Detect target framework version at compile time

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?

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.

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($([System.Version]::Parse('4.5.1')))) &gt;= 0">$(DefineConstants);NETFX_451</DefineConstants>
        <DefineConstants Condition="$([System.Version]::Parse('$(TargetFrameworkVersion.Substring(1))').CompareTo($([System.Version]::Parse('4.5'))))   &gt;= 0">$(DefineConstants);NETFX_45</DefineConstants>
        <DefineConstants Condition="$([System.Version]::Parse('$(TargetFrameworkVersion.Substring(1))').CompareTo($([System.Version]::Parse('4.0'))))   &gt;= 0">$(DefineConstants);NETFX_40</DefineConstants>
        <DefineConstants Condition="$([System.Version]::Parse('$(TargetFrameworkVersion.Substring(1))').CompareTo($([System.Version]::Parse('3.5'))))   &gt;= 0">$(DefineConstants);NETFX_35</DefineConstants>
        <DefineConstants Condition="$([System.Version]::Parse('$(TargetFrameworkVersion.Substring(1))').CompareTo($([System.Version]::Parse('3.0'))))   &gt;= 0">$(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#