foo1.cs

    [Pure, ContractAnnotation("null => false")]
    public static bool IsValueTuple([CanBeNull] this ITypeElement typeElement)
    {
      var structType = typeElement as IStruct;
      if (structType == null) return false;

      return typeElement.ShortName == "ValueTuple"
             && typeElement.GetContainingNamespace() is INamespace ns
             && ns.ShortName == "System"
             && ns.GetContainingNamespace() is INamespace root
             && root.IsRootNamespace;
    }

foo2.cs

    [Pure, ContractAnnotation("null => false")]
    public static bool IsValueTuple([CanBeNull] this ITypeElement typeElement)
    {
      return typeElement is IStruct { ShortName: "ValueTuple" }
             && typeElement.GetContainingNamespace() is { ShortName: "System" } ns
             && ns.GetContainingNamespace() is { IsRootNamespace: true };
    }

foo3.cs

    [Pure, ContractAnnotation("null => false")]
    public static bool IsValueTuple([CanBeNull] this ITypeElement typeElement)
    {
      return typeElement is IStruct {
               ShortName: "ValueTuple",
               ContainingNamespace: {
                 ShortName: "System",
                 ContainingNamespace: { IsRootNamespace: true }
               }
             };
    }