C# LINQ (语言集成查询)

概述

LINQ（语言集成查询）为处理来自不同来源的数据提供了统一的查询语法。它允许您直接在 C# 代码中编写查询。

基本 LINQ 概念

LINQ 查询语法

csharp

using System.Linq;

// 数据源
int[] numbers = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

// 查询语法
var evenNumbers = from num in numbers
                 where num % 2 == 0
                 select num;

// 方法语法（等价）
var evenNumbersMethod = numbers.Where(num => num % 2 == 0);

// 执行查询
foreach (var num in evenNumbers)
{
    Console.WriteLine(num); // 2, 4, 6, 8, 10
}

LINQ 方法语法

csharp

List<string> words = new List<string>
{
    "apple", "banana", "cherry", "date", "elderberry"
};

// 筛选
var longWords = words.Where(w => w.Length > 5);

// 投影
var wordLengths = words.Select(w => new { Word = w, Length = w.Length });

// 排序
var sortedWords = words.OrderBy(w => w);

// 分组
var wordsByLength = words.GroupBy(w => w.Length);

LINQ 操作符

筛选操作符

csharp

public class LinqFiltering
{
    public static void DemonstrateFiltering()
    {
        var products = new[]
        {
            new { Id = 1, Name = "Laptop", Price = 999.99m, Category = "Electronics" },
            new { Id = 2, Name = "Book", Price = 19.99m, Category = "Books" },
            new { Id = 3, Name = "Phone", Price = 699.99m, Category = "Electronics" },
            new { Id = 4, Name = "Pen", Price = 2.99m, Category = "Office" }
        };

        // Where 子句
        var electronics = products.Where(p => p.Category == "Electronics");
        var expensive = products.Where(p => p.Price > 500);
        var cheapBooks = products.Where(p => p.Category == "Books" && p.Price < 20);

        Console.WriteLine("Electronics:");
        foreach (var item in electronics)
        {
            Console.WriteLine($"{item.Name}: ${item.Price}");
        }
    }
}

投影操作符

csharp

public class LinqProjection
{
    public static void DemonstrateProjection()
    {
        var students = new[]
        {
            new { FirstName = "John", LastName = "Doe", Age = 20, Grade = 85 },
            new { FirstName = "Jane", LastName = "Smith", Age = 22, Grade = 92 },
            new { FirstName = "Bob", LastName = "Johnson", Age = 21, Grade = 78 }
        };

        // Select - 转换为新类型
        var studentNames = students.Select(s => $"{s.FirstName} {s.LastName}");
        var studentInfo = students.Select(s => new
        {
            FullName = $"{s.FirstName} {s.LastName}",
            IsAdult = s.Age >= 18
        });

        // SelectMany - 展平集合
        var classes = new[]
        {
            new { Name = "Math", Students = new[] { "Alice", "Bob" } },
            new { Name = "Science", Students = new[] { "Charlie", "David", "Eve" } }
        };

        var allStudents = classes.SelectMany(c => c.Students);

        Console.WriteLine("All students:");
        foreach (var student in allStudents)
        {
            Console.WriteLine(student);
        }
    }
}

排序操作符

csharp

public class LinqOrdering
{
    public static void DemonstrateOrdering()
    {
        var people = new[]
        {
            new { Name = "Alice", Age = 30, Salary = 50000 },
            new { Name = "Bob", Age = 25, Salary = 45000 },
            new { Name = "Charlie", Age = 35, Salary = 60000 },
            new { Name = "David", Age = 28, Salary = 52000 }
        };

        // OrderBy
        var byAge = people.OrderBy(p => p.Age);
        var byName = people.OrderBy(p => p.Name);

        // OrderByDescending
        var bySalaryDesc = people.OrderByDescending(p => p.Salary);

        // ThenBy 用于次要排序
        var byAgeThenName = people
            .OrderBy(p => p.Age)
            .ThenBy(p => p.Name);

        Console.WriteLine("People ordered by age:");
        foreach (var person in byAge)
        {
            Console.WriteLine($"{person.Name}, Age: {person.Age}");
        }
    }
}

