LINQ（Language-Integrated Query）是C#语言中的一个强大的查询技术，它提供了一种统一的查询语法，可以用于查询和操作各种数据源，包括集合、数据库、XML等。下面详细描述了LINQ的20个功能点，并提供了相应的源代码。
- 查询集合中的所有元素：
int[] numbers = { 1, 2, 3, 4, 5 };
var query = from num in numbers
select num;
foreach (var num in query)
{
Console.WriteLine(num);
}
- 使用条件过滤集合中的元素：
int[] numbers = { 1, 2, 3, 4, 5 };
var query = from num in numbers
where num % 2 == 0
select num;
foreach (var num in query)
{
Console.WriteLine(num);
}
- 对集合中的元素进行排序：
int[] numbers = { 5, 3, 1, 4, 2 };
var query = from num in numbers
orderby num
select num;
foreach (var num in query)
{
Console.WriteLine(num);
}
- 对集合中的元素进行分组：
string[] names = { "Alice", "Bob", "Charlie", "David", "Eve" };
var query = from name in names
group name by name[0];
foreach (var group in query)
{
Console.WriteLine(group.Key);
foreach (var name in group)
{
Console.WriteLine(name);
}
}
- 对集合中的元素进行投影：
string[] names = { "Alice", "Bob", "Charlie", "David", "Eve" };
var query = from name in names
select name.Length;
foreach (var length in query)
{
Console.WriteLine(length);
}
- 连接两个集合的元素：
string[] names1 = { "Alice", "Bob", "Charlie" };
string[] names2 = { "David", "Eve", "Frank" };
var query = from name1 in names1
join name2 in names2 on name1[0] equals name2[0]
select new { Name1 = name1, Name2 = name2 };
foreach (var pair in query)
{
Console.WriteLine($"{pair.Name1}, {pair.Name2}");
}
- 使用LINQ查询数据库：
using (var dbContext = new MyDbContext())
{
var query = from user in dbContext.Users
where user.Age > 18
select user;
foreach (var user in query)
{
Console.WriteLine(user.Name);
}
}
- 使用LINQ查询XML文档：
XDocument doc = XDocument.Load("data.xml");
var query = from element in doc.Descendants("book")
where (int)element.Element("price") > 10
select element.Element("title").Value;
foreach (var title in query)
{
Console.WriteLine(title);
}
- 使用LINQ查询JSON数据：
string json = File.ReadAllText("data.json");
var data = JsonConvert.DeserializeObject>(json);
var query = from person in data
where person.Age > 18
select person.Name;
foreach (var name in query)
{
Console.WriteLine(name);
}
- 使用LINQ查询并转换结果：
int[] numbers = { 1, 2, 3, 4, 5 };
var query = from num in numbers
where num % 2 == 0
select num * 2;
foreach (var num in query)
{
Console.WriteLine(num);
}
- 使用LINQ查询返回匿名类型：
string[] names = { "Alice", "Bob", "Charlie", "David", "Eve" };
var query = from name in names
select new { Name = name, Length = name.Length };
foreach (var item in query)
{
Console.WriteLine($"{item.Name}, {item.Length}");
}
- 使用LINQ查询返回自定义类型：
public class Person
{
public string Name { get; set; }
public int Age { get; set; }
}
List people = new List
{
new Person { Name = "Alice", Age = 20 },
new Person { Name = "Bob", Age = 25 },
new Person { Name = "Charlie", Age = 30 }
};
var query = from person in people
where person.Age > 25
select person.Name;
foreach (var name in query)
{
Console.WriteLine(name);
}
- 使用LINQ查询返回集合：
int[] numbers = { 1, 2, 3, 4, 5 };
var query = from num in numbers
where num % 2 == 0
select num * 2;
List result = query.ToList();
foreach (var num in result)
{
Console.WriteLine(num);
}
- 使用LINQ进行分页查询：
int pageNumber = 2;
int pageSize = 10;
var query = from product in dbContext.Products
orderby product.Name
select product;
var page = query.Skip((pageNumber - 1) * pageSize).Take(pageSize);
foreach (var product in page)
{
Console.WriteLine(product.Name);
}
- 使用LINQ进行聚合操作：
int[] numbers = { 1, 2, 3, 4, 5 };
int sum = numbers.Sum();
int max = numbers.Max();
int min = numbers.Min();
double average = numbers.Average();
Console.WriteLine($"Sum: {sum}");
Console.WriteLine($"Max: {max}");
Console.WriteLine($"Min: {min}");
Console.WriteLine($"Average: {average}");
- 使用LINQ进行集合的交集操作：
int[] numbers1 = { 1, 2, 3, 4, 5 };
int[] numbers2 = { 4, 5, 6, 7, 8 };
var query = numbers1.Intersect(numbers2);
foreach (var num in query)
{
Console.WriteLine(num);
}
- 使用LINQ进行集合的并集操作：
int[] numbers1 = { 1, 2, 3, 4, 5 };
int[] numbers2 = { 4, 5, 6, 7, 8 };
var query = numbers1.Union(numbers2);
foreach (var num in query)
{
Console.WriteLine(num);
}
- 使用LINQ进行集合的差集操作：
int[] numbers1 = { 1, 2, 3, 4, 5 };
int[] numbers2 = { 4, 5, 6, 7, 8 };
var query = numbers1.Except(numbers2);
foreach (var num in query)
{
Console.WriteLine(num);
}
- 使用LINQ进行集合的连接操作：
var query = from order in dbContext.Orders
join customer in dbContext.Customers on order.CustomerId equals customer.Id
select new { OrderId = order.Id, CustomerName = customer.Name };
foreach (var result in query)
{
Console.WriteLine($"{result.OrderId}, {result.CustomerName}");
}
- 使用LINQ进行集合的逆序操作：
int[] numbers = { 1, 2, 3, 4, 5 };
var query = numbers.Reverse();
foreach (var num in query)
{
Console.WriteLine(num);
}
以上是20个常用的LINQ功能点的示例代码，它们展示了LINQ在C#中的强大查询能力和灵活性。通过使用LINQ，开发人员可以更加方便地进行各种数据操作和查询，提高代码的可读性和开发效率。
原文地址：https://www.toutiao.com/article/7290327773384835624/
