gistfile1.cs

public DeliveryService(ModelContext context)
{
    _modelContext = context;
    _itemSender = context.Get<EmailDeliverItemSender>();

    if (TestMode)
    {
        _itemSender = context.Get<FileDeliveryItemSender>();
    }
}

// Лучше так:
public DeliveryService(ModelContext context, IDeliverItemSender sender)
{
    _modelContext = context;
    _itemSender = sender;
}