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 strings) Prior to .NET 4.0
// 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
MSBuild Property Evaluation
Can I make a preprocessor directive dependent on the .NET framework version?
Conditional compilation depending on the framework version in C#
