Dynamics 365 Plugin for lookup values

Error: 0x80040203 Invalid Argument

I'm new to Dynamics. In preparation for getting lookup values from external data source into Dynamics via a plugin, I wanted to test first with hard coded values with this code. But after I registered the assembly, data provider and data source, I created a virtual entity in dynamics which I linked up to a field (lookup type) on a form. After publishing, clicking on the field throws error - Invalid Argument

using System;
using Microsoft.Xrm.Sdk;

namespace Hardcoded.Names.Plugin
{
public class NamesLookupPlugin : IPlugin
{
    public void Execute(IServiceProvider serviceProvider)
    {
        var context = (IPluginExecutionContext)serviceProvider.GetService(typeof(IPluginExecutionContext));
        var serviceFactory = (IOrganizationServiceFactory)serviceProvider.GetService(typeof(IOrganizationServiceFactory));
        serviceFactory.CreateOrganizationService(context.UserId);
        var tracingService = (ITracingService)serviceProvider.GetService(typeof(ITracingService));

        try
        {
            var results = new EntityCollection();
            var itemOne = new Entity
            {
                ["ID"] = Guid.NewGuid(),
                ["Name"] = "First Item"
            };

            var itemTwo = new Entity()
            {
                ["ID"] = Guid.NewGuid(),
                ["Name"] = "Second Item"
            };
            results.Entities.Add(itemOne);
            results.Entities.Add(itemTwo);

            context.OutputParameters["BusinessEntityCollection"] = results;
        }
        catch (Exception e)
        {
            tracingService.Trace($"{e.Message} {e.StackTrace}");
            if (e.InnerException != null)
                tracingService.Trace($"{e.InnerException.Message} {e.InnerException.StackTrace}");

            throw new InvalidPluginExecutionException(e.Message);
        }
     }
   }
}

I expect to see "First Item" and "Second Item" selectable in the lookup field.

You’re hooking on RetrieveMultiple message in your plug-in. There is also Retrieve message (get single record by EntityReference) - you need to hook on that message as well for your entity.