Designer Filtering using Reference Assemblies

Visual Studio 2010 expands multi-targeting support to include metadata filtering in features such as intellisense and the property grid.  Notice how the property grid filters out the new ClientIDMode property added to web controls in .NET 4 when targeting .NET 2:

So, how is this happening when Visual Studio itself is running on .NET 4?  The answer is reference assemblies!

Reference assemblies are metadata-only assemblies that have method bodies and non-public members stripped out in order to make them more compact.  As such, they can’t be loaded for execution by the CLR.  However, the multi-targeting infrastructure is able to load and unload these assemblies as needed and has all the information it needs to provide target-aware reflection.

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