How to Detect the Target .NET Framework Version at Compile Time?

How to Detect Target Framework Version at Compile Time

Many developers encounter scenarios where they need to support multiple .NET framework versions while using features introduced in later versions. One common example is utilizing extension methods, which were introduced in .NET 3.0. However, if you want to use extension methods in a project targeting .NET 2.0, you would need to define the ExtensionAttribute class, which can lead to conflicts when targeting higher framework versions.

The question arises: is there a way to conditionally include the ExtensionAttribute class only when compiling for .NET 2.0? The answer lies in using conditional compilation directives.

Conditional Compilation Directives

Visual Studio provides conditional compilation directives that allow you to include or exclude code blocks based on preprocessor symbols. The TargetFrameworkVersion property is one such preprocessor symbol that indicates the target framework version.

Define Constants

To use conditional compilation directives, you can add DefineConstants elements to your project's .csproj file after the existing DefineConstants elements. For example:

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

The above code defines two constants: RUNNING_ON_4 when targeting .NET 4.0 and NOT_RUNNING_ON_4 otherwise.

Conditional Code Blocks

Once the constants are defined, you can conditionally include or exclude code blocks using #if, #elseif, and #else directives. For example:

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

This code will print "RUNNING_ON_4 was set" when compiling for .NET 4.0 and "NOT_RUNNING_ON_4 was set" for any other framework version.

Conclusion

By using conditional compilation directives and defining constants based on the TargetFrameworkVersion preprocessor symbol, you can selectively include code blocks depending on the target framework version. This allows you to support multiple versions of the .NET framework without encountering build errors or warnings.

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