Category Archives: Entity Framework Core

Using Entity Framework with IoC

One thing to bear in mind about using entity framework is that the DbContext object is not thread safe. This threw me when I first discovered it; it also confused why I was getting this error, as I was running in an environment that I thought was pretty much single threaded (in fact, it was an Azure function). I was using IoC and so the DbContext is shared across instances.

Because the error is based on, effectively, a race condition, your mileage may vary, but you’ll typically get one of the following errors:

System.InvalidOperationException: ‘The context cannot be used while the model is being created. This exception may be thrown if the context is used inside the OnModelCreating method or if the same context instance is accessed by multiple threads concurrently. Note that instance members of DbContext and related classes are not guaranteed to be thread safe.’

System.InvalidOperationException: ‘A second operation started on this context before a previous operation completed. Any instance members are not guaranteed to be thread safe.’

I had a good idea what the cause of this might be, but in order to investigate, I set-up a console app accessing Entity Framework; in this case, EF Core.

If you’re here to find a solution, then you can probably scroll down to the section labelled To Fix.

Reproducing the error

To reproduce the error, we need to introduce Unity (I imagine the same would be true of any IoC provider, as the problem is more with the concept than the implementation):

Install-Package Unity

The next step is to abstract away the data access layer, in order to provide a base for our dependency injection:

As you can see, we’re introducing a data access layer – and we’re creating an interface for our DbContext. The idea being that this can subsequently be resolved by Unity. Here are our interfaces:

public interface IDataAccess
{
    List<string> GetData();
 
    void AddData(List<string> newData);
}

public interface IMyDbContext
{
    DbSet<MyData> MyData { get; set; }
 
    void SaveChanges();
}

public interface IDoStuff
{
    void DoStuffQuickly();
}

Finally, we can implement the interfaces:

public class DataAccess : IDataAccess
{
    private readonly IMyDbContext _myDbContext;
 
    public DataAccess(IMyDbContext myDbContext)
    {
        _myDbContext = myDbContext;
    }
    public void AddData(List<string> newData)
    {
        foreach (var data in newData)
        {
            MyData myData = new MyData()
            {
                FieldOne = data
            };
            _myDbContext.MyData.Add(myData);
        }
        _myDbContext.SaveChanges();
    }
 
    public List<string> GetData()
    {
        List<string> data = 
            _myDbContext.MyData.Select(a => a.FieldOne).ToList();
 
        return data;
    }
}

The DbContext:

public class MyDbContext : DbContext, IMyDbContext
{
    public DbSet<MyData> MyData { get; set; }
 
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        string cn = @"Server=.\SQLEXPRESS;Database=test-db . . .";
        optionsBuilder.UseSqlServer(cn);
 
        base.OnConfiguring(optionsBuilder);
    }
 
    void IMyDbContext.SaveChanges()
    {
        base.SaveChanges();
    }
}

Let’s register a class to actually start the process:

public class DoStuff : IDoStuff
{
    private readonly IDataAccess _dataAccess;
    private readonly ILogger _logger;
 
    public DoStuff(IDataAccess dataAccess, ILogger logger)
    {
        _dataAccess = dataAccess;
        _logger = logger;
    }
 
    public void DoStuffQuickly()
    {
        _dataAccess.AddData(new List<string>()
        {
            "testing new data",
            "second test",
            "last test"
        });
 
        Parallel.For(1, 100, (a) =>
        {
            List<string> data = _dataAccess.GetData();
            foreach(string text in data)
            {
                _logger.Log(text);
            }
        });
    }
}

Now let’s register the types in out IoC container:

static void Main(string[] args)
{
    Console.WriteLine("Hello World!");
 
    UnityContainer container = new UnityContainer();
    container.RegisterType<IDataAccess, DataAccess.DataAccess>();
    container.RegisterType<IMyDbContext, MyDbContext>();
    container.RegisterType<IDoStuff, DoStuff>();
    container.RegisterType<ILogger, ConsoleLogger>();
 
    container.Resolve<IDoStuff>().DoStuffQuickly();
}

(Console logger is just a class that calls Console.WriteLine()).

Now, when you run it, you’ll get the same error as above (or something to the same effect).

To fix

One possible fix is to simply instantiate a DbContext as you need it; for example:

public List<string> GetData()
{
    using (var myDbContext = new MyDbContext())
    {
        List<string> data =
            myDbContext.MyData.Select(a => a.FieldOne).ToList();
        return data;
    }
    
}

However, the glaring problem that this creates is unit testing. The solution is a simple factory class:

public interface IGenerateDbContext
{
    IMyDbContext GenerateNewContext();
}

public class GenerateDbContext : IGenerateDbContext
{
    public IMyDbContext GenerateNewContext()
    {
        IMyDbContext myDbContext = new MyDbContext();
 
        return myDbContext;
    }
}

We’ll need to make the DbContext implementation disposable:

public interface IMyDbContext : IDisposable
{
    DbSet<MyData> MyData { get; set; }
 
    void SaveChanges();
}

And, finally, we can change the data access code:

public void AddData(List<string> newData)
{
    using (IMyDbContext myDbContext = _generateDbContext.GenerateNewContext())
    {
        foreach (string data in newData)
        {
            MyData myData = new MyData()
            {
                FieldOne = data
            };
            myDbContext.MyData.Add(myData);
        }
        myDbContext.SaveChanges();
    }
}
 
public List<string> GetData()
{
    using (IMyDbContext myDbContext = _generateDbContext.GenerateNewContext())
    {
        List<string> data =
            myDbContext.MyData.Select(a => a.FieldOne).ToList();
        return data;
    }
    
}

Now we can use IoC to generate the DBContext, and therefore it is testable, but we don’t pass the DbContext itself around, and therefore there are no threading issues.

