify the NET45 variable on build. This way the code in the #if block will only be used in one of the versions.

Conditional code execution in C# based on Targetframework of hosting project

IMO the correct way to do this is at the csproj level via multi-targeting - <TargetFrameworks> etc, and either #if (in the C#) or conditional file includes (again in the csproj). Ultimately, what you're describing is exactly what multi-targeting is designed to do, and any solution other than "just use multi-targeting" is missing out on a huge feature explicitly aimed at solving these problems. The build mechanism has a "bait and switch" layer whose job it is to ask "what is the target framework of the host application? ok, I'll give them {this version} of the dependency" - which is why the job of transitive dependency resolution is deferred to the top-level application build, rather than library build (libraries don't reliably know the host TFM, for the reasons I'm about to get to).


However, putting that aside:

Your biggest problem here is going to be that: .NET Standard doesn't exist at runtime, only at compile-time; I mean multiple things by this, including the reality that .NET Framework can host .NET Standard, so when you ask:

what is the target framework of the hosting process.

it will never be .NET Standard. If that is just a typo and you mean .NET Core, then you can probably be reasonably safe borrowing these 2 lines from PlatformDetection.cs:

using System.Runtime.InteropServices;
// ...
public static bool IsFullFramework => RuntimeInformation.FrameworkDescription.StartsWith(
".NET Framework", StringComparison.OrdinalIgnoreCase);
public static bool IsNetCore => RuntimeInformation.FrameworkDescription.StartsWith(
".NET Core", StringComparison.OrdinalIgnoreCase);

is it clean? no. Will it work: yes.



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