分组操作符

csharp

public class LinqGrouping
{
    public static void DemonstrateGrouping()
    {
        var products = new[]
        {
            new { Name = "Laptop", Category = "Electronics", Price = 999.99m },
            new { Name = "Mouse", Category = "Electronics", Price = 29.99m },
            new { Name = "Book", Category = "Books", Price = 19.99m },
            new { Name = "Pen", Category = "Office", Price = 2.99m },
            new { Name = "Phone", Category = "Electronics", Price = 699.99m }
        };

        // GroupBy
        var groupedByCategory = products.GroupBy(p => p.Category);

        foreach (var group in groupedByCategory)
        {
            Console.WriteLine($"Category: {group.Key}");
            foreach (var product in group)
            {
                Console.WriteLine($"  {product.Name}: ${product.Price}");
            }
        }

        // GroupBy 带投影
        var categoryStats = products
            .GroupBy(p => p.Category)
            .Select(g => new
            {
                Category = g.Key,
                Count = g.Count(),
                TotalPrice = g.Sum(p => p.Price),
                AveragePrice = g.Average(p => p.Price)
            });

        Console.WriteLine("\nCategory statistics:");
        foreach (var stat in categoryStats)
        {
            Console.WriteLine($"{stat.Category}: {stat.Count} items, " +
                          $"Avg: ${stat.AveragePrice:F2}");
        }
    }
}

集合操作

csharp

public class LinqSetOperations
{
    public static void DemonstrateSetOperations()
    {
        var set1 = new[] { 1, 2, 3, 4, 5 };
        var set2 = new[] { 4, 5, 6, 7, 8 };

        // Union 并集
        var union = set1.Union(set2);
        Console.WriteLine($"Union: {string.Join(", ", union)}");

        // Intersection 交集
        var intersection = set1.Intersect(set2);
        Console.WriteLine($"Intersection: {string.Join(", ", intersection)}");

        // Except 差集
        var except = set1.Except(set2);
        Console.WriteLine($"Except: {string.Join(", ", except)}");

        // Distinct 去重
        var numbers = new[] { 1, 2, 2, 3, 3, 3, 4 };
        var distinct = numbers.Distinct();
        Console.WriteLine($"Distinct: {string.Join(", ", distinct)}");
    }
}

LINQ 与集合

LINQ 与列表

csharp

public class LinqWithLists
{
    public static void DemonstrateListOperations()
    {
        var employees = new List<Employee>
        {
            new Employee { Id = 1, Name = "Alice", Department = "IT", Salary = 60000 },
            new Employee { Id = 2, Name = "Bob", Department = "HR", Salary = 50000 },
            new Employee { Id = 3, Name = "Charlie", Department = "IT", Salary = 65000 },
            new Employee { Id = 4, Name = "David", Department = "Finance", Salary = 55000 }
        };

        // 查找特定员工
        var employee = employees.FirstOrDefault(e => e.Id == 2);

        // 按部门筛选
        var itEmployees = employees.Where(e => e.Department == "IT").ToList();

        // 计算统计数据
        var totalSalary = employees.Sum(e => e.Salary);
        var averageSalary = employees.Average(e => e.Salary);
        var maxSalary = employees.Max(e => e.Salary);

        // 检查条件
        var hasHighEarner = employees.Any(e => e.Salary > 70000);
        var allHaveSalary = employees.All(e => e.Salary > 0);

        Console.WriteLine($"Total salary: ${totalSalary:N2}");
        Console.WriteLine($"Average salary: ${averageSalary:N2}");
        Console.WriteLine($"Max salary: ${maxSalary:N2}");
    }
}

public class Employee
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Department { get; set; }
    public decimal Salary { get; set; }
}

LINQ 与字典

csharp

public class LinqWithDictionaries
{
    public static void DemonstrateDictionaryOperations()
    {
        var productPrices = new Dictionary<string, decimal>
        {
            ["Laptop"] = 999.99m,
            ["Mouse"] = 29.99m,
            ["Keyboard"] = 79.99m,
            ["Monitor"] = 299.99m
        };