Setting up Entity Framework Core for a Console Application – One Error at a Time

Entity Framework can be difficult to get started with: especially if you come from a background of accessing the database directly, it can seem like there are endless meaningless errors that appear. In this post, I try to set-up EF Core using a .Net Core Console application. In order to better understand the errors, we’ll just do the minimum in each step; and be guided by the errors.

The first step is to create a .Net Core Console Application.

NuGet Packages

To use Entity Framework, you’ll first need to install the NuGet packages; to follow this post, you’ll need these two (initially) 1:

PM> Install-Package Microsoft.EntityFrameworkCore
PM> Install-Package Microsoft.EntityFrameworkCore.Tools

Model / Entities

The idea behind Entity Framework is that you represent database entities, or tables as they used to be known, with in memory objects. So the first step is to create a model:

namespace ConsoleApp1.Model
{
    public class MyData
    {
        public string FieldOne { get; set; }
 
    }
}

We’ve created the model, so the next thing is to create the DB:

PM> Update-Database

In the package manager console.

First Error – DbContext

The first error you get is:

No DbContext was found in assembly ‘ConsoleApp1’. Ensure that you’re using the correct assembly and that the type is neither abstract nor generic.

Okay, so let’s create a DbContext. The recommended pattern (as described here) is to inherit from DbContext:

namespace ConsoleApp1
{
    public class MyDbContext : DbContext
    {
    }
}

Okay, we’ve created a DbContext – let’s go again:

PM> Update-Database

Second Error – Database Provider

The next error is:

System.InvalidOperationException: No database provider has been configured for this DbContext. A provider can be configured by overriding the DbContext.OnConfiguring method or by using AddDbContext on the application service provider.

So we’ve moved on a little. The next thing we need to do is to configure a provider. Because in this case, I’m using SQL Server, I’ll need another NuGet package:

PM> Install-Package Microsoft.EntityFrameworkCore.SqlServer

Then configure the DbContext to use it:

public class MyDbContext : DbContext
{
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        string cn = @"Server=.\SQLEXPRESS;Database=test-db;User Id= . . .";
        optionsBuilder.UseSqlServer(cn);
 
        base.OnConfiguring(optionsBuilder);
    }
}

And again:

PM> Update-Database

Third Error – No Migrations

Strictly speaking this isn’t an actual error. It’s more a sign that nothing has happened:

No migrations were applied. The database is already up to date.
Done.

A quick look in SSMS shows that, whilst it has created the DB, it hasn’t created the table:

So we need to add a migration? Well, if we call Add-Migration here, we’ll get this: 2

That’s because we need to tell EF what data we care about. So, in the DbContext, we can let it know that we’re interested in a data set (or, if you like, table) called MyData:

public class MyDbContext : DbContext
{
    public DbSet<MyData> MyData { get; set; }

Right – now we can call:

PM> Add-Migration FirstMigration

Fourth Error – Primary Key

The next error is more about EF’s inner workings.:

System.InvalidOperationException: The entity type ‘MyData’ requires a primary key to be defined.

Definitely progress. Now we’re being moaned at because EF wants to know what the primary key for the table is, and we haven’t told it (Entity Framework, unlike SQL Server insists on a primary key). That requires a small change to the model:

using System.ComponentModel.DataAnnotations;
 
namespace ConsoleApp1.Model
{
    public class MyData
    {
        [Key]
        public string FieldOne { get; set; }
 
    }
}

This time,

PM> Add-Migration FirstMigration

Produces this:

    public partial class FirstMigration : Migration
    {
        protected override void Up(MigrationBuilder migrationBuilder)
        {
            migrationBuilder.CreateTable(
                name: "MyData",
                columns: table => new
                {
                    FieldOne = table.Column<string>(nullable: false)
                },
                constraints: table =>
                {
                    table.PrimaryKey("PK_MyData", x => x.FieldOne);
                });
        }
 
        protected override void Down(MigrationBuilder migrationBuilder)
        {
            migrationBuilder.DropTable(
                name: "MyData");
        }
    }

Which looks much more like we’ll get a table – let’s try:

PM> update-database
Applying migration '20180224075857_FirstMigration'.
Done.
PM> 

Success

And it has, indeed, created a table!

References

https://docs.microsoft.com/en-us/ef/core/miscellaneous/cli/powershell

https://docs.microsoft.com/en-us/ef/core/miscellaneous/configuring-dbcontext

https://www.learnentityframeworkcore.com/walkthroughs/console-application

Foot Notes

Adding Identity Capabilities to an ASP.NET Core App

If you create a new ASP.Net application, you get a built-in log-in feature – it provides the log-in page, all the back end services and even the DB tables. It does assume that your DB and your web-site are physically located on the same server (or at least that the web site can directly access the DB). Asp.Net Core also provides this, but it’s slightly different. It does still use Entity Framework (Core), and it does still assume direct access to the DB.

