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
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。
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);
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
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; }
}
假设你有一组国家
var countries = new[] { "France", "Italy" };
如果你对国家执行Select,你将得到数组中的每个元素IEnumerable<T>
IEnumerable<string> selectQuery = countries.Select(country => country);
在上面的代码中,国家表示指向数组中每个国家的字符串。现在迭代selectQuery以获得国家:
foreach(var country in selectQuery)
Console.WriteLine(country);
// output
//
// France
// Italy
如果你想打印每个国家的字符,你必须使用嵌套foreach
foreach (var country in selectQuery)
{
foreach (var charOfCountry in country)
{
Console.Write(charOfCountry + ", ");
}
}
// output
// F, r, a, n, c, e, I, t, a, l, y,
好的。现在尝试对国家执行SelectMany。这一次SelectMany获取每个国家作为字符串(和以前一样),因为字符串类型是一个字符的集合,SelectMany尝试将每个国家分为它的组成部分(字符),然后返回一个字符的集合IEnumerable<T>
IEnumerable<char> selectManyQuery = countries.SelectMany(country => country);
在上面的代码中,country表示一个字符串,该字符串引用数组中的每个国家,但返回值是每个国家的字符
实际上SelectMany喜欢在集合中获取两层,并将第二层平铺为IEnumerable<T>
现在遍历selectManyQuery以获得每个国家的字符:
foreach(var charOfCountry in selectManyQuery)
Console.Write(charOfCountry + ", ");
// output
// F, r, a, n, c, e, I, t, a, l, y,
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。