        // 筛选字典
        var expensive = productPrices.Where(kvp => kvp.Value > 100);

        // 转换为不同类型
        var productList = productPrices
            .Select(kvp => new { Name = kvp.Key, Price = kvp.Value })
            .OrderBy(p => p.Price);

        // 按价格范围分组
        var priceGroups = productPrices
            .GroupBy(kvp => kvp.Value < 50 ? "Cheap" :
                           kvp.Value < 200 ? "Medium" : "Expensive");

        Console.WriteLine("Products by price range:");
        foreach (var group in priceGroups)
        {
            Console.WriteLine($"{group.Key}:");
            foreach (var item in group)
            {
                Console.WriteLine($"  {item.Key}: ${item.Value}");
            }
        }
    }
}

高级 LINQ

使用自定义比较器的 LINQ

csharp

public class CustomComparer : IEqualityComparer<string>
{
    public bool Equals(string x, string y)
    {
        return string.Equals(x, y, StringComparison.OrdinalIgnoreCase);
    }

    public int GetHashCode(string obj)
    {
        return obj?.ToUpperInvariant().GetHashCode() ?? 0;
    }
}

public class LinqCustomComparers
{
    public static void DemonstrateCustomComparers()
    {
        var words = new[] { "Apple", "apple", "Banana", "banana", "Cherry" };

        // 不区分大小写的去重
        var distinctWords = words.Distinct(new CustomComparer());

        Console.WriteLine("Distinct words (case-insensitive):");
        foreach (var word in distinctWords)
        {
            Console.WriteLine(word);
        }
    }
}

使用匿名类型的 LINQ

csharp

public class LinqAnonymousTypes
{
    public static void DemonstrateAnonymousTypes()
    {
        var people = new[]
        {
            new { FirstName = "John", LastName = "Doe", Age = 30, City = "New York" },
            new { FirstName = "Jane", LastName = "Smith", Age = 25, City = "Los Angeles" },
            new { FirstName = "Bob", LastName = "Johnson", Age = 35, City = "Chicago" }
        };

        // 创建匿名类型
        var summary = people
            .GroupBy(p => p.City)
            .Select(g => new
            {
                City = g.Key,
                Count = g.Count(),
                AverageAge = g.Average(p => p.Age),
                Names = g.Select(p => $"{p.FirstName} {p.LastName}").ToList()
            });

        foreach (var city in summary)
        {
            Console.WriteLine($"City: {city.City}");
            Console.WriteLine($"  Count: {city.Count}");
            Console.WriteLine($"  Average Age: {city.AverageAge:F1}");
            Console.WriteLine($"  Names: {string.Join(", ", city.Names)}");
        }
    }
}

LINQ to XML

使用 LINQ 查询 XML

csharp

using System.Xml.Linq;

public class LinqToXml
{
    public static void DemonstrateXmlQuerying()
    {
        // 创建 XML 文档
        var xml = XElement.Parse(@"
            <books>
                <book id='1' category='Fiction'>
                    <title>The Great Gatsby</title>
                    <author>F. Scott Fitzgerald</author>
                    <price>12.99</price>
                </book>
                <book id='2' category='Non-Fiction'>
                    <title>Thinking, Fast and Slow</title>
                    <author>Daniel Kahneman</author>
                    <price>16.99</price>
                </book>
                <book id='3' category='Fiction'>
                    <title>1984</title>
                    <author>George Orwell</author>
                    <price>14.99</price>
                </book>
            </books>");

        // 使用 LINQ 查询 XML
        var fictionBooks = from book in xml.Elements("book")
                         where (string)book.Attribute("category") == "Fiction"
                         select new
                         {
                             Title = (string)book.Element("title"),
                             Author = (string)book.Element("author"),
                             Price = (decimal)book.Element("price"),
                             Id = (int)book.Attribute("id")
                         };

        Console.WriteLine("Fiction books:");
        foreach (var book in fictionBooks)
        {
            Console.WriteLine($"{book.Title} by {book.Author} - ${book.Price}");
        }
    }
}

性能考虑

延迟执行与立即执行

csharp

public class LinqPerformance
{
    public static void DemonstrateExecution()
    {
        var numbers = Enumerable.Range(1, 1000000);

