Unable to resolve service for type 'Microsoft.AspNetCore.Identity.UserManager' while attempting to activate 'Management.Controllers.RoleController'

Question

I want to create Roles and assign users to those Roles, I have created Roles but I didn't know how to assign users to them, I tried to add UserManager to services in startup but it doesn't work. I have this error:

InvalidOperationException: Unable to resolve service for type 'Microsoft.AspNetCore.Identity.UserManager`1[PFE_Management.Models.AppUser]' while attempting to activate 'PFE_Management.Controllers.RoleController'.

This the controller:

namespace PFE_Management.Controllers
{
    public class RoleController : Controller
    {
        private RoleManager<IdentityRole> roleManager;
        private UserManager<AppUser> userManager;
        public RoleController(RoleManager<IdentityRole> roleMgr, UserManager<AppUser> userMrg)
        {
            roleManager = roleMgr;
            userManager = userMrg;
        }

        //some code here
        // le code qui suive c'est ajouter ou supprimer un utilisateur pour un Role
        public async Task<IActionResult> Update(string id)
        {
            //some code here
        }
        [HttpPost]
        public async Task<IActionResult> Update(RoleModification model)
        {
            IdentityResult result;
            if (ModelState.IsValid)
            {
                foreach (string userId in model.AddIds ?? new string[] { })
                {
                    AppUser user = await userManager.FindByIdAsync(userId);
                    if (user != null)
                    {
                        result = await userManager.AddToRoleAsync(user, model.RoleName);
                        if (!result.Succeeded)
                            Errors(result);
                    }
                }
                foreach (string userId in model.DeleteIds ?? new string[] { })
                {
                    AppUser user = await userManager.FindByIdAsync(userId);
                    if (user != null)
                    {
                        result = await userManager.RemoveFromRoleAsync(user, model.RoleName);
                        if (!result.Succeeded)
                            Errors(result);
                    }
                }
            }

            if (ModelState.IsValid)
                return RedirectToAction(nameof(Index));
            else
                return await Update(model.RoleId);
        }
    }
}

This is services in startup.cs:

public void ConfigureServices(IServiceCollection services)
    {
        services.AddDbContext<ApplicationDbContext>(options =>
            options.UseSqlServer(
                Configuration.GetConnectionString("DefaultConnection")));
        services.AddDefaultIdentity<IdentityUser>(options => options.SignIn.RequireConfirmedAccount = true)                
            .AddRoles<IdentityRole>()
            .AddEntityFrameworkStores<ApplicationDbContext>();

        services.AddControllersWithViews();
        services.AddRazorPages();    
    }

Answer 1

AddDefaultIdentity doesn't add the user manager into the service collection. You need to add it yourself. For example:

services.AddDefaultIdentity<IdentityUser>(...)                
    .AddRoles<IdentityRole>()
    .AddUserManager<IdentityUser>() // <-- add this
    .AddEntityFrameworkStores<ApplicationDbContext>();