Reference Assemblies folder and different assemblies with the same version

I have a project that uses System.Runtime.Serialization assembly. I am using the type DataContractSerializer from that assembly, but I have a problem. There are two assemblies:

C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework.NETFramework\v4.0\System.Runtime.Serialization.dll

C:\Windows\Microsoft.net\Framework\v4.0.30319\System.Runtime.Serialization.dll

Both of them have the same version - v4.0.30319. The first one have 429kb size, and the second one 1037kb. I used reflector to see the list of classes, and the first one doesn't have the class that I need (DataContractSerializerSettings). However, the second one does have it.

Why are there some big difference in size and classes for that assembly? Will it be ok, if I use the second one, instead of the first?

.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.