c# - 在 Controller 内构建模型时如何将 Byte[] 隐式转换为字符串

Question

我正在使用请求 product/index/[Category ID] 加载索引页面上属于唯一类别的所有产品。

我有一个 ProductViewModel 类，其中包含用于在两者之间转换类型的隐式方法，还有一个 Product 实体模型类。将 Product 实体转换为 ProductViewModel 的隐式方法包含将字节转换为 base64 字符串的方法，我在 Controller 中使用它成功地创造了新的类别和产品。

public class ProductViewModel
    {
        public int Id { get; set; }
        [Required, Display(Name="Product Name")]
        public string Name { get; set; }
        [Required, DataType(DataType.Upload)]
        public HttpPostedFileBase Image { get; set; }
        public string OutputImage { get; set; }
        [Required]
        public Decimal Price { get; set; }

        public static byte[] ConvertToByte(ProductViewModel model)
        {
            if (model.Image != null)
            {
                byte[] imageByte = null;
                BinaryReader rdr = new BinaryReader(model.Image.InputStream);
                imageByte = rdr.ReadBytes((int)model.Image.ContentLength);

                return imageByte;
            }

            return null;
        }

        // ViewModel => Model | Implicit type Operator
        public static implicit operator Product(ProductViewModel viewModel)
        {
            var model = new Product
            {
                Id = viewModel.Id,
                Name = viewModel.Name,
                Image = ConvertToByte(viewModel),
                Price = viewModel.Price
            };

            return model;
        }

        // Model => ViewModel | Implicit type Operator
        public static implicit operator ProductViewModel(Product model)
        {
            var viewModel = new ProductViewModel
            {
                Id = model.Id,
                Name = model.Name,
                OutputImage = string.Format("data:image/jpg;base64,{0}", Convert.ToBase64String(model.Image)),
                Price = model.Price
            };

            return viewModel;
        }

    }

但是，当传递包含属于要在产品 View 上显示的唯一类别 ID 的所有产品的模型时，我无法将字节隐式转换为字符串。我用作替代方法的方法不被接受，并出现错误:

LINQ to Entities does not recognize the method 'System.String Format(System.String, System.Object)' method, and this method cannot be translated into a store expression.

Controller 中的模型如下:

var products = (await db.Categories.Where(c => c.Id == id).Select(p => p.Products.Select(x => new 
    ProductViewModel { Id = x.Id, Name = x.Name, OutputImage = (string.Format("data:image/jpg;base64,{0}", Convert.ToBase64String(x.Image))), Price = x.Price})).ToListAsync());
return View(products);

我为View提供的Model类型如下:

@model List<IEnumerable<ValueVille.Models.ProductViewModel>>

Answer 1

您不能在 LINQ 表达式中使用 string.Format()，而是可以在 Name setter 中使用它:

public class ProductViewModel {

    public string Name{
      get
      {
         return this.Name;
      }
      set{
           this.Name = value;
           this.OutputImage  = string.Format("data:image/jpg;base64,{0}", Convert.ToBase64String(value))
      }
    }
}

在 Controller 中:

var products = (await db.Categories.Where(c => c.Id == id).Select(p => p.Products.Select(x => new 
                    ProductViewModel { Id = x.Id, Name = x.Name, Convert.ToBase64String(x.Image))), Price = x.Price})).ToListAsync());