the web app

.gitignore

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+################################################################################
+# This .gitignore file was automatically created by Microsoft(R) Visual Studio.
+################################################################################
+
+/obj
+/bin
+/.vs

Controllers/HomeController.cs

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+using System.Linq;
+using aspnetcoreapp_efcore_inherited_entity_id_problem.Data;
+using Microsoft.AspNetCore.Mvc;
+
+namespace aspnetcoreapp_efcore_inherited_entity_id_problem.Controllers
+{
+    public class HomeController : Controller
+    {
+        private readonly AppDbContext _dbContext;
+
+        /// <summary>
+        /// Constructor
+        /// </summary>
+        public HomeController(AppDbContext dbContext)
+        {
+            _dbContext = dbContext;
+        }
+
+        public IActionResult Index()
+        {
+            // Uncomment the case you would like to test...
+
+            // 1st problem:
+            // This causes error "ArgumentException: An item with the same key has already been added. Key: 1".
+            _dbContext.Cat.Add(
+                    new Cat
+                    {
+                        Name = "Tom",
+                    });
+            _dbContext.SaveChanges();
+
+            // 2nd problem:
+            // This inserts new Dog with id 1 and replaces Cat with id 1.
+            //_dbContext.Dog.Add(
+            //    new Dog
+            //    {
+            //        Name = "Laika"
+            //    });
+            //_dbContext.SaveChanges();
+
+
+            return View(_dbContext.Animal.ToList());
+        }
+    }
+}

Data/Animal.cs

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+namespace aspnetcoreapp_efcore_inherited_entity_id_problem.Data
+{
+    public abstract class AnimalBase : Entity<int>
+    {
+        public string Name { get; set; }
+    }
+
+    public class Cat : AnimalBase
+    {
+    }
+
+    public class Dog : AnimalBase
+    {
+    }
+}

Data/AppDbContext.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using Microsoft.EntityFrameworkCore;
+
+namespace aspnetcoreapp_efcore_inherited_entity_id_problem.Data
+{
+    public class AppDbContext : DbContext
+    {
+        public DbSet<AnimalBase> Animal { get; set; }
+        public DbSet<Cat> Cat { get; set; }
+        public DbSet<Dog> Dog { get; set; }
+
+        /// <summary>
+        /// Constructor.
+        /// </summary>
+        public AppDbContext(DbContextOptions<AppDbContext> options)
+            : base(options)
+        {
+        }
+
+        protected override void OnModelCreating(ModelBuilder modelBuilder)
+        {
+            base.OnModelCreating(modelBuilder);
+
+            // Seed data
+            modelBuilder.Entity<Cat>().HasData(
+                new Cat
+                {
+                    Id = 1,
+                    Name = "Mourek",
+                },
+                new Cat
+                {
+                    Id = 2,
+                    Name = "Líza",
+                }
+            );
+        }
+    }
+}

Data/Entity.cs

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+namespace aspnetcoreapp_efcore_inherited_entity_id_problem.Data
+{
+    public class Entity<T>
+    {
+        public T Id { get; set; }
+    }
+}

Program.cs

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+using Microsoft.AspNetCore.Hosting;
+using Microsoft.Extensions.Hosting;
+
+namespace aspnetcoreapp_efcore_inherited_entity_id_problem
+{
+    public class Program
+    {
+        public static void Main(string[] args)
+        {
+            CreateHostBuilder(args).Build().Run();
+        }
+
+        public static IHostBuilder CreateHostBuilder(string[] args) =>
+            Host.CreateDefaultBuilder(args)
+                .ConfigureWebHostDefaults(webBuilder =>
+                {
+                    webBuilder.UseStartup<Startup>();
+                });
+    }
+}

Properties/launchSettings.json

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+{
+  "iisSettings": {
+    "windowsAuthentication": false, 
+    "anonymousAuthentication": true, 
+    "iisExpress": {
+      "applicationUrl": "http://localhost:61907",
+      "sslPort": 44353
+    }
+  },
+  "profiles": {
+    "IIS Express": {
+      "commandName": "IISExpress",
+      "launchBrowser": true,
+      "environmentVariables": {
+        "ASPNETCORE_ENVIRONMENT": "Development"
+      }
+    },
+    "aspnetcoreapp_efcore_inherited_entity_id_problem": {
+      "commandName": "Project",
+      "launchBrowser": true,
+      "applicationUrl": "https://localhost:5001;http://localhost:5000",
+      "environmentVariables": {
+        "ASPNETCORE_ENVIRONMENT": "Development"
+      }
+    }
+  }
+}