For a new application

Adding this functionality to a new application is very straightforward…

Step One – Create a new Asp.Net Core Web App

Step Two – Add authentication

Select “Change Authentication”:

If you’re creating a standard self-authenticating web page, then Individual is the answer. “Windows Authentication” allows you to defer authentication to your domain, and “Work or School Account” allows you to use Microsoft’s own security using AD, Azure or Office 365.

Step Three – Log-in

Now, just log-in:

So far so good; but what if you have already created a web app using ASP.Net Core and want to retrospectively fit this functionality?

For Existing Applications

Obviously, adding this functionality can depend on what you’re adding it to. The following was compiled from an ASP.Net Core app created without identity services, and then retrofitted with them. In order to do this, I strongly recommend starting with a dummy app created as above, as there’s a lot of cutting and pasting coming up.

Step One – Add Entity Framework

The identity service is built on top of EF (Core in this case); so add:

Microsoft.AspNetCore.Identity.EntityFrameworkCore

Step Two – The ApplicationUser Model

You need to add the concept of IdentityUser to your application to use the ASP.Net Core Identity functionality; so you will need a model to represent your user:

This should inherit from IdentityUser:

using Microsoft.AspNetCore.Identity.EntityFrameworkCore;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace MyWebApp.Models
{
    public class ApplicationUser : IdentityUser
    {
    }
}

Step Three – ApplicationDbContext

You need a DBContext; this provides an abstraction for EF and allows it to work out how to create your DB, etc.; create a Data folder:

And add a class similar to the following:

using MyWebApp.Models;
using Microsoft.AspNetCore.Identity.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace MyWebApp.Data
{
    public class ApplicationDbContext : IdentityDbContext<ApplicationUser>
    {
        public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options)
            : base(options)
        {
        }

        protected override void OnModelCreating(ModelBuilder builder)
        {
            base.OnModelCreating(builder);
            // Customize the ASP.NET Identity model and override the defaults if needed.
            // For example, you can rename the ASP.NET Identity table names and more.
            // Add your customizations after calling base.OnModelCreating(builder);
        }
    }
}

Step Four – Startup.cs

With ASP.Net Core there is an opt-in policy; so all the functionality that you might need is registered in an IoC first (including MVC). The identity service needs to be registered in Startup.ConfigureServices:

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddIdentity<ApplicationUser, IdentityRole>()
                .AddEntityFrameworkStores<ApplicationDbContext>()
                .AddDefaultTokenProviders();

            // Add framework services.
            services.AddMvc();
        }

Step Five – Services

To deal with two factor authentication, you’ll need an implementation of a message sender. I initially became confused with this naming, and it refers to a class that sends messages (e-mails, etc), and not message in any of the many other senses you may imagine.

    public interface IEmailSender
    {
        Task SendEmailAsync(string email, string subject, string message);
    }
    public interface ISmsSender
    {
        Task SendSmsAsync(string number, string message);
    }
    public class AuthMessageSender : IEmailSender, ISmsSender
    {
        public Task SendEmailAsync(string email, string subject, string message)
        {
            // Plug in your email service here to send an email.
            return Task.FromResult(0);
        }

        public Task SendSmsAsync(string number, string message)
        {
            // Plug in your SMS service here to send a text message.
            return Task.FromResult(0);
        }
    }

Step Six – ViewModels and Views

I won’t detail them all here, but you’ll need view models and views to cover all the basic functionality (register, reset, login, etc…):

Step Seven – AccountController

The controllers are the drivers for functionality in MVC; the following details how the log-in system will function.

    [Authorize]
    public class AccountController : Controller
    {
        private readonly UserManager<ApplicationUser> _userManager;
        private readonly SignInManager<ApplicationUser> _signInManager;
        private readonly IEmailSender _emailSender;
        private readonly ISmsSender _smsSender;
        private readonly ILogger _logger;
        private readonly string _externalCookieScheme;

        public AccountController(
            UserManager<ApplicationUser> userManager,
            SignInManager<ApplicationUser> signInManager,
            IOptions<IdentityCookieOptions> identityCookieOptions,
            IEmailSender emailSender,
            ISmsSender smsSender,
            ILoggerFactory loggerFactory)
        {
            _userManager = userManager;
            _signInManager = signInManager;
            _externalCookieScheme = identityCookieOptions.Value.ExternalCookieAuthenticationScheme;
            _emailSender = emailSender;
            _smsSender = smsSender;
            _logger = loggerFactory.CreateLogger<AccountController>();
        }

        //
        // GET: /Account/Login
        [HttpGet]
        [AllowAnonymous]
        public async Task<IActionResult> Login(string returnUrl = null)
        {
            // Clear the existing external cookie to ensure a clean login process
            await HttpContext.Authentication.SignOutAsync(_externalCookieScheme);

