Detect target framework version at compile time
C#.NetVisual StudioMsbuildExtension MethodsC# Problem Overview
I have some code which makes use of Extension Methods, but compiles under .NET 2.0 using the compiler in VS2008. To facilitate this, I had to declare ExtensionAttribute:
/// <summary>
/// ExtensionAttribute is required to define extension methods under .NET 2.0
/// </summary>
public sealed class ExtensionAttribute : Attribute
{
}
However, I'd now like the library in which that class is contained to also be compilable under .NET 3.0, 3.5 and 4.0 - without the 'ExtensionAttribute is defined in multiple places' warning.
Is there any compile time directive I can use to only include the ExtensionAttribute when the framework version being targetted is .NET 2?
C# Solutions
Solution 1 - C#
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.
Solution 2 - C#
I have a few suggestions for improving on the answers given so far:
-
Use Version.CompareTo(). Testing for equality will not work for later framework versions, yet to be named. E.g.
<CustomConstants Condition=" '$(TargetFrameworkVersion)' == 'v4.0' ">
will not match v4.5 or v4.5.1, which typically you do want.
-
Use an import file so that these additional properties only need to be defined once. I recommend keeping the imports file under source control, so that changes are propagated along with the project files, without extra effort.
-
Add the import element at the end of your project file, so that it is independent of any configuration specific property groups. This also has the benefit of requiring a single additional line in your project file.
###Here is the import file (VersionSpecificSymbols.Common.prop)###
<!--
******************************************************************
Defines the Compile time symbols Microsoft forgot
Modelled from https://msdn.microsoft.com/en-us/library/ms171464.aspx
*********************************************************************
-->
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<PropertyGroup>
<DefineConstants Condition="$([System.Version]::Parse('$(TargetFrameworkVersion.Substring(1))').CompareTo($([System.Version]::Parse('4.5.1')))) >= 0">$(DefineConstants);NETFX_451</DefineConstants>
<DefineConstants Condition="$([System.Version]::Parse('$(TargetFrameworkVersion.Substring(1))').CompareTo($([System.Version]::Parse('4.5')))) >= 0">$(DefineConstants);NETFX_45</DefineConstants>
<DefineConstants Condition="$([System.Version]::Parse('$(TargetFrameworkVersion.Substring(1))').CompareTo($([System.Version]::Parse('4.0')))) >= 0">$(DefineConstants);NETFX_40</DefineConstants>
<DefineConstants Condition="$([System.Version]::Parse('$(TargetFrameworkVersion.Substring(1))').CompareTo($([System.Version]::Parse('3.5')))) >= 0">$(DefineConstants);NETFX_35</DefineConstants>
<DefineConstants Condition="$([System.Version]::Parse('$(TargetFrameworkVersion.Substring(1))').CompareTo($([System.Version]::Parse('3.0')))) >= 0">$(DefineConstants);NETFX_30</DefineConstants>
</PropertyGroup>
</Project>
###Add Import Element to Project File### Reference it from your .csproj file by adding at the end, before the tag.
…
<Import Project="VersionSpecificSymbols.Common.prop" />
</Project>
You will need to fix up the path to point to the common/shared folder where you put this file.
###To Use Compile Time Symbols### namespace VersionSpecificCodeHowTo { using System;
internal class Program
{
private static void Main(string[] args)
{
#if NETFX_451
Console.WriteLine("NET_451 was set");
#endif
#if NETFX_45
Console.WriteLine("NET_45 was set");
#endif
#if NETFX_40
Console.WriteLine("NET_40 was set");
#endif
#if NETFX_35
Console.WriteLine("NETFX_35 was set");
#endif
#if NETFX_30
Console.WriteLine("NETFX_30 was set");
#endif
#if NETFX_20
Console.WriteLine("NETFX_20 was set");
#else
The Version specific symbols were not set correctly!
#endif
#if DEBUG
Console.WriteLine("DEBUG was set");
#endif
#if MySymbol
Console.WriteLine("MySymbol was set");
#endif
Console.ReadKey();
}
}
}
###A Common “Real Life” Example###
Implementing Join(string delimiter, IEnumerable
// string Join(this IEnumerable<string> strings, string delimiter)
// was not introduced until 4.0. So provide our own.
#if ! NETFX_40 && NETFX_35
public static string Join( string delimiter, IEnumerable<string> strings)
{
return string.Join(delimiter, strings.ToArray());
}
#endif
###References### Property Functions
Can I make a preprocessor directive dependent on the .NET framework version?
Conditional compilation depending on the framework version in C#
Solution 3 - C#
Property groups are overwrite only so this would knock out your settings for DEBUG
, TRACE
, or any others. - See MSBuild Property Evaluation
Also if the DefineConstants
property is set from the command line anything you do to it inside the project file is irrelevant as that setting becomes global readonly. This means your changes to that value fail silently.
