Hi,
The problem is that the System.Threading namespace is not available on all the target platforms that a portable class library supports. I think for example Windows Phone 7.x do not have a Threading namespace.
The one and only reason why you should use a portable class library is that you want to use it on different target plattforms, such as Windows 7, WinRT (Store App), Windows Phone 8.1, etc...
So it unites all the common features. If one plattform does not have a Threading namespace, then you can not use it.
Do you use a portable class library for the correct reason?
Please provide more information about the platforms that you are targetting!
Rgds MM
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