            ViewData["ReturnUrl"] = returnUrl;
            return View();
        }

        //
        // POST: /Account/Login
        [HttpPost]
        [AllowAnonymous]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> Login(LoginViewModel model, string returnUrl = null)
        {
            ViewData["ReturnUrl"] = returnUrl;
            if (ModelState.IsValid)
            {
                // This doesn't count login failures towards account lockout
                // To enable password failures to trigger account lockout, set lockoutOnFailure: true
                var result = await _signInManager.PasswordSignInAsync(model.Email, model.Password, model.RememberMe, lockoutOnFailure: false);
                if (result.Succeeded)
                {
                    _logger.LogInformation(1, "User logged in.");
                    return RedirectToLocal(returnUrl);
                }
                if (result.RequiresTwoFactor)
                {
                    return RedirectToAction(nameof(SendCode), new { ReturnUrl = returnUrl, RememberMe = model.RememberMe });
                }
                if (result.IsLockedOut)
                {
                    _logger.LogWarning(2, "User account locked out.");
                    return View("Lockout");
                }
                else
                {
                    ModelState.AddModelError(string.Empty, "Invalid login attempt.");
                    return View(model);
                }
            }

            // If we got this far, something failed, redisplay form
            return View(model);
        }

        //
        // GET: /Account/Register
        [HttpGet]
        [AllowAnonymous]
        public IActionResult Register(string returnUrl = null)
        {
            ViewData["ReturnUrl"] = returnUrl;
            return View();
        }

        //
        // POST: /Account/Register
        [HttpPost]
        [AllowAnonymous]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> Register(RegisterViewModel model, string returnUrl = null)
        {
            ViewData["ReturnUrl"] = returnUrl;
            if (ModelState.IsValid)
            {
                var user = new ApplicationUser { UserName = model.Email, Email = model.Email };
                var result = await _userManager.CreateAsync(user, model.Password);
                if (result.Succeeded)
                {
                    // For more information on how to enable account confirmation and password reset please visit https://go.microsoft.com/fwlink/?LinkID=532713
                    // Send an email with this link
                    //var code = await _userManager.GenerateEmailConfirmationTokenAsync(user);
                    //var callbackUrl = Url.Action(nameof(ConfirmEmail), "Account", new { userId = user.Id, code = code }, protocol: HttpContext.Request.Scheme);
                    //await _emailSender.SendEmailAsync(model.Email, "Confirm your account",
                    //    $"Please confirm your account by clicking this link: <a href='{callbackUrl}'>link</a>");
                    await _signInManager.SignInAsync(user, isPersistent: false);
                    _logger.LogInformation(3, "User created a new account with password.");
                    return RedirectToLocal(returnUrl);
                }
                AddErrors(result);
            }

            // If we got this far, something failed, redisplay form
            return View(model);
        }

        //
        // POST: /Account/Logout
        [HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> Logout()
        {
            await _signInManager.SignOutAsync();
            _logger.LogInformation(4, "User logged out.");
            return RedirectToAction(nameof(HomeController.Index), "Home");
        }

        //
        // POST: /Account/ExternalLogin
        [HttpPost]
        [AllowAnonymous]
        [ValidateAntiForgeryToken]
        public IActionResult ExternalLogin(string provider, string returnUrl = null)
        {
            // Request a redirect to the external login provider.
            var redirectUrl = Url.Action(nameof(ExternalLoginCallback), "Account", new { ReturnUrl = returnUrl });
            var properties = _signInManager.ConfigureExternalAuthenticationProperties(provider, redirectUrl);
            return Challenge(properties, provider);
        }

        //
        // GET: /Account/ExternalLoginCallback
        [HttpGet]
        [AllowAnonymous]
        public async Task<IActionResult> ExternalLoginCallback(string returnUrl = null, string remoteError = null)
        {
            if (remoteError != null)
            {
                ModelState.AddModelError(string.Empty, $"Error from external provider: {remoteError}");
                return View(nameof(Login));
            }
            var info = await _signInManager.GetExternalLoginInfoAsync();
            if (info == null)
            {
                return RedirectToAction(nameof(Login));
            }

            // Sign in the user with this external login provider if the user already has a login.
            var result = await _signInManager.ExternalLoginSignInAsync(info.LoginProvider, info.ProviderKey, isPersistent: false);
            if (result.Succeeded)
            {
                _logger.LogInformation(5, "User logged in with {Name} provider.", info.LoginProvider);
                return RedirectToLocal(returnUrl);
            }
            if (result.RequiresTwoFactor)
            {
                return RedirectToAction(nameof(SendCode), new { ReturnUrl = returnUrl });
            }
            if (result.IsLockedOut)
            {
                return View("Lockout");
            }
            else
            {
                // If the user does not have an account, then ask the user to create an account.
                ViewData["ReturnUrl"] = returnUrl;
                ViewData["LoginProvider"] = info.LoginProvider;
                var email = info.Principal.FindFirstValue(ClaimTypes.Email);
                return View("ExternalLoginConfirmation", new ExternalLoginConfirmationViewModel { Email = email });
            }
        }

        //
        // POST: /Account/ExternalLoginConfirmation
        [HttpPost]
        [AllowAnonymous]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> ExternalLoginConfirmation(ExternalLoginConfirmationViewModel model, string returnUrl = null)
        {
            if (ModelState.IsValid)
            {
                // Get the information about the user from the external login provider
                var info = await _signInManager.GetExternalLoginInfoAsync();
                if (info == null)
                {
                    return View("ExternalLoginFailure");
                }
                var user = new ApplicationUser { UserName = model.Email, Email = model.Email };
                var result = await _userManager.CreateAsync(user);
                if (result.Succeeded)
                {
                    result = await _userManager.AddLoginAsync(user, info);
                    if (result.Succeeded)
                    {
                        await _signInManager.SignInAsync(user, isPersistent: false);
                        _logger.LogInformation(6, "User created an account using {Name} provider.", info.LoginProvider);
                        return RedirectToLocal(returnUrl);
                    }
                }
                AddErrors(result);
            }

