使用 Linq，如何针对目标对象内列表中的对象进行分组

Question

可能措辞

不太正确，但这是我所拥有的，这是我想要实现的结果：

class Cake
{
public List<string> Ingrediants {get;set;}
public DateTime Baked {get;set;}
public string CakeName {get;set;}
}
List<Cake> cakes= new List<Cake>();
cakes.Add(new Cake() {CakeName = "Cake1", Ingrediants = new List<string>() {"Sugar", "Chocolate"}});
cakes.Add(new Cake() {CakeName = "Cake2", Ingrediants = new List<string>() {"Sugar", "Butter"}});
cakes.Add(new Cake() {CakeName = "Cake3", Ingrediants = new List<string>() {"Stevia", "Butter"}});

我想按成分对蛋糕进行分组。所以我想以这个结束：

 - Sugar
        Cake1
        Cake2
 - Stevia
        Cake3 
 - Chocolate
        Cake1
 - Butter
        Cake2
        Cake3

提前感谢！

Answer 1

如果您不介意查询推导，这里有一个替代方案（注意更正的拼写）：

IEnumerable<IGrouping<string,Cake>> query =
    from cake in cakes
    from ingredient in cake.Ingredients
    group cake by ingredient;

惊喜！这是一个有效的查询！语言规范允许理解以分组依据结束。从技术上讲，IGrouping<string,Cake>是一种具有string型Key特性的IEnumerable<Cake> - 在这种情况下是成分。编译器致力于将其转换为与其他答案几乎相同的代码。

我们可以通过引入一个into和一个select子句来更改查询以生成与其他答案类似的泛型类型：

var query =
    from cake in cakes
    from ingredient in cake.Ingredients
    group cake by ingredient into cakesGrouped
    select new { Ingredient = cakesGrouped.Key, 
        Cakes = cakesGrouped.ToList() };

恕我直言，流畅的语法和查询语法都值得了解。干杯！

Answer 2

var ingrediants = cakes.SelectMany(c => c.Ingrediants.Select(i => new { Cake = c, Ingrediant = i }))
    .GroupBy(ci => ci.Ingrediant)

然后组键是成分名称，组集合是匹配的蛋糕。

Answer 3

这里有一种方法可以做到这一点：

var result = cakes
    .SelectMany(c => c.Ingrediants.Select(i => new
        {
            c.CakeName,
            Ingredient = i
        }))
    .GroupBy(x => x.Ingredient)
    .Select(g => new
        {
            Ingredient = g.Key,
            Cakes = g.Select(x=>x.CakeName).ToArray()
        });

result将是一个匿名类型的IEnumerable;一个元素代表一种成分和所有包含它的蛋糕。