c# - 左连接中的错误空引用

Question

当我评论 dishes.Add(new Dishes { DishID = 8, DishName = "Name", DishTypeID = 2, IngredientID = 2 }); 我进入 ll 一项 Amount="1 cup"DishID=1 Ingridient="egg"Name ="Soup"。当取消注释该行引发错误时，b.IngredientTypeID 中出现空引用异常。主要问题是如何得到 ll 两项:

1) Amount="1 cup"DishID=1 Ingridient="egg"Name ="Soup"

2) Amount=null DishID=2 Ingridient=null Name =null

namespace ConsoleApplication1
{
class Program
{
    static void Main(string[] args)
    {
        List<Dishes> dishes = new List<Dishes>();
        List<Ingredients> ingredients = new List<Ingredients>();
        List<Amount> amount = new List<Amount>();
        List<Ingredient> ingredient = new List<Ingredient>();

        dishes.Add(new Dishes { DishID = 1, DishName = "Soup", DishTypeID = 1, IngredientID = 1 });
        //dishes.Add(new Dishes { DishID = 8, DishName = "Name", DishTypeID = 2, IngredientID = 2 }); 
        ingredients.Add(new Ingredients { AmountID = 2, IngredientID = 1, IngredientTypeID = 1, IngredientUniqID = 1 });
        amount.Add(new Amount { AmountID = 2, AmountName = "1 cup" });
        ingredient.Add(new Ingredient { IngredientID = 1, IngredientName = "egg" });

        var test = from dish in dishes
                   join ing in ingredients on dish.IngredientID equals ing.IngredientID into result
                   from b in result.DefaultIfEmpty()
                   join i in ingredient on b.IngredientTypeID equals i.IngredientID into r
                   from c in r.DefaultIfEmpty()
                   join am in amount on b.AmountID equals am.AmountID into s
                   from t in s.DefaultIfEmpty()
                   select new DisplayRecipe { Name = dish.DishName, Amount = t.AmountName, Ingredient = c.IngredientName, DishID = dish.DishID };

        List<DisplayRecipe> ll = test.ToList();
    }
}
public partial class Dishes
{
    public int DishID { get; set; }
    public string DishName { get; set; }
    public Nullable<int> DishTypeID { get; set; }
    public Nullable<int> IngredientID { get; set; }
}

public partial class Ingredients
{
    public int IngredientID { get; set; }
    public Nullable<int> AmountID { get; set; }
    public Nullable<int> IngredientTypeID { get; set; }
    public int IngredientUniqID { get; set; }
}

public partial class Amount
{
    public int AmountID { get; set; }
    public string AmountName { get; set; }
}
public partial class Ingredient
{
    public int IngredientID { get; set; }
    public string IngredientName { get; set; }
}
public class DisplayRecipe
{
    public string Name { get; set; }
    public string Ingredient { get; set; }
    public string Amount { get; set; }
    public int DishID { get; set; }
}
}

Answer 1

问题是 b、c、t 变量中的任何一个都可以是 null，因为 DefaultIfEmpty，您需要在任何成员访问中考虑到这一点，包括加入条件。

如果你使用的是 C#6 (VS2015)，你可以像这样使用 ?. 运算符

var test = from dish in dishes
           join ing in ingredients on dish.IngredientID equals ing.IngredientID into result
           from b in result.DefaultIfEmpty()
           join i in ingredient on b?.IngredientTypeID equals i.IngredientID into r
           from c in r.DefaultIfEmpty()
           join am in amount on b?.AmountID equals am.AmountID into s
           from t in s.DefaultIfEmpty()
           select new DisplayRecipe { Name = dish.DishName, Amount = t?.AmountName, Ingredient = c?.IngredientName, DishID = dish.DishID };

在 C#6 之前:

var test = from dish in dishes
           join ing in ingredients on dish.IngredientID equals ing.IngredientID into result
           from b in result.DefaultIfEmpty()
           join i in ingredient on b != null ? b.IngredientTypeID : null equals i.IngredientID into r
           from c in r.DefaultIfEmpty()
           join am in amount on b != null ? b.AmountID : null equals am.AmountID into s
           from t in s.DefaultIfEmpty()
           select new DisplayRecipe { Name = dish.DishName, Amount = t != null ? t.AmountName : null, Ingredient = c != null ? c.IngredientName : null, DishID = dish.DishID };