Target .Net 4.5.1 in VS2012? Yep, with the power of multi-targeting...

.NET Framework Blog - Building apps with the .NET Framework 4.5.1 Preview in Visual Studio 2012

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You can. Visual Studio 2013 preview provides built-in support for targeting the .NET Framework 4.5.1 Preview, but you can also build apps that target the .NET Framework 4.5.1 using Visual Studio 2012.

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The .NET Framework 4.5.1 Preview includes the .NET runtime, but doesn’t include the files needed by Visual Studio to build .NET 4.5.1 apps. Instead, there is a separate download that includes all of the reference assemblies, IntelliSense XML files, and anything else needed to provide a great targeting experience in Visual Studio. We call this download a targeting pack.

You can get the targeting pack in these two ways:

• Either install Visual Studio 2013 Preview on your machine that already includes Visual Studio 2012. You’ll be able to target .NET 4.5.1 from both versions of Visual Studio.
• Or install the Windows 8.1 Preview SDK, which includes the targeting pack. The next time you open Visual Studio 2012, .NET 4.5.1 will be available from the target framework list.

For future reference, this information is also listed on .NET Framework SDKs and Targeting Packs for Visual Studio 2012. Our Get the latest .NET bits post explains how you can get to the SDK and targeting pack pages, for each version of Visual Studio.

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.NET Framework SDKs and Targeting Packs for Visual Studio 2012

The following .NET Framework targeting packs and SDKs are available for use with Visual Studio 2012. Each download enables you to build apps for a specific Microsoft platform, such as Windows Azure. The SDKs and .NET Framework downloads may update Visual Studio and the .NET Framework on your machine.

The .NET team has also published many libraries, listed at .NET NuGet packages, that can be used when building apps that target these platforms.

Learn about .NET Framework SDKs and targeting packs available for Visual Studio 2013 Preview.

This shows off the power of the Multi-targeting feature of Visual Studio and helps round out the round-tripping, VS2012 <-> VS2013, feature.

I'd still love to see a Mono Target Pack... :)