            ViewData["ReturnUrl"] = returnUrl;
            return View(model);
        }

        // GET: /Account/ConfirmEmail
        [HttpGet]
        [AllowAnonymous]
        public async Task<IActionResult> ConfirmEmail(string userId, string code)
        {
            if (userId == null || code == null)
            {
                return View("Error");
            }
            var user = await _userManager.FindByIdAsync(userId);
            if (user == null)
            {
                return View("Error");
            }
            var result = await _userManager.ConfirmEmailAsync(user, code);
            return View(result.Succeeded ? "ConfirmEmail" : "Error");
        }

        //
        // GET: /Account/ForgotPassword
        [HttpGet]
        [AllowAnonymous]
        public IActionResult ForgotPassword()
        {
            return View();
        }

        //
        // POST: /Account/ForgotPassword
        [HttpPost]
        [AllowAnonymous]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> ForgotPassword(ForgotPasswordViewModel model)
        {
            if (ModelState.IsValid)
            {
                var user = await _userManager.FindByEmailAsync(model.Email);
                if (user == null || !(await _userManager.IsEmailConfirmedAsync(user)))
                {
                    // Don't reveal that the user does not exist or is not confirmed
                    return View("ForgotPasswordConfirmation");
                }

                // For more information on how to enable account confirmation and password reset please visit https://go.microsoft.com/fwlink/?LinkID=532713
                // Send an email with this link
                //var code = await _userManager.GeneratePasswordResetTokenAsync(user);
                //var callbackUrl = Url.Action(nameof(ResetPassword), "Account", new { userId = user.Id, code = code }, protocol: HttpContext.Request.Scheme);
                //await _emailSender.SendEmailAsync(model.Email, "Reset Password",
                //   $"Please reset your password by clicking here: <a href='{callbackUrl}'>link</a>");
                //return View("ForgotPasswordConfirmation");
            }

            // If we got this far, something failed, redisplay form
            return View(model);
        }

        //
        // GET: /Account/ForgotPasswordConfirmation
        [HttpGet]
        [AllowAnonymous]
        public IActionResult ForgotPasswordConfirmation()
        {
            return View();
        }

        //
        // GET: /Account/ResetPassword
        [HttpGet]
        [AllowAnonymous]
        public IActionResult ResetPassword(string code = null)
        {
            return code == null ? View("Error") : View();
        }

        //
        // POST: /Account/ResetPassword
        [HttpPost]
        [AllowAnonymous]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> ResetPassword(ResetPasswordViewModel model)
        {
            if (!ModelState.IsValid)
            {
                return View(model);
            }
            var user = await _userManager.FindByEmailAsync(model.Email);
            if (user == null)
            {
                // Don't reveal that the user does not exist
                return RedirectToAction(nameof(AccountController.ResetPasswordConfirmation), "Account");
            }
            var result = await _userManager.ResetPasswordAsync(user, model.Code, model.Password);
            if (result.Succeeded)
            {
                return RedirectToAction(nameof(AccountController.ResetPasswordConfirmation), "Account");
            }
            AddErrors(result);
            return View();
        }

        //
        // GET: /Account/ResetPasswordConfirmation
        [HttpGet]
        [AllowAnonymous]
        public IActionResult ResetPasswordConfirmation()
        {
            return View();
        }

        //
        // GET: /Account/SendCode
        [HttpGet]
        [AllowAnonymous]
        public async Task<ActionResult> SendCode(string returnUrl = null, bool rememberMe = false)
        {
            var user = await _signInManager.GetTwoFactorAuthenticationUserAsync();
            if (user == null)
            {
                return View("Error");
            }
            var userFactors = await _userManager.GetValidTwoFactorProvidersAsync(user);
            var factorOptions = userFactors.Select(purpose => new SelectListItem { Text = purpose, Value = purpose }).ToList();
            return View(new SendCodeViewModel { Providers = factorOptions, ReturnUrl = returnUrl, RememberMe = rememberMe });
        }

        //
        // POST: /Account/SendCode
        [HttpPost]
        [AllowAnonymous]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> SendCode(SendCodeViewModel model)
        {
            if (!ModelState.IsValid)
            {
                return View();
            }

            var user = await _signInManager.GetTwoFactorAuthenticationUserAsync();
            if (user == null)
            {
                return View("Error");
            }

            // Generate the token and send it
            var code = await _userManager.GenerateTwoFactorTokenAsync(user, model.SelectedProvider);
            if (string.IsNullOrWhiteSpace(code))
            {
                return View("Error");
            }

            var message = "Your security code is: " + code;
            if (model.SelectedProvider == "Email")
            {
                await _emailSender.SendEmailAsync(await _userManager.GetEmailAsync(user), "Security Code", message);
            }
            else if (model.SelectedProvider == "Phone")
            {
                await _smsSender.SendSmsAsync(await _userManager.GetPhoneNumberAsync(user), message);
            }

            return RedirectToAction(nameof(VerifyCode), new { Provider = model.SelectedProvider, ReturnUrl = model.ReturnUrl, RememberMe = model.RememberMe });
        }

