Hello, 

Here are a couple of tips that will hopefully help you 😀

1. Ensure any async calls are awaited and that a Task is always returned (not void).
2. In order to execute CreateAsync in parallel the UserManager cannot use the same DbContext instance. A new context can be provided when instantiating the UserManager, for example:
   

   var userManager = new UserManager<ApplicationUser>(new UserStore<ApplicationUser>(new ApplicationDbContext()));

   The main point is that in every thread the user manager should to use a new DbContext, preventing issues such as 'A second operation was started'. It should be possible to do this using dependency injection as well.
3. The UserManager will auto save your changes. You can possibly turn this off and bulk save the newly created users for better performane. See this post on how to do that. 

Hi ynze, thanks for the response

currently I am getting the user manager in scheduled job like this.

var userManager = ServiceLocator.Current.GetInstance<ApplicationUserManager<SiteUser>>();

when I changed this to

var userManager = new UserManager<ApplicationUser>(new UserStore<ApplicationUser>(new ApplicationDbContext()));

I am facing two issues

At UserManager:  There is no argument given that corresponds to the required formal parameter 'optionsAccessor' of 'UserManager<ApplicationUser>.UserManager(IUserStore<ApplicationUser>, IOptions<IdentityOptions>, IPasswordHasher<ApplicationUser>, IEnumerable<IUserValidator<ApplicationUser>>, IEnumerable<IPasswordValidator<ApplicationUser>>, ILookupNormalizer, IdentityErrorDescriber, IServiceProvider, ILogger<UserManager<ApplicationUser>>)

At ApplicationDbContext: I got the following errors

1. Using the generic type 'ApplicationDbContext<TUser>' requires 1 type arguments.

There is no argument given that corresponds to the required formal parameter 'options' of 'ApplicationDbContext<ApplicationUser>.ApplicationDbContext(DbContextOptions<ApplicationDbContext<ApplicationUser>>)

Currently modified as below but still facing issue with UserManager regarding 'optionsAccessor' parameter

DbContextOptions<ApplicationDbContext<ApplicationUser>> options = new();
var userManager = new UserManager<ApplicationUser>(new UserStore<ApplicationUser>(new ApplicationDbContext<ApplicationUser>(options)));

Can you please help me with these parameters which are required in the instantiation of the user manager

why do you need to run in parallel?  If is 100K users one time I would just create a schedule job and do synchronously

+1 to Mark.

await immediately CreateAsync one at a time.

As a rule of thumb, always try to max out single thread performance before thinking about running in parallel. DotTrace is a great tool to find bottleneck and optimize .

I am handling this migration in scheduled job only. The issue with importing users synchronously is that the process is slowing down with the number of migrated users. It completed only 16000 users in 4hrs and I am trying to speed up this process.

As I said above, try dotTrace to see if you can find the bottlenecks in your code

Praveen. I've just written this quick snippet. Try this and amend to your own needs if it works. Remember to call wait() on your entire method of logic inside the Execute() of the schedule job if you want the job not to finish before your async methods do.

                foreach (var user in userList)
                {
                    // Create new dbcontext for every user
                    var _configuration = ServiceLocator.Current.GetInstance<IConfiguration>();
                    var options = new DbContextOptionsBuilder<ApplicationDbContext<SiteUser>>().UseSqlServer(_configuration.GetConnectionString("EcfSqlConnection")).Options;
                    using (var dbContext = new ApplicationDbContext<SiteUser>(options))
                    {
                        var store = new UserStore<SiteUser>(dbContext);
                        var result = Task.Run(() => store.CreateAsync(user)).Result;
                    }
                }

Thanks, Surjit for the response. Very much helpful