我一直在搜索Select和SelectMany之间的区别,但我还没有找到合适的答案。我需要学习使用LINQ to SQL时的差异,但我所找到的都是标准数组示例。
有人能提供一个LINQ到SQL的例子吗?
我一直在搜索Select和SelectMany之间的区别,但我还没有找到合适的答案。我需要学习使用LINQ to SQL时的差异,但我所找到的都是标准数组示例。
有人能提供一个LINQ到SQL的例子吗?
当前回答
SelectMany方法将IEnumerable<IEnumerable<T>>分解为IEnumerable<T>,就像共产主义一样,每个元素都以相同的方式表现(愚蠢的家伙拥有与天才相同的权利)。
var words = new [] { "a,b,c", "d,e", "f" };
var splitAndCombine = words.SelectMany(x => x.Split(','));
// returns { "a", "b", "c", "d", "e", "f" }
其他回答
select操作符用于从集合中选择值,SelectMany操作符用于从集合的集合(即嵌套集合)中选择值。
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)}
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
...
没有太多的技术-数据库与许多组织,每个有许多用户:-
var orgId = "123456789";
var userList1 = db.Organizations
.Where(a => a.OrganizationId == orgId)
.SelectMany(a => a.Users)
.ToList();
var userList2 = db.Users
.Where(a => a.OrganizationId == orgId)
.ToList();
都为所选组织返回相同的ApplicationUser列表。
第一个“项目”从Organization到Users,第二个直接查询Users表。
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; }
}