        //
        // GET: /Account/VerifyCode
        [HttpGet]
        [AllowAnonymous]
        public async Task<IActionResult> VerifyCode(string provider, bool rememberMe, string returnUrl = null)
        {
            // Require that the user has already logged in via username/password or external login
            var user = await _signInManager.GetTwoFactorAuthenticationUserAsync();
            if (user == null)
            {
                return View("Error");
            }
            return View(new VerifyCodeViewModel { Provider = provider, ReturnUrl = returnUrl, RememberMe = rememberMe });
        }

        //
        // POST: /Account/VerifyCode
        [HttpPost]
        [AllowAnonymous]
        [ValidateAntiForgeryToken]
        public async Task<IActionResult> VerifyCode(VerifyCodeViewModel model)
        {
            if (!ModelState.IsValid)
            {
                return View(model);
            }

            // The following code protects for brute force attacks against the two factor codes.
            // If a user enters incorrect codes for a specified amount of time then the user account
            // will be locked out for a specified amount of time.
            var result = await _signInManager.TwoFactorSignInAsync(model.Provider, model.Code, model.RememberMe, model.RememberBrowser);
            if (result.Succeeded)
            {
                return RedirectToLocal(model.ReturnUrl);
            }
            if (result.IsLockedOut)
            {
                _logger.LogWarning(7, "User account locked out.");
                return View("Lockout");
            }
            else
            {
                ModelState.AddModelError(string.Empty, "Invalid code.");
                return View(model);
            }
        }

        //
        // GET /Account/AccessDenied
        [HttpGet]
        public IActionResult AccessDenied()
        {
            return View();
        }

        #region Helpers

        private void AddErrors(IdentityResult result)
        {
            foreach (var error in result.Errors)
            {
                ModelState.AddModelError(string.Empty, error.Description);
            }
        }

        private IActionResult RedirectToLocal(string returnUrl)
        {
            if (Url.IsLocalUrl(returnUrl))
            {
                return Redirect(returnUrl);
            }
            else
            {
                return RedirectToAction(nameof(HomeController.Index), "Home");
            }
        }

        #endregion
    }

Step Eight – Adding the Log-in Button

The next step is to change the master page, this is typically Layout.cshtml. Here, we just add a reference to another file (_LoginPartial):

            <div class="navbar-collapse collapse">
                <ul class="nav navbar-nav">
                    <li><a asp-area="" asp-controller="Home" asp-action="Index">Home</a></li>
                    <li><a asp-area="" asp-controller="Home" asp-action="About">About</a></li>
                    <li><a asp-area="" asp-controller="Home" asp-action="Contact">Contact</a></li>
                </ul>
                @await Html.PartialAsync("_LoginPartial")
            </div>
        </div>
    </nav>
    <div class="container body-content">
        @RenderBody()

LoginPartial looks like this:

@using Microsoft.AspNetCore.Identity
@using MyWebApp.Models

@inject SignInManager<ApplicationUser> SignInManager
@inject UserManager<ApplicationUser> UserManager

@if (SignInManager.IsSignedIn(User))
{
    <form asp-area="" asp-controller="Account" asp-action="Logout" method="post" id="logoutForm" class="navbar-right">
        <ul class="nav navbar-nav navbar-right">
            <li>
                <a asp-area="" asp-controller="Manage" asp-action="Index" title="Manage">Hello @UserManager.GetUserName(User)!</a>
            </li>
            <li>
                <button type="submit" class="btn btn-link navbar-btn navbar-link">Log out</button>
            </li>
        </ul>
    </form>
}
else
{
    <ul class="nav navbar-nav navbar-right">
        <li><a asp-area="" asp-controller="Account" asp-action="Register">Register</a></li>
        <li><a asp-area="" asp-controller="Account" asp-action="Login">Log in</a></li>
    </ul>
}

… and that’s it. When you’re done, your website should provide basic log-in and register functionality; the following section has some suggestions about what to do if it does not.

Errors

The following are errors you may encounter at this stage, depending on what state your project was in before you started this.

DbContext Error

An unhandled exception occurred while processing the request.
InvalidOperationException: Unable to resolve service for type ‘MyWebApp.Data.ApplicationDbContext’ while attempting to activate ‘Microsoft.AspNetCore.Identity.EntityFrameworkCore.UserStore`4[MyWebApp.Models.ApplicationUser,Microsoft.AspNetCore.Identity.EntityFrameworkCore.IdentityRole,MyWebApp.Data.ApplicationDbContext,System.String]’.
Microsoft.Extensions.DependencyInjection.ServiceLookup.Service.PopulateCallSites(ServiceProvider provider, ISet callSiteChain, ParameterInfo[] parameters, bool throwIfCallSiteNotFound)

This is simply because the DbContext was never registered; the fix is:

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddDbContext<ApplicationDbContext>(options =>
                options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));

            services.AddIdentity<ApplicationUser, IdentityRole>()
                .AddEntityFrameworkStores<ApplicationDbContext>()
                .AddDefaultTokenProviders();

