DetailItemLoaderWithProgressItem.cs

public class DetailItemLoaderWithProgressItem<T> : IDetailItemLoader<T>
{
    private readonly IDetailItemLoader<T> _itemLoader;
    private readonly ProgressItem _progressItem;

    public DetailItemLoaderWithProgressItem(IDetailItemLoader<T> itemLoader, ProgressItem progressItem)
    {
        _itemLoader = itemLoader;
        _progressItem = progressItem;
    }

    public T GetDetailItemFrom(Row row)
    {
        _progressItem.IncrementProgress();
        return _itemLoader.GetDetailItemFrom(row);
    }
}

DetailLoader.cs

public class DetailLoader<T> : IDetailLoader<T>
{
    public DetailLoader(IDetailItemLoader<T> itemLoader)
    {
        ItemLoader = itemLoader;
    }

    public IDetailItemLoader<T> ItemLoader { get; set; }

    public virtual void Load(SpreadsheetDocument spreadsheet, Action<T> addToBatch)
    {
        var rows = spreadsheet.Rows();

        var details = GetDetailItems(rows);
        foreach (var detail in details)
            addToBatch(detail);
    }

    public virtual IEnumerable<T> GetDetailItems(IEnumerable<Row> rows)
    {
        foreach (var row in rows)
        {
            var detail = ItemLoader.GetDetailItemFrom(row);
            yield return detail;
        }
    }
}

DetailLoaderWithProgressItem.cs

public class DetailLoaderWithProgressItem<T> : IDetailLoader<T>
{
    private IDetailLoader<T> _loader;
    private ProgressItem _progressItem;

    public DetailLoaderWithProgressItem(IDetailLoader<T> loader, ProgressItem progressItem)
    {
        _progressItem = progressItem;
        _loader = loader;
        ItemLoader = new DetailItemLoaderWithProgressItem<T>(_loader.ItemLoader, _progressItem);
        _loader.ItemLoader = ItemLoader;
    }

    public IDetailItemLoader<T> ItemLoader { get; set; }

    public void Load(SpreadsheetDocument spreadsheet, Action<T> addToBatch)
    {
        var rows = spreadsheet.Rows();
        _progressItem.Start(rows.Count());

        _loader.Load(spreadsheet, addToBatch);
        _progressItem.EndWithSuccess();
    }

    public IEnumerable<T> GetDetailItems(IEnumerable<Row> rows)
    {
        // This should never actually be called.
        return _loader.GetDetailItems(rows);
    }
}

FakeDetailItemLoader.cs

public class Fake { };

public class FakeDetailItemLoader : IDetailItemLoader<Fake>
{
    public Fake GetDetailItemFrom(Row row)
    {
        return new Fake();
    }
}