        // 延迟执行 - 查询尚未执行
        var query = numbers.Where(n => n % 2 == 0)
                        .Select(n => n * 2);

        // 立即执行 - 现在执行查询
        var result = query.ToList();

        // 使用 ToList()、ToArray()、Count() 等强制立即执行
        var count = numbers.Count(n => n > 500000);

        Console.WriteLine($"Count: {count}");
        Console.WriteLine($"First 5 results: {string.Join(", ", result.Take(5))}");
    }
}

优化 LINQ 查询

csharp

public class LinqOptimization
{
    public static void DemonstrateOptimization()
    {
        var largeList = Enumerable.Range(1, 1000000).ToList();

        // 不好：多次枚举
        // var count = largeList.Where(n => n > 500000).Count();
        // var sum = largeList.Where(n => n > 500000).Sum();

        // 好：单次枚举
        var filtered = largeList.Where(n => n > 500000);
        var count = filtered.Count();
        var sum = filtered.Sum();

        // 更好：使用特定的操作符
        var count2 = largeList.Count(n => n > 500000);
        var sum2 = largeList.Where(n => n > 500000).Sum();

        Console.WriteLine($"Count: {count}, Sum: {sum}");
    }
}

实际示例

数据分析示例

csharp

public class DataAnalysis
{
    public static void AnalyzeSalesData()
    {
        var sales = new[]
        {
            new { Date = DateTime.Parse("2023-01-01"), Product = "Laptop", Amount = 1200, Region = "North" },
            new { Date = DateTime.Parse("2023-01-02"), Product = "Mouse", Amount = 30, Region = "South" },
            new { Date = DateTime.Parse("2023-01-03"), Product = "Laptop", Amount = 1100, Region = "East" },
            new { Date = DateTime.Parse("2023-01-04"), Product = "Keyboard", Amount = 80, Region = "North" },
            new { Date = DateTime.Parse("2023-01-05"), Product = "Mouse", Amount = 35, Region = "West" }
        };

        // 按地区销售
        var salesByRegion = sales
            .GroupBy(s => s.Region)
            .Select(g => new
            {
                Region = g.Key,
                TotalSales = g.Sum(s => s.Amount),
                AverageSale = g.Average(s => s.Amount),
                Count = g.Count()
            })
            .OrderByDescending(r => r.TotalSales);

        Console.WriteLine("Sales by Region:");
        foreach (var region in salesByRegion)
        {
            Console.WriteLine($"{region.Region}: Total=${region.TotalSales:N2}, " +
                          $"Avg=${region.AverageSale:N2}, Count={region.Count}");
        }

        // 按产品销售
        var salesByProduct = sales
            .GroupBy(s => s.Product)
            .Select(g => new
            {
                Product = g.Key,
                TotalRevenue = g.Sum(s => s.Amount),
                UnitsSold = g.Count(),
                AveragePrice = g.Average(s => s.Amount)
            });

        Console.WriteLine("\nSales by Product:");
        foreach (var product in salesByProduct)
        {
            Console.WriteLine($"{product.Product}: Revenue=${product.TotalRevenue:N2}, " +
                          $"Units={product.UnitsSold}, Avg Price=${product.AveragePrice:N2}");
        }
    }
}

报告生成

csharp

public class ReportGenerator
{
    public static void GenerateEmployeeReport()
    {
        var employees = new[]
        {
            new { Name = "Alice", Department = "IT", Salary = 60000, HireDate = DateTime.Parse("2020-01-15") },
            new { Name = "Bob", Department = "HR", Salary = 50000, HireDate = DateTime.Parse("2019-03-20") },
            new { Name = "Charlie", Department = "IT", Salary = 65000, HireDate = DateTime.Parse("2021-06-10") },
            new { Name = "David", Department = "Finance", Salary = 55000, HireDate = DateTime.Parse("2018-11-05") }
        };

