iOS and Android dependency classes' inheritance causes "You must add a reference to assembly" error

In my Xamarin.Forms project I have a dependency interface

    `/*
 * Interface to get device IP address on network, implemented natively in .Droid's and .iOS's IPAddressManager.cs
 */
public interface IIPAddressManager
{
    List<string> GetConnectedHostIPAddresses();
}`

which is then inherited from in my .iOS and .Droid's IPAddressManager.cs classes.

using System.Collections.Generic;
using System.Net;
using System.Net.NetworkInformation;
using System.Threading.Tasks;
using Java.Lang;
using NDO.PortableDataViewer.Android.Android.DependencyServices;

// assembly registers this class to IIPAdressManager in NetworkService.cs
[assembly: Xamarin.Forms.Dependency (typeof(IPAddressManager))] 

namespace NDO.PortableDataViewer.Android.Android.DependencyServices
{
    public class IPAddressManager : Object, IIPAddressManager

I was initially getting 2 errors telling me I had to add a reference to assemblies. The first one was on the [assembly: Xamarin.Forms.Dependency (typeof(IPAddressManager))] and was resolved using ReSharper per the instruction of this thread.

However, the second one is on IPAddressManager inheriting IIPAddressManager and has persisted. The error reads: The type 'List<>' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Collections, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. I can't use ReSharper to resolve this one.