Linq羣體加入

Question

如何在recipe=中寫Description從Recipe。 我試圖使用join r in Recipe on d.DishID equals r.DishID，但它給出了錯誤的結果。它擦除result1中的一個項目。

Exptected輸出：

namespace ConsoleApplication3 
{ 
    class Program 
    { 
     static void Main(string[] args) 
     { 
      List<Component> Component = new List<Component>(); 
      List<Dish> Dish = new List<Dish>(); 
      List<Recipe> Recipe = new List<Recipe>(); 
      Dish.Add(new Dish { DishID = 9, CategoryID = 6, DishName = "Pork" }); 
      Dish.Add(new Dish { DishID = 10, CategoryID = 6, DishName = "Beef" }); 
      Component.Add(new Component { ComponentID = 1, DishID = 9, AmountID = "1", NameID = "1" }); 
      Recipe.Add(new Recipe { DishID = 9, RecipeID = 0, Description = "Test" }); 

      var result1 = (
      from d in Dish 
      //join r in Recipe on d.DishID equals r.DishID 
      join c in Component on d.DishID equals c.DishID into items 
      select new Item { DishID = d.DishID, components = items.ToList() recipe=} 
      ).ToList(); 
     } 
    } 
    public class Item 
    { 
     public int DishID { get; set; } 

     public string DishName { get; set; } 

     public string recipe { get; set; } 

     public List<Component> components { get; set; } 
    } 

    public partial class Component 
    { 
     public int ComponentID { get; set; } 
     public int DishID { get; set; } 
     public string AmountID { get; set; } 
     public string NameID { get; set; } 
    } 

    public partial class Dish 
    { 
     public int DishID { get; set; } 
     public int CategoryID { get; set; } 
     public string DishName { get; set; } 
    } 
    public partial class Recipe 
    { 
     public int RecipeID { get; set; } 
     public int DishID { get; set; } 
     public string Description { get; set; } 
    } 
} 

Answer 1

你想要做一個左連接

要做到這一點，你需要做一些額外的東西在LINQ，不知道爲什麼它不是本地支持，但它不是。

 var result1 = (
     from d in Dish 
     join c in Component on d.DishID equals c.DishID into items 
     join r in Recipe on d.DishID equals r.DishID into recipes 
     select new Item { 

        DishID = d.DishID, 

        components = items.DefaultIfEmpty() 
             .Where(a=>a!=null) 
             .ToArray(), 

        recipe = recipes.DefaultIfEmpty() 
            .Where(a=>a!=null) 
            .Select(a=>a.Description) 
            .FirstOrDefault() 
     }).ToList();