            // Add framework services.
            services.AddMvc();
        }

You’ll need the following NuGet package installed:

Microsoft.EntityFrameworkCore.SqlServer

And you’ll need to add:

using Microsoft.EntityFrameworkCore;

ConnectionString Error

An unhandled exception occurred while processing the request.
ArgumentNullException: Value cannot be null.
Parameter name: connectionString
Microsoft.EntityFrameworkCore.Utilities.Check.NotEmpty(string value, string parameterName)

Admittedly, it’s not rocket science to work this one out; your appsettings.json needs a connection string. By default, this uses SQLExpress, but you can actually point it to any DB:

{
  "ConnectionStrings": {
    "DefaultConnection": "Server=ServerName\\InstanceName;Database=MyDatabase;Trusted_Connection=True;MultipleActiveResultSets=true"
  },
  "Logging": {
    "IncludeScopes": false,
    "LogLevel": {
      "Default": "Warning"
    }
  }
}

Identity.External Error

An unhandled exception occurred while processing the request.
InvalidOperationException: No authentication handler is configured to handle the scheme: Identity.External
Microsoft.AspNetCore.Http.Authentication.Internal.DefaultAuthenticationManager+d__15.MoveNext()

In Startup.cs, change the Configure function to include the following:

    . . .
    app.UseStaticFiles();

    app.UseIdentity();

    app.UseMvc(routes =>
    {
        routes.MapRoute(
                    name: "default",
                    template: "{controller=Home}/{action=Index}/{id?}");
    });

Error in Compilation of Required Resource

An error occurred during the compilation of a resource required to process this request. Please review the following specific error details and modify your source code appropriately.

Check the _ViewImports.cshtml – this is where all the using statements for the views are held; it should include all the necessary namespaces; for example:

@using MyApp.Web.Core
@using MyApp
@using MyApp.Web.Core.Models
@using MyApp.Web.Core.Models.AccountViewModels
@using MyApp.Web.Core.Models.ManageViewModels
@using Microsoft.AspNetCore.Identity
@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers

Could not find Table AspNetUsers

In Visual Studio, you can use the Package Manager Console to apply pending migrations to the database:

PM> Update-Database

Alternatively, you can apply pending migrations from a command prompt at your project directory:

> dotnet ef database update

To set-up a migration, you need the package:

Microsoft.EntityFrameworkCore.Design

Set-up a migration:

Add an identity migration (this is the default one):

    public partial class CreateIdentitySchema : Migration
    {
        protected override void Up(MigrationBuilder migrationBuilder)
        {
            migrationBuilder.CreateTable(
                name: "AspNetRoles",
                columns: table => new
                {
                    Id = table.Column<string>(nullable: false),
                    ConcurrencyStamp = table.Column<string>(nullable: true),
                    Name = table.Column<string>(maxLength: 256, nullable: true),
                    NormalizedName = table.Column<string>(maxLength: 256, nullable: true)
                },
                constraints: table =>
                {
                    table.PrimaryKey("PK_AspNetRoles", x => x.Id);
                });

            migrationBuilder.CreateTable(
                name: "AspNetUserTokens",
                columns: table => new
                {
                    UserId = table.Column<string>(nullable: false),
                    LoginProvider = table.Column<string>(nullable: false),
                    Name = table.Column<string>(nullable: false),
                    Value = table.Column<string>(nullable: true)
                },
                constraints: table =>
                {
                    table.PrimaryKey("PK_AspNetUserTokens", x => new { x.UserId, x.LoginProvider, x.Name });
                });

            migrationBuilder.CreateTable(
                name: "AspNetUsers",
                columns: table => new
                {
                    Id = table.Column<string>(nullable: false),
                    AccessFailedCount = table.Column<int>(nullable: false),
                    ConcurrencyStamp = table.Column<string>(nullable: true),
                    Email = table.Column<string>(maxLength: 256, nullable: true),
                    EmailConfirmed = table.Column<bool>(nullable: false),
                    LockoutEnabled = table.Column<bool>(nullable: false),
                    LockoutEnd = table.Column<DateTimeOffset>(nullable: true),
                    NormalizedEmail = table.Column<string>(maxLength: 256, nullable: true),
                    NormalizedUserName = table.Column<string>(maxLength: 256, nullable: true),
                    PasswordHash = table.Column<string>(nullable: true),
                    PhoneNumber = table.Column<string>(nullable: true),
                    PhoneNumberConfirmed = table.Column<bool>(nullable: false),
                    SecurityStamp = table.Column<string>(nullable: true),
                    TwoFactorEnabled = table.Column<bool>(nullable: false),
                    UserName = table.Column<string>(maxLength: 256, nullable: true)
                },
                constraints: table =>
                {
                    table.PrimaryKey("PK_AspNetUsers", x => x.Id);
                });

            migrationBuilder.CreateTable(
                name: "AspNetRoleClaims",
                columns: table => new
                {
                    Id = table.Column<int>(nullable: false)
                        .Annotation("SqlServer:ValueGenerationStrategy", SqlServerValueGenerationStrategy.IdentityColumn),
                    ClaimType = table.Column<string>(nullable: true),
                    ClaimValue = table.Column<string>(nullable: true),
                    RoleId = table.Column<string>(nullable: false)
                },
                constraints: table =>
                {
                    table.PrimaryKey("PK_AspNetRoleClaims", x => x.Id);
                    table.ForeignKey(
                        name: "FK_AspNetRoleClaims_AspNetRoles_RoleId",
                        column: x => x.RoleId,
                        principalTable: "AspNetRoles",
                        principalColumn: "Id",
                        onDelete: ReferentialAction.Cascade);
                });

