c# - 使用 Linq，如何对目标对象中的列表中的对象进行 GroupBy

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。所以我想结束这个:

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

提前致谢!

Answer 1

如果您不介意查询理解，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结束。 IGrouping<string,Cake>技术上是 IEnumerable<Cake>有一个Key类型属性 string - 在这种情况下是成分。编译器会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 into 来更改查询以生成与其他答案相似的相同泛型类型。和一个 select查询继续的子句:

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，流式语法和查询语法都值得了解。干杯!