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
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; }
}
其他回答
我理解SelectMany工作起来像一个连接快捷方式。
所以你可以:
var orders = customers
.Where(c => c.CustomerName == "Acme")
.SelectMany(c => c.Orders);
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。
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; }
}
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 }
再举一个如何使用SelectMany + Select来累积子数组对象数据的例子。
假设我们有一些带着手机的用户:
class Phone {
public string BasePart = "555-xxx-xxx";
}
class User {
public string Name = "Xxxxx";
public List<Phone> Phones;
}
现在我们需要选择所有用户的所有手机的basepart:
var usersArray = new List<User>(); // array of arrays
List<string> allBaseParts = usersArray.SelectMany(ua => ua.Phones).Select(p => p.BasePart).ToList();
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