LINQ教程 (更新中...)

关于教程

LINQ是英文Language Integrated Query的缩写，中文译为语言集成查询，发音"link"(我们听到有部分人发作"link-queue"其实是不正确的)。

LINQ是微软的一项技术，新增一种自然查询的SQL语法到.NET Framework的编程语言中，于2007年随.NET 3.5和Visual Studio 2008中引入的一种功能强大的查询语言。

LINQ可以与C#或Visual Basic、F#等语言一起从不同的数据源查询数据。

作为开发人员，理解LINQ是非常重要的，因为在大多数实时应用程序中，你将使用LINQ执行不同类型的操作。在本系列教程中，我们将使用C#语言编写LINQ查询。

《LINQ教程》将帮助你从基本到高级，从入门到实战，由浅入深地系统学习LINQ语言。这些教程分为一系列相关的主题，让你从一个必须先理解的主题开始，然后循序渐进地学习LINQ的其他特性。《LINQ教程》包含了大量易于理解的真实示例，帮助读者朋友们将知识点迅速转化到实际生产。

在这个《LINQ教程》系列文章中，我们将涵盖C#中LINQ的所有基本和高级概念。

前提条件

由于本教程系列并不是入门级的，所以在进行本教程学习之前，你需要准备或者掌握：

• C# 3.5或以上版本的基础知识
• Visual Studio 2019或以上版本(或者VS Code,Rider等)IDE
• .NET 5或以上版本SDK

本系列教程用到的示例模型(类)及模拟数据：

/// <summary>
/// 订单实体
/// </summary>
public class Order
{
    /// <summary>
    /// ID
    /// </summary>
    public int Id { get; set; }
    /// <summary>
    /// 客户名称
    /// </summary>
    public string Customer { get; set; }
    /// <summary>
    /// 订单金额
    /// </summary>
    public double Price { get; set; }
    /// <summary>
    /// 订单来源
    /// </summary>
    public string Source { get; set; }
    /// <summary>
    /// 下单时间
    /// </summary>
    public DateTime CreatedAt { get; set; }
    /// <summary>
    /// 收货地址
    /// </summary>
    public ShippingAddress Address { get; set; }
}

public class Customer
{
    public Customer()
    {
    }

    public Customer(int id, int age, string gender, string name, int addressId)
    {
        Id = id;
        Age = age;
        Gender = gender;
        Name = name;
        AddressId = addressId;
    }

    public int Id { get; set; }
    public string Name { get; set; }
    public int Age { get; set; }
    public string Gender { get; set; }
    public int AddressId { get; set; }
}

/// <summary>
/// 收货地址
/// </summary>
public class ShippingAddress
{
    public int Id { get; set; }
    /// <summary>
    /// 省份
    /// </summary>
    public string Province { get; set; }
    /// <summary>
    /// 城市
    /// </summary>
    public string City { get; set; }
    /// <summary>
    /// 地区
    /// </summary>
    public string District { get; set; }
    /// <summary>
    /// 街道
    /// </summary>
    public string Street { get; set; }
}


/// <summary>
/// 模拟数据
/// </summary>
public static class FakeData
{
    /// <summary>
    /// 模拟订单集合数据
    /// </summary>
    public static List<Order> Orders => new()
    {
        new Order
        {
            Id = 1,
            Customer = "Rector",
            Price = 3699.00D,
            Source = "线上",
            CreatedAt = new DateTime(2021, 5, 1, 20, 30, 25),
            Address = Address1
        },
        new Order
        {
            Id = 2,
            Customer = "James",
            Price = 2699.00D,
            Source = "线上",
            CreatedAt = new DateTime(2021, 6, 15, 18, 06, 05),
            Address = Address2
        },
        new Order
        {
            Id = 3,
            Customer = "Chris",
            Price = 1999.00D,
            Source = "线下",
            CreatedAt = new DateTime(2021, 7, 21, 20, 10, 02),
            Address = Address3
        },
        new Order
        {
            Id = 4,
            Customer = "Steven",
            Price = 5699.00D,
            Source = "线上",
            CreatedAt = new DateTime(2021, 8, 30, 09, 30, 25),
            Address = Address4
        },
        new Order
        {
            Id = 5,
            Customer = "Jo",
            Price = 2569.00D,
            Source = "线下",
            CreatedAt = new DateTime(2021, 9, 11, 10, 28, 25),
            Address = Address1
        },
        new Order
        {
            Id = 6,
            Customer = "Rector",
            Price = 5699.00D,
            Source = "线上",
            CreatedAt = new DateTime(2021, 10, 16, 16, 30, 08),
            Address = Address1
        }
    };

    public static List<Customer> Customers => new()
    {
        new Customer(id: 1, age: 20, gender: "男", name: "Rector", addressId: 1),
        new Customer(id: 2, age: 24, gender: "女", name: "Anna", addressId: 2),
        new Customer(id: 3, age: 26, gender: "女", name: "Xi", addressId: 1),
        new Customer(id: 4, age: 30, gender: "男", name: "Curry", addressId: 5)
    };

    public static ShippingAddress Address1 => new()
    {
        Id = 1,
        Province = "重庆",
        City = "重庆",
        District = "渝北区",
        Street = "回兴街道"
    };
    public static ShippingAddress Address2 => new()
    {
        Id = 2,
        Province = "重庆",
        City = "重庆",
        District = "江北区",
        Street = "观音桥街道"
    };
    public static ShippingAddress Address3 => new()
    {
        Id = 3,
        Province = "四川",
        City = "成都",
        District = "金牛区",
        Street = "西华街道"
    };
    public static ShippingAddress Address4 => new()
    {
        Id = 4,
        Province = "四川",
        City = "成都",
        District = "青羊区",
        Street = "草市街街道"
    };
    public static ShippingAddress Address5 => new()
    {
        Id = 5,
        Province = "上海",
        City = "上海",
        District = "黄浦区",
        Street = "外滩街道"
    };

    public static List<ShippingAddress> Addresses =>
        new()
        {
            Address1,
            Address2,
            Address3,
            Address4,
            Address5
        };
}

注：示例模型仅为了演示，不代表实际生产的代码和逻辑(仅供学习参考)。