Visual Studio 2008's multi-targeting support for compiling projects to different versions of the .NET Framework is very powerful. Multi-targeting is a compelling feature because it enables users to continue working on solutions that target .NET Framework 2.0 and 3.0 while upgrading to the latest and greatest IDE. What isn't obvious is that all projects, regardless of target, are compiled with the C# 3.0 compiler. That means users can employ many of the new C# 3.0 language features in legacy projects. The only language features that can't be used are those that require library support from .NET Framework 3.5, in essence, LINQ, Expression Trees and Extension Methods. Implicitly-typed local variables, lambda expressions, auto-implemented properties, object and collection initializers, and anonymous types are all fair game. It's sort of like having C# 3.0-lite or C# 2.5.

Interestingly, it has recently been discovered that even Extension Methods can be used in projects targeting .NET Framework 2.0 and 3.0. All that must be done to enable this support is to create a new System.Runtime.CompilerServices.ExtensionAttribute.

This trick does have flaws. There are potential scoping issues that occur when an assembly containing a custom System.Runtime.CompilerServices.ExtensionAttribute is referenced by a project that targets .NET Framework 3.5. A compiler warning is generated stating that "the predefined type 'System.Runtime.CompilerServices.ExtensionAttribute' is defined in multiple assemblies in the global alias." However, this is only a minor irritation. In my tests, Extension Methods still worked properly despite the warning.

The ability to use C# 3.0 features in .NET Framework 2.0 or 3.0 projects is very powerful. It helps users get comfortable with the new syntax without having to upgrade projects to .NET Framework 3.5. Viva la C# 2.5!