SelectMany()的正式描述是:

将序列的每个元素投射到IEnumerable上并展开 产生的序列变成一个序列。

SelectMany()将结果序列平铺成一个序列，并对其中的每个元素调用结果选择器函数。

class PetOwner
{
    public string Name { get; set; }
    public List<String> Pets { get; set; }
}

public static void SelectManyEx()
{
     PetOwner[] petOwners =
         { new PetOwner { Name="Higa, Sidney",
              Pets = new List<string>{ "Scruffy", "Sam" } },
           new PetOwner { Name="Ashkenazi, Ronen",
              Pets = new List<string>{ "Walker", "Sugar" } },
           new PetOwner { Name="Price, Vernette",
              Pets = new List<string>{ "Scratches", "Diesel" } } };

// Query using SelectMany().
IEnumerable<string> query1 = petOwners.SelectMany(petOwner => petOwner.Pets);

Console.WriteLine("Using SelectMany():");

// Only one foreach loop is required to iterate
// through the results since it is a
// one-dimensional collection.
foreach (string pet in query1)
{
    Console.WriteLine(pet);
}

// This code shows how to use Select()
// instead of SelectMany().
IEnumerable<List<String>> query2 =
    petOwners.Select(petOwner => petOwner.Pets);

Console.WriteLine("\nUsing Select():");

// Notice that two foreach loops are required to
// iterate through the results
// because the query returns a collection of arrays.
foreach (List<String> petList in query2)
{
    foreach (string pet in petList)
    {
        Console.WriteLine(pet);
    }
    Console.WriteLine();
}
}

/*
   This code produces the following output:

    Using SelectMany():
    Scruffy
    Sam
    Walker
    Sugar
    Scratches
    Diesel

   Using Select():
   Scruffy
   Sam

   Walker
   Sugar

   Scratches
   Diesel
  */

主要的区别是每个方法的结果，而SelectMany()返回一个扁平的结果;Select()返回一个列表的列表，而不是一个扁平的结果集。

因此，SelectMany的结果是一个列表

{斯库菲，山姆，沃克，甜甜，划痕，迪塞尔}

你可以迭代每一项。但是对于select的结果，您需要一个额外的foreach循环来遍历结果，因为查询返回一个数组集合。

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

...

Some SelectMany可能不需要。以下2个查询给出相同的结果。

Customers.Where(c=>c.Name=="Tom").SelectMany(c=>c.Orders)

Orders.Where(o=>o.Customer.Name=="Tom")

对于一对多关系，

如果从“1”开始，则需要SelectMany，它将平展many。 如果从“Many”开始，则不需要SelectMany。(仍然能够从“1”中过滤，这也比以下标准的连接查询更简单)

from o in Orders
join c in Customers on o.CustomerID equals c.ID
where c.Name == "Tom"
select o

我认为这是理解的最好方式。

            var query =
            Enumerable
                .Range(1, 10)
                .SelectMany(ints => Enumerable.Range(1, 10), (a, b) => $"{a} * {b} = {a * b}")
                .ToArray();

        Console.WriteLine(string.Join(Environment.NewLine, query));

        Console.Read();

乘法表的例子。

SelectMany()的正式描述是:

将序列的每个元素投射到IEnumerable上并展开 产生的序列变成一个序列。

SelectMany()将结果序列平铺成一个序列，并对其中的每个元素调用结果选择器函数。

class PetOwner
{
    public string Name { get; set; }
    public List<String> Pets { get; set; }
}

public static void SelectManyEx()
{
     PetOwner[] petOwners =
         { new PetOwner { Name="Higa, Sidney",
              Pets = new List<string>{ "Scruffy", "Sam" } },
           new PetOwner { Name="Ashkenazi, Ronen",
              Pets = new List<string>{ "Walker", "Sugar" } },
           new PetOwner { Name="Price, Vernette",
              Pets = new List<string>{ "Scratches", "Diesel" } } };

// Query using SelectMany().
IEnumerable<string> query1 = petOwners.SelectMany(petOwner => petOwner.Pets);

Console.WriteLine("Using SelectMany():");

// Only one foreach loop is required to iterate
// through the results since it is a
// one-dimensional collection.
foreach (string pet in query1)
{
    Console.WriteLine(pet);
}

// This code shows how to use Select()
// instead of SelectMany().
IEnumerable<List<String>> query2 =
    petOwners.Select(petOwner => petOwner.Pets);

Console.WriteLine("\nUsing Select():");

// Notice that two foreach loops are required to
// iterate through the results
// because the query returns a collection of arrays.
foreach (List<String> petList in query2)
{
    foreach (string pet in petList)
    {
        Console.WriteLine(pet);
    }
    Console.WriteLine();
}
}

/*
   This code produces the following output:

    Using SelectMany():
    Scruffy
    Sam
    Walker
    Sugar
    Scratches
    Diesel

   Using Select():
   Scruffy
   Sam

   Walker
   Sugar

   Scratches
   Diesel
  */

主要的区别是每个方法的结果，而SelectMany()返回一个扁平的结果;Select()返回一个列表的列表，而不是一个扁平的结果集。

因此，SelectMany的结果是一个列表

{斯库菲，山姆，沃克，甜甜，划痕，迪塞尔}

你可以迭代每一项。但是对于select的结果，您需要一个额外的foreach循环来遍历结果，因为查询返回一个数组集合。

SelectMany有几个重载。其中之一允许您在遍历层次结构时跟踪父节点和子节点之间的任何关系。

示例:假设您有以下结构:League -> Teams -> Player。

您可以很容易地返回一个平坦的播放器集合。但是你可能会失去任何关于玩家所在团队的信息。

幸运的是，有一个重载用于此目的:

var teamsAndTheirLeagues = 
         from helper in leagues.SelectMany
               ( l => l.Teams
                 , ( league, team ) => new { league, team } )
                      where helper.team.Players.Count > 2 
                           && helper.league.Teams.Count < 10
                           select new 
                                  { LeagueID = helper.league.ID
                                    , Team = helper.team 
                                   };

前面的例子来自Dan的IK博客。我强烈建议你看一看。