下面是一个用于测试的初始化小集合的代码示例:

class Program
{
    static void Main(string[] args)
    {
        List<Order> orders = new List<Order>
        {
            new Order
            {
                OrderID = "orderID1",
                OrderLines = new List<OrderLine>
                {
                    new OrderLine
                    {
                        ProductSKU = "SKU1",
                        Quantity = 1
                    },
                    new OrderLine
                    {
                        ProductSKU = "SKU2",
                        Quantity = 2
                    },
                    new OrderLine
                    {
                        ProductSKU = "SKU3",
                        Quantity = 3
                    }
                }
            },
            new Order
            {
                OrderID = "orderID2",
                OrderLines = new List<OrderLine>
                {
                    new OrderLine
                    {
                        ProductSKU = "SKU4",
                        Quantity = 4
                    },
                    new OrderLine
                    {
                        ProductSKU = "SKU5",
                        Quantity = 5
                    }
                }
            }
        };

        //required result is the list of all SKUs in orders
        List<string> allSKUs = new List<string>();

        //With Select case 2 foreach loops are required
        var flattenedOrdersLinesSelectCase = orders.Select(o => o.OrderLines);
        foreach (var flattenedOrderLine in flattenedOrdersLinesSelectCase)
        {
            foreach (OrderLine orderLine in flattenedOrderLine)
            {
                allSKUs.Add(orderLine.ProductSKU);
            }
        }

        //With SelectMany case only one foreach loop is required
        allSKUs = new List<string>();
        var flattenedOrdersLinesSelectManyCase = orders.SelectMany(o => o.OrderLines);
        foreach (var flattenedOrderLine in flattenedOrdersLinesSelectManyCase)
        {
            allSKUs.Add(flattenedOrderLine.ProductSKU);
        }

       //If the required result is flattened list which has OrderID, ProductSKU and Quantity,
       //SelectMany with selector is very helpful to get the required result
       //and allows avoiding own For loops what according to my experience do code faster when
       // hundreds of thousands of data rows must be operated
        List<OrderLineForReport> ordersLinesForReport = (List<OrderLineForReport>)orders.SelectMany(o => o.OrderLines,
            (o, ol) => new OrderLineForReport
            {
                OrderID = o.OrderID,
                ProductSKU = ol.ProductSKU,
                Quantity = ol.Quantity
            }).ToList();
    }
}
class Order
{
    public string OrderID { get; set; }
    public List<OrderLine> OrderLines { get; set; }
}
class OrderLine
{
    public string ProductSKU { get; set; }
    public int Quantity { get; set; }
}
class OrderLineForReport
{
    public string OrderID { get; set; }
    public string ProductSKU { get; set; }
    public int Quantity { get; set; }
}

var players = db.SoccerTeams.Where(c => c.Country == "Spain")
                            .SelectMany(c => c.players);

foreach(var player in players)
{
    Console.WriteLine(player.LastName);
}

De Gea 阿尔芭 科斯塔 别墅 Busquets

...

SelectMany()方法用于抚平序列，其中序列的每个元素都是独立的。

我的类用户是这样的

class User
    {
        public string UserName { get; set; }
        public List<string> Roles { get; set; }
    }

主要:

var users = new List<User>
            {
                new User { UserName = "Reza" , Roles = new List<string>{"Superadmin" } },
                new User { UserName = "Amin" , Roles = new List<string>{"Guest","Reseption" } },
                new User { UserName = "Nima" , Roles = new List<string>{"Nurse","Guest" } },
            };

var query = users.SelectMany(user => user.Roles, (user, role) => new { user.UserName, role });

foreach (var obj in query)
{
    Console.WriteLine(obj);
}
//output

//{ UserName = Reza, role = Superadmin }
//{ UserName = Amin, role = Guest }
//{ UserName = Amin, role = Reseption }
//{ UserName = Nima, role = Nurse }
//{ UserName = Nima, role = Guest }

您可以对序列的任何项使用操作

int[][] numbers = {
                new[] {1, 2, 3},
                new[] {4},
                new[] {5, 6 , 6 , 2 , 7, 8},
                new[] {12, 14}
            };

IEnumerable<int> result = numbers
                .SelectMany(array => array.Distinct())
                .OrderBy(x => x);

//output

//{ 1, 2 , 2 , 3, 4, 5, 6, 7, 8, 12, 14 }

 List<List<int>> numbers = new List<List<int>> {
                new List<int> {1, 2, 3},
                new List<int> {12},
                new List<int> {5, 6, 5, 7},
                new List<int> {10, 10, 10, 12}
            };

 IEnumerable<int> result = numbers
                .SelectMany(list => list)
                .Distinct()
                .OrderBy(x=>x);

//output

// { 1, 2, 3, 5, 6, 7, 10, 12 }

想想这个例子:

        var array = new string[2]
        {
            "I like what I like",
            "I like what you like"
        };
        //query1 returns two elements sth like this:
        //fisrt element would be array[5]  :[0] = "I" "like" "what" "I" "like"
        //second element would be array[5] :[1] = "I" "like" "what" "you" "like"
        IEnumerable<string[]> query1 = array.Select(s => s.Split(' ')).Distinct();

        //query2 return back flat result sth like this :
        // "I" "like" "what" "you"
        IEnumerable<string> query2 = array.SelectMany(s => s.Split(' ')).Distinct();

因此，正如你所看到的，像“I”或“like”这样的重复值已从query2中删除，因为“SelectMany”将在多个序列上平坦和投影。 但是query1返回字符串数组序列。由于query1中有两个不同的数组(第一个和第二个元素)，因此不会删除任何东西。

当查询返回一个字符串(一个char数组)时更清楚:

例如，如果列表" Fruits "包含" apple "

'Select'返回字符串:

Fruits.Select(s=>s) 

[0]: "apple"

'SelectMany'将字符串平展:

Fruits.SelectMany(s=>s)

[0]: 97  'a'
[1]: 112 'p'
[2]: 112 'p'
[3]: 108 'l'
[4]: 101 'e'

Select many就像SQL中的交叉连接操作，它需要叉乘。 例如，如果我们有

Set A={a,b,c}
Set B={x,y}

选择许多可以用来得到下面的集合

{ (x,a) , (x,b) , (x,c) , (y,a) , (y,b) , (y,c) }

注意，这里我们取集合A和集合B中的元素的所有可能组合。

下面是一个你可以尝试的LINQ示例

List<string> animals = new List<string>() { "cat", "dog", "donkey" };
List<int> number = new List<int>() { 10, 20 };

var mix = number.SelectMany(num => animals, (n, a) => new { n, a });

混合将有以下元素在平面结构，如

{(10,cat), (10,dog), (10,donkey), (20,cat), (20,dog), (20,donkey)}