How Can I Use Conditional Compilation to Target Different .NET Framework Versions?

Targeting Multiple .NET Frameworks with Conditional Compilation

Conditional compilation offers a powerful mechanism for tailoring your C# code to specific .NET Framework versions, ensuring compatibility and avoiding version-specific errors. This is achieved through preprocessor directives.

Leveraging Preprocessor Directives

Preprocessor directives allow you to conditionally include or exclude code blocks based on defined symbols. For example:

#if NET40
using FooXX = Foo40;
#elif NET35
using FooXX = Foo35;
#else
using FooXX = Foo20; // Default if NET40 and NET35 aren't defined
#endif

Note that NET40, NET35, and NET20 are not automatically defined; you must explicitly set them.

Defining Symbols via MSBuild

You can inject these symbols using the /p:DefineConstants MSBuild property:

<code>/p:DefineConstants="NET40"</code>

This adds the NET40 symbol to the project's build configuration.

Managing Build Configurations

Alternatively, create distinct build configurations within your project file. Each configuration can define its own DefineConstants value:

<PropertyGroup Condition="'$(Framework)' == 'NET20'">
  <DefineConstants>NET20</DefineConstants>
</PropertyGroup>
<PropertyGroup Condition="'$(Framework)' == 'NET35'">
  <DefineConstants>NET35</DefineConstants>
</PropertyGroup>

Set a default framework version in one of your configurations, for example:

<PropertyGroup>
  <Framework>NET35</Framework>
</PropertyGroup>

Automated Recompilation for Different Versions

After defining your build configurations, use an AfterBuild target to automatically recompile for other framework versions:

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

This will recompile your project for .NET 2.0 after the initial build (assuming .NET 3.5 is the default). Each compilation will utilize the appropriate conditional defines.

Advanced Techniques

Conditional compilation extends beyond simple using statements. You can also conditionally include or exclude entire files or references based on the target framework, providing fine-grained control over your build process.

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