Microsoft.Threading.Tasks not found

Sounds like you're using the Google API .Net Client. I suggest using Nuget to install the assemblies as described on the linked page. However, if you download the source, the Microsoft.Threading.Task assmeblies are included and so it seems the code your calling is trying to access those assemblies.

You could manually move that assembly into your directory but I'd usually opt for the Nuget method unless you need to be using a particular build.

This is what worked for me:

Open the NuGet console via the Tools menu > NuGet Package Manager > Package Manager Console

From the console type in: update-package Microsoft.Bcl.Async -reinstall

After that, you may be prompted to re-start Visual Studio to finish uninstalling the package. Once you re-start, the package should be re-installed and everything should work.

Just install Microsoft.Bcl.Async nuget package!

(if you are using Google.Apis.Oauth2.v2 with UWP app)

I expect you are using the "google-api-dotnet-client". Microsoft.Threading.Tasks is a dll used by this client according to google code: https://code.google.com/p/google-api-dotnet-client/source/browse/ThirdParty/Microsoft.Threading.Tasks.dll

You probably just have to move this file into your bin directory.