        // 部门摘要
        var departmentSummary = employees
            .GroupBy(e => e.Department)
            .Select(g => new
            {
                Department = g.Key,
                EmployeeCount = g.Count(),
                TotalSalary = g.Sum(e => e.Salary),
                AverageSalary = g.Average(e => e.Salary),
                NewestHire = g.Max(e => e.HireDate),
                Employees = g.OrderBy(e => e.Name).ToList()
            });

        Console.WriteLine("=== Employee Report ===");
        foreach (var dept in departmentSummary)
        {
            Console.WriteLine($"\nDepartment: {dept.Department}");
            Console.WriteLine($"Employees: {dept.EmployeeCount}");
            Console.WriteLine($"Total Salary: ${dept.TotalSalary:N2}");
            Console.WriteLine($"Average Salary: ${dept.AverageSalary:N2}");
            Console.WriteLine($"Newest Hire: {dept.NewestHire:yyyy-MM-dd}");
            Console.WriteLine("Employees:");

            foreach (var emp in dept.Employees)
            {
                int yearsOfService = (int)((DateTime.Now - emp.HireDate).TotalDays / 365);
                Console.WriteLine($"  {emp.Name} - {yearsOfService} years");
            }
        }
    }
}

最佳实践

LINQ 指南

csharp

// 好：使用适当的 LINQ 方法
var result = data.Where(x => x.IsValid)
               .OrderBy(x => x.Name)
               .Take(10);

// 不好：复杂的嵌套查询
var badResult = data.Where(x =>
    data.Where(y => y.Id == x.ParentId)
         .Select(y => y.Name)
         .FirstOrDefault() == "Expected");

// 好：将复杂查询分解为步骤
var parentIds = data.Where(x => x.IsValid).Select(x => x.ParentId);
var parentNames = data.Where(x => parentIds.Contains(x.Id))
                   .ToDictionary(x => x.Id, x => x.Name);

性能提示

csharp

// 好：尽可能使用特定的操作符
var count = collection.Count(x => x.IsActive);

// 不好：当存在带谓词的 Count() 时使用 Where().Count()
var badCount = collection.Where(x => x.IsActive).Count();

// 好：对数据库查询使用 AsEnumerable()
var dbResults = dbContext.Products
    .Where(p => p.Price > 100)
    .AsEnumerable()
    .Select(p => new { p.Name, FormattedPrice = $"${p.Price:N2}" });

总结

在本章中，您学习了：

LINQ 查询语法与方法语法
筛选、投影、排序和分组操作符
集合操作和聚合
不同集合类型的 LINQ
自定义比较器等高级功能
LINQ to XML
性能考虑和最佳实践

LINQ 提供了一种强大、一致的方式来查询和操作来自各种来源的数据。掌握 LINQ 将使您在处理 C# 中的集合和数据时更加高效。

C# LINQ (语言集成查询) ​

概述 ​

基本 LINQ 概念 ​

LINQ 查询语法 ​

LINQ 方法语法 ​

LINQ 操作符 ​

筛选操作符 ​

投影操作符 ​

排序操作符 ​

分组操作符 ​

集合操作 ​

LINQ 与集合 ​

LINQ 与列表 ​

LINQ 与字典 ​

高级 LINQ ​

使用自定义比较器的 LINQ ​

使用匿名类型的 LINQ ​

LINQ to XML ​

使用 LINQ 查询 XML ​

性能考虑 ​

延迟执行与立即执行 ​

优化 LINQ 查询 ​

实际示例 ​

数据分析示例 ​

报告生成 ​

最佳实践 ​

LINQ 指南 ​

性能提示 ​

总结 ​

C# LINQ (语言集成查询)

概述

基本 LINQ 概念

LINQ 查询语法

LINQ 方法语法

LINQ 操作符

筛选操作符

投影操作符

排序操作符

分组操作符

集合操作

LINQ 与集合

LINQ 与列表

LINQ 与字典

高级 LINQ

使用自定义比较器的 LINQ

使用匿名类型的 LINQ

LINQ to XML

使用 LINQ 查询 XML

性能考虑

延迟执行与立即执行

优化 LINQ 查询

实际示例

数据分析示例

报告生成

最佳实践

LINQ 指南

性能提示

总结