linq - 选择和选择很多不同

Question

我已经阅读了许多关于 select 和 select many in LINQ 的站点/线程，但仍然不太明白。

select 是否返回集合中的一个元素并选择多个扁平化集合(例如 List>())？

谢谢

Answer 1

这是一个示例。希望它澄清一切:

static void MethodRun()
{
    List<Topping> testToppings = new List<Topping> { Topping.Cheese, Topping.Pepperoni, Topping.Sausage };

    var firstLetterofToppings = testToppings.Select(top => top.ToString().First());

    // returns "C, P, S"

    var singleToppingPizzas = testToppings.Select(top => new Pizza(top)).ToArray();

    // returns "Pizza(Cheese), Pizza(Pepperoni), Pizza(Sausage)"

    List<Topping> firstPizza = new List<Topping> { Topping.Cheese, Topping.Anchovies };
    List<Topping> secondPizza = new List<Topping> { Topping.Sausage, Topping.CanadianBacon, Topping.Pepperoni };
    List<Topping> thirdPizza = new List<Topping> { Topping.Ham, Topping.Pepperoni };

    List<IEnumerable<Topping>> toppingsPurchaseOrder = new List<IEnumerable<Topping>> { firstPizza, secondPizza, thirdPizza };

    var toppingsToOrder = toppingsPurchaseOrder.SelectMany(order => order);

    //returns "Cheese, Anchovies, Sausage, CanadianBacon, Pepperoni, Ham, Pepperoni"

}

class Pizza
{
    public List<Topping> Toppings { get; private set; }
    public Pizza(Topping topping) : this(new List<Topping> { topping }) { }
    public Pizza(IEnumerable<Topping> toppings)
    {
        this.Toppings = new List<Topping>();
        this.Toppings.AddRange(toppings);
    }
}

enum Topping
{
    Cheese,
    Pepperoni,
    Anchovies,
    Sausage,
    Ham,
    CanadianBacon
}

关键是 Select() 可以选择任何类型的对象。确实，您可以选择分配给集合的任何通用值的属性，但您也可以选择任何其他类型的对象。 SelectMany() 只是扁平化你的列表。