Reference Assemblies folder and different assemblies with the same version

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.NET version 4.0 made a big change in the way framework reference assemblies are done. Previously, the reference assembly was a simple copy of the runtime assembly, the one stored in the GAC. That however caused some painful problems. Notable is the WaitHandle.WaitOne(int) overload, it was added in the .NET 2.0 Service Pack 2 update (aka .NET 3.5). Programmers used it without noticing that it was an added method, the mscorlib assembly version number was still 2.0.0.0. But then discovered their program failed when running on an unpatched version of .NET 2.0. Very nasty kaboom, MissingMethodException without a hint why such a common method could be missing.

To prevent this kind of breakage, the .NET 4.0 reference assemblies are kept separate, in the “%programfiles%\Reference Assemblies” directory as you found out. And they are special assemblies, they only contain the metadata with all the IL stripped out. Which is why the assembly is so much smaller.

Microsoft now can improve the .NET 4 code and add public classes and methods without causing this kind of breakage. And have done so profusely, updates 4.01, 4.02 and 4.03 have shipped since the original 4.0 release.

The reason you are having trouble with the DataContractSerializerSetting class is thus easily explained, it just doesn’t appear in the reference assembly. It got added, probably in one of those incremental updates. And you should not try, your program will break on a machine that doesn’t have the update. You should wait until .NET 4.5, the version that added it to the reference assembly. You can invoke DLL Hell if you really want to.

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