关于LINQ：Select 和select many的差异

Select and select many difference

我已经阅读了许多关于选择和选择LINQ中的许多站点/线程，但仍然不太理解。

select是否返回集合中的一个元素并选择many flatten a collection(例如list>())？

谢谢

这是一个样品。希望它能澄清一切：

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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泛型值的属性，但也可以选择任何其他类型的对象。selectmany()只是平展您的列表。

selectmany为传递到表达式中的每个对象返回多个对象。SELECT ISTEAD为传递到表达式中的每个对象返回单个对象。

引用文档：

选择许多

Projects each element of a sequence to an IEnumerable<(Of <(T>)>) and flattens the resulting sequences into one sequence.

选择

Projects each element of a sequence into a new form.

如果您想平展层次结构，可以使用SelectMany。例如，如果您有Orders和OrderDetails。如果您希望根据订单进行选择，但您希望返回OrderDetails，请使用SelectMany。

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var result = db.Orders
.Where(x => x.CustomerId == 500) // input to next expression is IEnumerable<Order>
.SelectMany(x => x.OrderDetails) // input to next expression is IEnumerable<OrderDetails>
.Sum(x => x.PositionTotal);

var result = db.Orders
.Where(x => x.CustomerId == 500) // input to next expression is IEnumerable<Order>
.Select(x => CustomerName);