Single responsability example, step 4

ProductRepository.cs

class ProductRepository : IProductRepository
{
    private readonly IFileLoader loader; 
    private readonly IProductsMapper mapper;

    public ProductRepository()
    {
        loader = new FileLoader(); 
        mapper = new ProductsMapper();
    }

    public IEnumerable<Product> GetByFileName(string fileName)
    {
        IEnumerable<Product> products; 
        using (Stream file = loader.Load(fileName)) {
            products = mapper.Map(file);
        }
        return products;
    }
}

ProductsMapper.cs

class ProductsMapper : IProductsMapper
{
    private Product MapItem(XmlReader reader)
    {
        if (reader.Name != "product") throw new InvalidOperationException("XML reader is not on a product fragment.");

        var product = new Product(); 
        product.Id = int.Parse(reader.GetAttribute("id"));
        product.Name = reader.GetAttribute("name"); 
        product.UnitPrice = decimal.Parse(reader.GetAttribute("unitPrice"), CultureInfo.InvariantCulture); 
        product.Discontinued = bool.Parse(reader.GetAttribute("discontinued")); 
        return product; 
    }

    public IEnumerable<Product> Map(Stream stream)
    {
        if (stream == null) throw new ArgumentNullException("Stream used when mapping cannot be null.");
        var reader = XmlReader.Create(stream);

        var products = new List<Product>(); 
        while (reader.Read())
        {
            if (reader.Name != "product") continue;
            var product = MapItem(reader);
            products.Add(product);
        }
        return products;
    }
}