Well , the picture speaks for itself.

• Silverlight project
• VS 2015 ( tried also with 2013 and 2012 )

CodePudding user response：

It is a documented restriction, the constructor you are trying to use is not supported in Silverlight version 5. From the MSDN article:

Supported in: 4, 3

Not crystal why it was dropped, probably has something to do with it being [SecurityCritical].

Why your IntelliSense still shows it is hard to guess, I certainly can't repro that. You probably ought to verify the path to the mscorlib reference, mine points to C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\Silverlight\v5.0\mscorlib.dll