ListViewCachingStrategy - where is it defined?

When I try to use this, the enum can't be found: Error CS0246 The type or namespace name 'ListViewCachingStrategy' could not be found (are you missing a using directive or an assembly reference?)

Code Snipet:

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace MyAppNamespace
{
class ListViewHolder : ListView
{
List dataSource { get; set; }

    public ListViewHolder(ListViewCachingStrategy strategy) : base(strategy)
    {

    }

Am I using the wrong version of Xamarin.Forms or ???

Thanks.