AddIdentity vs AddIdentityCore

AddIdentityCore adds the services that are necessary for user-management actions, such as creating users, hashing passwords, etc. Here’s the relevant source:

public static IdentityBuilder AddIdentityCore<TUser>(this IServiceCollection services, Action<IdentityOptions> setupAction)
    where TUser : class
{
    // Services identity depends on
    services.AddOptions().AddLogging();

    // Services used by identity
    services.TryAddScoped<IUserValidator<TUser>, UserValidator<TUser>>();
    services.TryAddScoped<IPasswordValidator<TUser>, PasswordValidator<TUser>>();
    services.TryAddScoped<IPasswordHasher<TUser>, PasswordHasher<TUser>>();
    services.TryAddScoped<ILookupNormalizer, UpperInvariantLookupNormalizer>();
    
    // No interface for the error describer so we can add errors without rev'ing the interface
    services.TryAddScoped<IdentityErrorDescriber>();
    services.TryAddScoped<IUserClaimsPrincipalFactory<TUser>, UserClaimsPrincipalFactory<TUser>>();
    services.TryAddScoped<UserManager<TUser>>();

    ...
}

Essentially, this boils down to registering an instance of UserManager<TUser>, but first registers all of its dependencies. With these services registered, you can retrieve an instance of UserManager<TUser> from DI and create users, set passwords, change emails, etc.

AddIdentity registers the same services as AddIdentityCore, with a few extras:

• Cookie-based authentication schemes for the application itself, external sign-in (e.g. Facebook and Google), and 2FA.
• The SignInManager, which effectively sits on top of the UserManager as a sort of orchestrator. For example, PasswordSignInAsync uses UserManager to retrieve a user, verify the password (if set) and then takes care of cookie creation.
• AddIdentity itself also takes a TRole and registers the services that are necessary for supporting Roles.

Here’s the AddIdentity source for completeness:

public static IdentityBuilder AddIdentity<TUser, TRole>(this IServiceCollection services, Action<IdentityOptions> setupAction)
    where TUser : class
    where TRole : class
{
    // Services used by identity
    services.AddAuthentication(options =>
    {
        options.DefaultAuthenticateScheme = IdentityConstants.ApplicationScheme;
        options.DefaultChallengeScheme = IdentityConstants.ApplicationScheme;
        options.DefaultSignInScheme = IdentityConstants.ExternalScheme;
    })
    .AddCookie(IdentityConstants.ApplicationScheme, o =>
    {
        o.LoginPath = new PathString("/Account/Login");
        o.Events = new CookieAuthenticationEvents
        {
            OnValidatePrincipal = SecurityStampValidator.ValidatePrincipalAsync
        };
    })
    .AddCookie(IdentityConstants.ExternalScheme, o =>
    {
        o.Cookie.Name = IdentityConstants.ExternalScheme;
        o.ExpireTimeSpan = TimeSpan.FromMinutes(5);
    })
    .AddCookie(IdentityConstants.TwoFactorRememberMeScheme, o =>
    {
        o.Cookie.Name = IdentityConstants.TwoFactorRememberMeScheme;
        o.Events = new CookieAuthenticationEvents
        {
            OnValidatePrincipal = SecurityStampValidator.ValidateAsync<ITwoFactorSecurityStampValidator>
        };
    })
    .AddCookie(IdentityConstants.TwoFactorUserIdScheme, o =>
    {
        o.Cookie.Name = IdentityConstants.TwoFactorUserIdScheme;
        o.ExpireTimeSpan = TimeSpan.FromMinutes(5);
    });

    // Hosting doesn't add IHttpContextAccessor by default
    services.AddHttpContextAccessor();
    
    // Identity services
    services.TryAddScoped<IUserValidator<TUser>, UserValidator<TUser>>();
    services.TryAddScoped<IPasswordValidator<TUser>, PasswordValidator<TUser>>();
    services.TryAddScoped<IPasswordHasher<TUser>, PasswordHasher<TUser>>();
    services.TryAddScoped<ILookupNormalizer, UpperInvariantLookupNormalizer>();
    services.TryAddScoped<IRoleValidator<TRole>, RoleValidator<TRole>>();
    // No interface for the error describer so we can add errors without rev'ing the interface
    services.TryAddScoped<IdentityErrorDescriber>();
    services.TryAddScoped<ISecurityStampValidator, SecurityStampValidator<TUser>>();
    services.TryAddScoped<ITwoFactorSecurityStampValidator, TwoFactorSecurityStampValidator<TUser>>();
    services.TryAddScoped<IUserClaimsPrincipalFactory<TUser>, UserClaimsPrincipalFactory<TUser, TRole>>();
    services.TryAddScoped<UserManager<TUser>>();
    services.TryAddScoped<SignInManager<TUser>>();
    services.TryAddScoped<RoleManager<TRole>>();

    ...
}