You will need a link to System.Threading.Tasks.Parallel
You can use nuget to get this.
Also add the following to project.json
"dependencies": { // "System.Threading.Tasks.Parallel": "4.0.1" },
Note: This is now deprecated since there is more project.json
According to the comment, add this to your csproj:
<PackageReference Include="System.Threading.Tasks.Parallel" Version="4.3.0" />
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