Cant find namespace in Visual Studio 2017 RC

Source: stackoverflow.com --- Tuesday, January 31, 2017
I am actually working on an Outlook VSTO-Add In in VS 2017 RC and tried to access the namespace System.Security.Cryptography.Pkcs ( https://msdn.Microsoft.com/de-de/library/system.security.cryptography.pkcs(v=vs.110).aspx ), sadly VS says, that this namespace could not be found - I am quite new to Visual Studio, am I doing something wrong when trying to access this namespace? using System.Security.Cryptography.Pkcs.SignedCms; ...

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