Predefined type 'System.ValueTuple´2´ is not defined or imported
For .NET 4.6.2 or lower, .NET Core 1.x, and .NET Standard 1.x you need to install the NuGet package System.ValueTuple:
Install-Package "System.ValueTuple"
Or using a package reference in VS 2017:
<PackageReference Include="System.ValueTuple" Version="4.4.0" />
.NET Framework 4.7, .NET Core 2.0, and .NET Standard 2.0 include these types.
It's part of the .NET Framework 4.7.
As long as you don't target the above framework or higher (or .NET Core 2.0 / .NET Standard 2.0), you'll need to reference ValueTuple. Do this by adding the System.ValueTuple NuGet Package