选择许多困惑

根据我对 SelectMany 文档的理解，可以使用它生成1-many 关系的(扁平化)序列。

我有下列课程

  public class Customer
{
public int Id { get; set; }
public string Name { get; set; }
}


class Order
{
public int Id { get; set; }
public int CustomerId { get; set; }
public string Description { get; set; }
}

然后，我尝试使用这样的查询表达式语法来使用它们

  var customers = new Customer[]
{
new Customer() { Id=1, Name ="A"},
new Customer() { Id=2, Name ="B"},
new Customer() { Id=3, Name ="C"}
};


var orders = new Order[]
{
new Order { Id=1, CustomerId=1, Description="Order 1"},
new Order { Id=2, CustomerId=1, Description="Order 2"},
new Order { Id=3, CustomerId=1, Description="Order 3"},
new Order { Id=4, CustomerId=1, Description="Order 4"},
new Order { Id=5, CustomerId=2, Description="Order 5"},
new Order { Id=6, CustomerId=2, Description="Order 6"},
new Order { Id=7, CustomerId=3, Description="Order 7"},
new Order { Id=8, CustomerId=3, Description="Order 8"},
new Order { Id=9, CustomerId=3, Description="Order 9"}
};


var customerOrders = from c in customers
from o in orders
where o.CustomerId == c.Id
select new
{
CustomerId = c.Id
, OrderDescription = o.Description
};


foreach (var item in customerOrders)
Console.WriteLine(item.CustomerId + ": " + item.OrderDescription);

这就是我想要的。

1: Order 1
1: Order 2
1: Order 3
1: Order 4
2: Order 5
2: Order 6
3: Order 7
3: Order 8
3: Order 9

我假设这转化为在不使用查询表达式语法时使用 SelectMany 方法？

无论哪种方式，我都在尝试使用 SelectMany。因此，即使我上面的查询没有转换成 SelectMany，考虑到这两个类和模拟数据，是否有人可以为我提供一个使用 SelectMany 的 linq 查询？

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小开

SelectMany() works like Select, but with that extra feature of flattening a collection that is selected. It should be used whenever you want a projection of elements of sub-collections, and don't care about the sub-collection's containing element.

For example, let's say your domain looked like this:

public class Customer
{
public int Id { get; set; }
public string Name { get; set; }
public List<Order> Orders { get; set; }
}


class Order
{
public int Id { get; set; }
public Customer Customer { get; set; }
public string Description { get; set; }
}

To get the same list you wanted, your Linq would look something like this:

var customerOrders = Customers
.SelectMany(c=>c.Orders)
.Select(o=> new { CustomerId = o.Customer.Id,
OrderDescription = o.Description });

... which will produce the same result without needing the flat collection of Orders. The SelectMany takes each Customer's Orders collection and iterates through that to produce an IEnumerable<Order> from an IEnumerable<Customer>.

小开
最佳答案

Here is your query using SelectMany, modeled exactly after your example. Same output!

        var customerOrders2 = customers.SelectMany(
c => orders.Where(o => o.CustomerId == c.Id),
(c, o) => new { CustomerId = c.Id, OrderDescription = o.Description });

The first argument maps each customer to a collection of orders (completely analagous to the 'where' clause you already have).

The second argument transforms each matched pair {(c1, o1), (c1, o2) .. (c3, o9)} into a new type, which I've made the same as your example.

So:

  • arg1 maps each element in the base collection to another collection.
  • arg2 (optional) transforms each pair into a new type

The resulting collection is flat like you'd expect in your original example.

If you were to omit the second argument, you would end up with a collection of all orders the match up to a customer. It'd be just that, a flat collection of Order objects.

Using it takes a lot of getting used to, I still have trouble wrapping my head around it sometimes. :(

小开

Though this is an old question, I thought I would improve the excellent answers a little:

SelectMany returns a list (which may be empty) for each element of the controlling list. Each element in these result lists are enumerated into the expressions' output sequence and so are concatenated into the result. Hence, a' list -> b' list[] -> concatenate -> b' list.

using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using System.Linq;
using System.Diagnostics;
namespace Nop.Plugin.Misc.WebServices.Test
{
[TestClass]
public class TestBase
{
[TestMethod]
public void TestMethod1()
{  //See result in TestExplorer - test output
var a = new int[]{7,8};
var b = new int[]
{12,23,343,6464,232,75676,213,1232,544,86,97867,43};
Func<int, int, bool> numberHasDigit =
(number
, digit) =>
( number.ToString().Contains(digit.ToString()) );


Debug.WriteLine("Unfiltered: All elements of 'b' for each element of 'a'");
foreach(var l in a.SelectMany(aa => b))
Debug.WriteLine(l);
Debug.WriteLine(string.Empty);
Debug.WriteLine("Filtered:" +
"All elements of 'b' for each element of 'a' filtered by the 'a' element");
foreach(var l in a.SelectMany(aa => b.Where(bb => numberHasDigit(bb, aa))))
Debug.WriteLine(l);
}
}
}
小开

Here is another option using SelectMany

var customerOrders = customers.SelectMany(
c => orders.Where(o => o.CustomerId == c.Id)
.Select(p => new {CustomerId = c.Id, OrderDescription = p.Description}));

If you use the Entity Framework or LINQ to Sql and you have an association (relationship) between the entities, then you can do so:

var customerOrders = customers.SelectMany(
c => c.orders
.Select(p => new {CustomerId = c.Id, OrderDescription = p.Description}));

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