To return any object as an Enumerable...

ObjectHelper.cs

using System.Collections.Generic;

public static class ObjectHelper
{
	public static IEnumerable<T> AsEnumerable<T>(this T self)
	{
		yield return self;
	}
}

/*
class Data { }

interface IWorker
{
    public Data GetData();
    public List<Data> GetAllData();
}

class ResultsObject
{
    public IEnumerable<Data> Results { get; set; }
}

...
public bool SetResults(IWorker worker, out ResultsObject results)
{
    if (ConditionMet())
    {
        results.Data = worker.GetData().AsEnumerable();
    }
    else
    {
        results.Data = worker.GetAllData();
    }
    
    return true;
}

*/