Example maintaining existing defined constants:
<CustomConstants Condition=" '$(TargetFrameworkVersion)' == 'v2.0' ">V2</CustomConstants>
<CustomConstants Condition=" '$(TargetFrameworkVersion)' == 'v4.0' ">V4</CustomConstants>
<DefineConstants Condition=" '$(DefineConstants)' != '' And '$(CustomConstants)' != '' ">$(DefineConstants);</DefineConstants>
<DefineConstants>$(DefineConstants)$(CustomConstants)</DefineConstants>
This section MUST come after any other defined constants since those are unlikely to be set up in an additive manner
I only defined those 2 because that's mostly what I'm interested in on my project, ymmv.
See Also: Common MsBuild Project Properties
Solution 4 - C#
Pre-defined symbols for target frameworks are now built into the version of MSBuild that is used by the dotnet
tool and by VS 2017 onwards. See https://docs.microsoft.com/en-us/dotnet/standard/frameworks#how-to-specify-a-target-framework for the full list.
#if NET47
Console.WriteLine("Running on .Net 4.7");
#elif NETCOREAPP2_0
Console.WriteLine("Running on .Net Core 2.0");
#endif
Solution 5 - C#
I would like to contribute with an updated answer which solves some issues.
If you set DefineConstants instead of CustomConstants you'll end up, in the Conditional Compilation Symbols Debug command line, after some framework version switch, with duplicated conditional constants (i.e.: NETFX_451;NETFX_45;NETFX_40;NETFX_35;NETFX_30;NETFX_20;NETFX_35;NETFX_30;NETFX_20;). This is the VersionSpecificSymbols.Common.prop which solves any issue.
<!--
*********************************************************************
Defines the Compile time symbols Microsoft forgot
Modelled from https://msdn.microsoft.com/en-us/library/ms171464.aspx
*********************************************************************
Author: Lorenzo Ruggeri ([email protected])
-->
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<Choose>
<When Condition=" $(TargetFrameworkVersion) == 'v2.0' ">
<PropertyGroup>
<CustomConstants >$(CustomConstants);NETFX_20</CustomConstants>
</PropertyGroup>
</When>
<When Condition=" $(TargetFrameworkVersion) == 'v3.0' ">
<PropertyGroup>
<CustomConstants >$(CustomConstants);NETFX_30</CustomConstants>
<CustomConstants >$(CustomConstants);NETFX_20</CustomConstants>
</PropertyGroup>
</When>
<When Condition=" $(TargetFrameworkVersion) == 'v3.5' ">
<PropertyGroup>
<CustomConstants >$(CustomConstants);NETFX_35</CustomConstants>
<CustomConstants >$(CustomConstants);NETFX_30</CustomConstants>
<CustomConstants >$(CustomConstants);NETFX_20</CustomConstants>
</PropertyGroup>
</When>
<Otherwise>
<PropertyGroup>
<CustomConstants Condition="$([System.Version]::Parse('$(TargetFrameworkVersion.Substring(1))').CompareTo($([System.Version]::Parse('4.5.1')))) >= 0">$(CustomConstants);NETFX_451</CustomConstants>
<CustomConstants Condition="$([System.Version]::Parse('$(TargetFrameworkVersion.Substring(1))').CompareTo($([System.Version]::Parse('4.5')))) >= 0">$(CustomConstants);NETFX_45</CustomConstants>
<CustomConstants Condition="$([System.Version]::Parse('$(TargetFrameworkVersion.Substring(1))').CompareTo($([System.Version]::Parse('4.0')))) >= 0">$(CustomConstants);NETFX_40</CustomConstants>
<CustomConstants Condition="$([System.Version]::Parse('$(TargetFrameworkVersion.Substring(1))').CompareTo($([System.Version]::Parse('3.5')))) >= 0">$(CustomConstants);NETFX_35</CustomConstants>
<CustomConstants Condition="$([System.Version]::Parse('$(TargetFrameworkVersion.Substring(1))').CompareTo($([System.Version]::Parse('3.0')))) >= 0">$(CustomConstants);NETFX_30</CustomConstants>
<CustomConstants Condition="$([System.Version]::Parse('$(TargetFrameworkVersion.Substring(1))').CompareTo($([System.Version]::Parse('2.0')))) >= 0">$(CustomConstants);NETFX_20</CustomConstants>
</PropertyGroup>
</Otherwise>
</Choose>
<PropertyGroup>
<DefineConstants>$(DefineConstants);$(CustomConstants)</DefineConstants>
</PropertyGroup>
</Project>
Solution 6 - C#
Use reflection to determine if the class exists. If it does, then dynamically create and use it, otherwise use the .Net2 workaround class which can be defined, but not used for all other .net versions.
Here is code I used for an AggregateException
which is .Net 4 and greater only:
var aggregatException = Type.GetType("System.AggregateException");
if (aggregatException != null) // .Net 4 or greater
{
throw ((Exception)Activator.CreateInstance(aggregatException, ps.Streams.Error.Select(err => err.Exception)));
}
// Else all other non .Net 4 or less versions
throw ps.Streams.Error.FirstOrDefault()?.Exception
?? new Exception("Powershell Exception Encountered."); // Sanity check operation, should not hit.