Changing .NET target framework makes my code doesn't work.

I was trying changing targeted .NET framework from 4.5 to 3.5 and got some of messages like below:

1.The type or namespace name 'Tasks' does not exist in the namespace 'System.Threading' (are you missing an assembly reference?)

my code used -> using System.Threading.Tasks;

2.The primary reference "Microsoft.CSharp", which is a framework assembly, could not be resolved in the currently targeted framework. ".NETFramework,Version=v3.5". To resolve this problem, either remove the reference "Microsoft.CSharp" or retarget your application to a framework version which contains "Microsoft.CSharp".

Any solution??
thanks..

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I'm Okie Eko Wardoyo Owner of http://www.mediatutorial.web.id