EF Core 共變查詢（TPH 繼承映射).linq

<Query Kind="Program">
  <NuGetReference>Microsoft.EntityFrameworkCore</NuGetReference>
  <NuGetReference>Microsoft.EntityFrameworkCore.InMemory</NuGetReference>
  <Namespace>Microsoft.EntityFrameworkCore</Namespace>
</Query>

// 1️⃣ 定義基底與衍生類別
public class Person
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class Employee : Person
{
	public string Department { get; set; }
}

// 2️⃣ 建立 DbContext
public class MyAppContext : DbContext
{
	public DbSet<Person> People { get; set; }

	protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
	{
		// 使用本地 SQLite 測試資料庫
		optionsBuilder.UseInMemoryDatabase("TestDb");
	}

	protected override void OnModelCreating(ModelBuilder modelBuilder)
	{
		// 預設是 TPH (Table-Per-Hierarchy) 模式
		// EF Core 會自動建立 Discriminator 欄位
		modelBuilder.Entity<Person>()
			.HasDiscriminator<string>("PersonType")
			.HasValue<Person>("Person")
			.HasValue<Employee>("Employee");
	}
}

// 3️⃣ 測試程式
class Program
{
	static void Main()
	{
		using var context = new MyAppContext();
		context.Database.EnsureDeleted();
		context.Database.EnsureCreated();

		// 新增一些資料
		context.People.AddRange(new Person { Name = "Ray" },
								new Employee { Name = "Emma", Department = "IT" });
		context.SaveChanges();

		// 共變查詢：從 Person DbSet 取出 Employee
		var employees = context.People.OfType<Employee>().ToList();

		Console.WriteLine("Employees:");
		foreach (var e in employees)
		{
			Console.WriteLine($"Name: {e.Name}, Department: {e.Department}");
		}
	}
}