            migrationBuilder.CreateTable(
                name: "AspNetUserClaims",
                columns: table => new
                {
                    Id = table.Column<int>(nullable: false)
                        .Annotation("SqlServer:ValueGenerationStrategy", SqlServerValueGenerationStrategy.IdentityColumn),
                    ClaimType = table.Column<string>(nullable: true),
                    ClaimValue = table.Column<string>(nullable: true),
                    UserId = table.Column<string>(nullable: false)
                },
                constraints: table =>
                {
                    table.PrimaryKey("PK_AspNetUserClaims", x => x.Id);
                    table.ForeignKey(
                        name: "FK_AspNetUserClaims_AspNetUsers_UserId",
                        column: x => x.UserId,
                        principalTable: "AspNetUsers",
                        principalColumn: "Id",
                        onDelete: ReferentialAction.Cascade);
                });

            migrationBuilder.CreateTable(
                name: "AspNetUserLogins",
                columns: table => new
                {
                    LoginProvider = table.Column<string>(nullable: false),
                    ProviderKey = table.Column<string>(nullable: false),
                    ProviderDisplayName = table.Column<string>(nullable: true),
                    UserId = table.Column<string>(nullable: false)
                },
                constraints: table =>
                {
                    table.PrimaryKey("PK_AspNetUserLogins", x => new { x.LoginProvider, x.ProviderKey });
                    table.ForeignKey(
                        name: "FK_AspNetUserLogins_AspNetUsers_UserId",
                        column: x => x.UserId,
                        principalTable: "AspNetUsers",
                        principalColumn: "Id",
                        onDelete: ReferentialAction.Cascade);
                });

            migrationBuilder.CreateTable(
                name: "AspNetUserRoles",
                columns: table => new
                {
                    UserId = table.Column<string>(nullable: false),
                    RoleId = table.Column<string>(nullable: false)
                },
                constraints: table =>
                {
                    table.PrimaryKey("PK_AspNetUserRoles", x => new { x.UserId, x.RoleId });
                    table.ForeignKey(
                        name: "FK_AspNetUserRoles_AspNetRoles_RoleId",
                        column: x => x.RoleId,
                        principalTable: "AspNetRoles",
                        principalColumn: "Id",
                        onDelete: ReferentialAction.Cascade);
                    table.ForeignKey(
                        name: "FK_AspNetUserRoles_AspNetUsers_UserId",
                        column: x => x.UserId,
                        principalTable: "AspNetUsers",
                        principalColumn: "Id",
                        onDelete: ReferentialAction.Cascade);
                });

            migrationBuilder.CreateIndex(
                name: "RoleNameIndex",
                table: "AspNetRoles",
                column: "NormalizedName");

            migrationBuilder.CreateIndex(
                name: "IX_AspNetRoleClaims_RoleId",
                table: "AspNetRoleClaims",
                column: "RoleId");

            migrationBuilder.CreateIndex(
                name: "IX_AspNetUserClaims_UserId",
                table: "AspNetUserClaims",
                column: "UserId");

            migrationBuilder.CreateIndex(
                name: "IX_AspNetUserLogins_UserId",
                table: "AspNetUserLogins",
                column: "UserId");

            migrationBuilder.CreateIndex(
                name: "IX_AspNetUserRoles_RoleId",
                table: "AspNetUserRoles",
                column: "RoleId");

            migrationBuilder.CreateIndex(
                name: "IX_AspNetUserRoles_UserId",
                table: "AspNetUserRoles",
                column: "UserId");

            migrationBuilder.CreateIndex(
                name: "EmailIndex",
                table: "AspNetUsers",
                column: "NormalizedEmail");

            migrationBuilder.CreateIndex(
                name: "UserNameIndex",
                table: "AspNetUsers",
                column: "NormalizedUserName",
                unique: true);
        }

        protected override void Down(MigrationBuilder migrationBuilder)
        {
            migrationBuilder.DropTable(
                name: "AspNetRoleClaims");

            migrationBuilder.DropTable(
                name: "AspNetUserClaims");

            migrationBuilder.DropTable(
                name: "AspNetUserLogins");

            migrationBuilder.DropTable(
                name: "AspNetUserRoles");

            migrationBuilder.DropTable(
                name: "AspNetUserTokens");

            migrationBuilder.DropTable(
                name: "AspNetRoles");

            migrationBuilder.DropTable(
                name: "AspNetUsers");
        }
    }

You’ll need the .designer.cs file, too:

Now, if you run:

 Update-Database

Or, run

dotnet ef database update

From powershell (project directory); it should update your database:

Disclaimer

Just to point out the obvious here: I didn’t create this identity system, I simply took what was supplied by default, and applied it to an existing project. The code above is not mine – it’s all copied and pasted by simply creating a new project with Identity Services and copying the relevant parts.

References

https://docs.microsoft.com/en-us/aspnet/core/security/authentication/identity