AccessingADerivedDbSetPropertyUsingAbaseDbContext.linq

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

// ---- 測試 ----
using BaseContext ctx = new AppContext();
ctx.Add(new Person { Name = "Ray" });
ctx.Add(new Employee { Name = "Emma", Department = "IT" });
ctx.SaveChanges();

BaseContext baseCtx = ctx;

// ✅ 以基底參考拿衍生實體（不用轉型）
var e1 = baseCtx.Set<Employee>().ToList();
e1.Dump();
// ✅ 共變查詢
var e2 = baseCtx.Set<Person>().OfType<Employee>().ToList();
e2.Dump();
// ❌ 看不到衍生屬性
// baseCtx.Employees  // 編譯錯誤

// ✅ 需要屬性才轉型
var e3 = (ctx as AppContext).Employees.ToList();
e3.Dump();

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

public abstract class BaseContext : DbContext
{
	public DbSet<Person> People { get; set; }

	protected override void OnConfiguring(DbContextOptionsBuilder b)
		=> b.UseInMemoryDatabase("demo");

	protected override void OnModelCreating(ModelBuilder m)
	{
		m.Entity<Person>()
		 .HasDiscriminator<string>("Discriminator")
		 .HasValue<Person>("Person")
		 .HasValue<Employee>("Employee");

		// 確保 Employee 被模型認得（若沒有 DbSet<Employee> 也沒關係）
		m.Entity<Employee>();
	}
}

public class AppContext : BaseContext
{
	public DbSet<Employee> Employees { get; set; }
}