是否有其他方法将列表的模型类添加到c＃中的另一个模型类列表中？

Question

In the application I have two model class category and product. Shows below

public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int CategoryId { get; set; }
}
public class Category
{
    public Category()
    {
        Products = new List<Product>();
    }
    public int Id { get; set; }
    public string Name { get; set; }
    public List<Product> Products { get; set; }
}

I added the data into the category and product list as below.

List<Category> data = new List<Category>();
data.Add(new Category { Id = 1, Name = "Plastic"});
data.Add(new Category { Id = 2, Name = "Metal"});
data.Add(new Category { Id = 3, Name = "Paper" });
foreach (var item in data)
{
    switch (item.Id)
    {
        case 1: 
            item.Products.Add(new Product { Id = 1, Name = "Doll", CategoryId = item.Id });
            item.Products.Add(new Product { Id = 2, Name = "Pen", CategoryId = item.Id });
            break;
        case 2: 
            item.Products.Add(new Product { Id = 3, Name = "Nail", CategoryId = item.Id });
            item.Products.Add(new Product { Id = 4, Name = "Key", CategoryId = item.Id });
            break;
        case 3: 
            item.Products.Add(new Product { Id = 5, Name = "Book", CategoryId = item.Id });
            break;
        default: 
            item.Products.Add(new Product { Id = 0, Name = null });
            break;
    }
}

Is there any other way to write with minimum lines of code? I would appreciate if anyone suggest another solution for this.

Answer 1

You can provide fluent builder methods to add products to category (it can be either extension method or instance method of category). These methods should return category which product was added to. And usage will look like (there are three different options to add products to category):

var data = new List<Category> {       
   new Category { Id = 1, Name = "Plastic" } 
      .WithProducts(  
          new Product { Id = 1, Name = "Doll" }
          new Product { Id = 2, Name = "Pen" }
      ),

   new Category { Id = 3, Name = "Paper" }
      .WithProduct( new Product { Id = 2, Name = "Book" } ),

   new Category { Id = 2, Name = "Plastic" }
      .WithProduct(3, "Nail")
      .WithProduct(4, "Key"),
};

And fluent extension methods (you can use all, or pick those which syntax you like more):

public static Category WithProduct(this Category category, int id, string product)
{
    category.Products.Add(new Product { Id = id, Name = name, CategoryId = category.Id });
    return category;
}

public static Category WithProduct(this Category category, Product product)
{
    product.CategoryId = category.Id;
    category.Products.Add(product);
    return category;
}

public static Category WithProducts(this Category category, params Product[] products)
{
    foreach(var product in products)
    {
        product.CategoryId = category.Id;
        category.Products.Add(product);
    };

    return category;
}

Note that category id will be set up automatically when you add a product to the category.

---

Further notes: there is one problem with your design - when you expose category Products and id of category for modification, then it's possible that someone can add a product from another category to the products list:

var plastic = new Category { Id = 1, Name = "Plastic"};
plastic.Products.Add(new Product {Id = 1, Name = "Doll", CategoryId = 1});
plastic.Products.Add(new Product {Id = 5, Name = "Nail", CategoryId = 2}); // plastic nail

I would at least consider disabling direct modification of category products and moved setting category id of product there:

private List<Product> products = new List<Product>();
public IEnumerable<Product> Products => products;

public void AddProduct(Product product)
{
    // probably check if product was not added to some other category
    product.CategoryId = Id;
    products.Add(product);
}

Answer 2

Nothing going to be significantly shorter because it deals with object creation, so you have to make those new calls anyway. You could do something thing like...

On you Product class create a static method called Create:

public static Product Create(int id, string name, Category cat)
{
    return new Product { Id = id, Name = name, cat.Id };
}

then instead of writing:

item.Products.Add(new Product { Id = 1, Name = "Doll", CategoryId = item.Id });

write:

item.Products.Add(Product.Create(1, "Doll", item);

Answer 3

I would do something similar to what Sergey Berezovskiy suggested but I would also add constructors to both classes. Extension methods are fine but, alternatively, you could create a public method to the Category class.

Here's the output:

var categories = new List<Category>
{
    new Category(1, "Plastic").WithProduct(1, "Doll").WithProduct(2, "Pen"),
    new Category(2, "Metal").WithProduct(3, "Nail").WithProduct(4, "Key"),
    new Category(3, "Paper").WithProduct(5, "Book")
};

Here's the amended Product class:

public class Product
{
    public Product(int id, string name, int categoryId)
    {
        Id = id;
        Name = name;
        CategoryId = categoryId;
    }

    public int Id { get; set; }
    public string Name { get; set; }
    public int CategoryId { get; set; }
}

Here's the amended Category class:

public class Category
{
    public Category(int id, string name)
    {
        Id = id;
        Name = name;
        Products = new List<Product>();
    }

    public int Id { get; set; }
    public string Name { get; set; }
    public List<Product> Products { get; set; }

    public Category WithProduct(int id, string name)
    {
        Products.Add(new Product(id, name, Id));
        return this;
    }
}