Startup.cs

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+using aspnetcoreapp_efcore_inherited_entity_id_problem.Data;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace aspnetcoreapp_efcore_inherited_entity_id_problem
+{
+    public class Startup
+    {
+        public Startup(IConfiguration configuration)
+        {
+            Configuration = configuration;
+        }
+
+        public IConfiguration Configuration { get; }
+
+        // This method gets called by the runtime. Use this method to add services to the container.
+        public void ConfigureServices(IServiceCollection services)
+        {
+            services.AddDbContext<AppDbContext>(options => options.UseInMemoryDatabase("appDb"));
+
+            services.AddControllersWithViews();
+        }
+
+        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
+        public void Configure(IApplicationBuilder app, IWebHostEnvironment env, AppDbContext context)
+        {
+            app.UseDeveloperExceptionPage();
+            app.UseHttpsRedirection();
+
+            app.UseRouting();
+
+            app.UseEndpoints(endpoints =>
+            {
+                endpoints.MapControllerRoute(
+                    name: "default",
+                    pattern: "{controller=Home}/{action=Index}/{id?}");
+            });
+
+            context.Database.EnsureCreated();
+        }
+    }
+}

Views/Home/Index.cshtml

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+@using aspnetcoreapp_efcore_inherited_entity_id_problem.Data;
+
+@model IEnumerable<AnimalBase>;
+
+<p>
+    @Model.Count()
+</p>
+
+<table border="1" cellpadding="10">
+    <thead>
+    <tr>
+        <th>Id</th>
+        <th>Name</th>
+        <th>Type</th>
+    </tr>
+    </thead>
+    <tbody>
+    @foreach (var obj in Model)
+    {
+        <tr>
+            <td>@obj.Id</td>
+            <td>@obj.Name</td>
+            <td>@obj.GetType()</td>
+        </tr>
+    }
+    </tbody>
+</table>

appsettings.Development.json

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+{
+  "Logging": {
+    "LogLevel": {
+      "Default": "Information",
+      "Microsoft": "Warning",
+      "Microsoft.Hosting.Lifetime": "Information"
+    }
+  }
+}

appsettings.json

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+{
+  "Logging": {
+    "LogLevel": {
+      "Default": "Information",
+      "Microsoft": "Warning",
+      "Microsoft.Hosting.Lifetime": "Information"
+    }
+  },
+  "AllowedHosts": "*"
+}

aspnetcoreapp-efcore-inherited-entity-id-problem.csproj

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+<Project Sdk="Microsoft.NET.Sdk.Web">
+
+  <PropertyGroup>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <RootNamespace>aspnetcoreapp_efcore_inherited_entity_id_problem</RootNamespace>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.EntityFrameworkCore.InMemory" Version="3.1.0" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Folder Include="wwwroot\" />
+  </ItemGroup>
+
+</Project>

aspnetcoreapp-efcore-inherited-entity-id-problem.sln

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+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 16
+VisualStudioVersion = 16.0.29728.190
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "aspnetcoreapp-efcore-inherited-entity-id-problem", "aspnetcoreapp-efcore-inherited-entity-id-problem.csproj", "{87E6CC8B-FE9D-4B35-AF96-6965950DB682}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{87E6CC8B-FE9D-4B35-AF96-6965950DB682}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{87E6CC8B-FE9D-4B35-AF96-6965950DB682}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{87E6CC8B-FE9D-4B35-AF96-6965950DB682}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{87E6CC8B-FE9D-4B35-AF96-6965950DB682}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {F2894353-5A28-4E6F-9B5B-BAF478255113}
+	EndGlobalSection
+EndGlobal

aspnetcoreapp-efcore-inherited-entity-id-problem.sln.DotSettings.user

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+<wpf:ResourceDictionary xml:space="preserve" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:s="clr-namespace:System;assembly=mscorlib" xmlns:ss="urn:shemas-jetbrains-com:settings-storage-xaml" xmlns:wpf="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
+	<s:String x:Key="/Default/CodeInspection/Highlighting/AnalysisEnabled/@EntryValue">SOLUTION</s:String></wpf:ResourceDictionary>