Custom Authentication in ASP.Net-Core

@Manish Jain, I suggest to implement the method with boolean return:

public class UserManager
{

    // Additional code here...            

    public async Task<bool> SignIn(HttpContext httpContext, UserDbModel user)
    {
        // Additional code here...            

        // Here the real authentication against a DB or Web Services or whatever 
        if (user.Email != null)
            return false;                    

        ClaimsIdentity identity = new ClaimsIdentity(this.GetUserClaims(dbUserData), CookieAuthenticationDefaults.AuthenticationScheme);
        ClaimsPrincipal principal = new ClaimsPrincipal(identity);

        // This is for give the authentication cookie to the user when authentication condition was met
        await httpContext.SignInAsync(CookieAuthenticationDefaults.AuthenticationScheme, principal);
        return true;
    }
}

From what I learned after several days of research, Here is the Guide for ASP .Net Core MVC 2.x Custom User Authentication

In Startup.cs :

Add below lines to ConfigureServices method :

public void ConfigureServices(IServiceCollection services)
{

services.AddAuthentication(
    CookieAuthenticationDefaults.AuthenticationScheme
).AddCookie(CookieAuthenticationDefaults.AuthenticationScheme,
    options =>
    {
        options.LoginPath = "/Account/Login";
        options.LogoutPath = "/Account/Logout";
    });

    services.AddMvc();

    // authentication 
    services.AddAuthentication(options =>
    {
       options.DefaultScheme = CookieAuthenticationDefaults.AuthenticationScheme;
    });

    services.AddTransient(
        m => new UserManager(
            Configuration
                .GetValue<string>(
                    DEFAULT_CONNECTIONSTRING //this is a string constant
                )
            )
        );
     services.AddDistributedMemoryCache();
}

keep in mind that in above code we said that if any unauthenticated user requests an action which is annotated with [Authorize] , they well force redirect to /Account/Login url.

Add below lines to Configure method :

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
        app.UseBrowserLink();
        app.UseDatabaseErrorPage();
    }
    else
    {
        app.UseExceptionHandler(ERROR_URL);
    }
     app.UseStaticFiles();
     app.UseAuthentication();
     app.UseMvc(routes =>
    {
        routes.MapRoute(
            name: "default",
            template: DEFAULT_ROUTING);
    });
}

Create your UserManager class that will also manage login and logout. it should look like below snippet (note that i'm using dapper):

public class UserManager
{
    string _connectionString;

    public UserManager(string connectionString)
    {
        _connectionString = connectionString;
    }

    public async void SignIn(HttpContext httpContext, UserDbModel user, bool isPersistent = false)
    {
        using (var con = new SqlConnection(_connectionString))
        {
            var queryString = "sp_user_login";
            var dbUserData = con.Query<UserDbModel>(
                queryString,
                new
                {
                    UserEmail = user.UserEmail,
                    UserPassword = user.UserPassword,
                    UserCellphone = user.UserCellphone
                },
                commandType: CommandType.StoredProcedure
            ).FirstOrDefault();

            ClaimsIdentity identity = new ClaimsIdentity(this.GetUserClaims(dbUserData), CookieAuthenticationDefaults.AuthenticationScheme);
            ClaimsPrincipal principal = new ClaimsPrincipal(identity);

            await httpContext.SignInAsync(CookieAuthenticationDefaults.AuthenticationScheme, principal);
        }
    }

    public async void SignOut(HttpContext httpContext)
    {
        await httpContext.SignOutAsync();
    }

    private IEnumerable<Claim> GetUserClaims(UserDbModel user)
    {
        List<Claim> claims = new List<Claim>();

        claims.Add(new Claim(ClaimTypes.NameIdentifier, user.Id().ToString()));
        claims.Add(new Claim(ClaimTypes.Name, user.UserFirstName));
        claims.Add(new Claim(ClaimTypes.Email, user.UserEmail));
        claims.AddRange(this.GetUserRoleClaims(user));
        return claims;
    }

    private IEnumerable<Claim> GetUserRoleClaims(UserDbModel user)
    {
        List<Claim> claims = new List<Claim>();

        claims.Add(new Claim(ClaimTypes.NameIdentifier, user.Id().ToString()));
        claims.Add(new Claim(ClaimTypes.Role, user.UserPermissionType.ToString()));
        return claims;
    }
}

Then maybe you have an AccountController which has a Login Action that should look like below :

public class AccountController : Controller
{
    UserManager _userManager;

    public AccountController(UserManager userManager)
    {
        _userManager = userManager;
    }

    [HttpPost]
    public IActionResult LogIn(LogInViewModel form)
    {
        if (!ModelState.IsValid)
            return View(form);
         try
        {
            //authenticate
            var user = new UserDbModel()
            {
                UserEmail = form.Email,
                UserCellphone = form.Cellphone,
                UserPassword = form.Password
            };
            _userManager.SignIn(this.HttpContext, user);
             return RedirectToAction("Search", "Home", null);
         }
         catch (Exception ex)
         {
            ModelState.AddModelError("summary", ex.Message);
            return View(form);
         }
    }
}

Now you are able to use [Authorize] annotation on any Action or Controller.

Feel free to comment any questions or bug's.

I would like to add something to brilliant @AmiNadimi answer for everyone who going implement his solution in .NET Core 3:

First of all, you should change signature of SignIn method in UserManager class from:

public async void SignIn(HttpContext httpContext, UserDbModel user, bool isPersistent = false)

to:

public async Task SignIn(HttpContext httpContext, UserDbModel user, bool isPersistent = false)

It's because you should never use async void, especially if you work with HttpContext. Source: Microsoft Docs

The last, but not least, your Configure() method in Startup.cs should contains app.UseAuthorization and app.UseAuthentication in proper order:

if (env.IsDevelopment())
{
    app.UseDeveloperExceptionPage();
}
else
{
    app.UseExceptionHandler("/Home/Error");
    // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
    app.UseHsts();
}

app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseAuthentication();
app.UseRouting();
app.UseAuthorization();
app.UseEndpoints(endpoints =>
{
    endpoints.MapControllerRoute(
        name: "default",
        pattern: "{controller=Home}/{action=Index}/{id?}");
});

Creating custom authentication in ASP.NET Core can be done in a variety of ways. If you want to build off existing components (but don't want to use identity), checkout the "Security" category of docs on docs.asp.net. https://docs.asp.net/en/latest/security/index.html

Some articles you might find helpful:

Using Cookie Middleware without ASP.NET Identity

Custom Policy-Based Authorization

And of course, if that fails or docs aren't clear enough, the source code is at https://github.com/dotnet/aspnetcore/tree/master/src/Security which includes some samples.