No service for type 'Microsoft.AspNetCore.Identity.UserManager`1[Microsoft.AspNetCore.Identity.IdentityUser]' has been registered

What is the possible cause of this error:

> InvalidOperationException: No service for type 'Microsoft.AspNetCore.Identity.UserManager [Microsoft.AspNetCore.Identity.IdentityUser]' has been registered.

My target framework is netcoreapp2.1.

This is my user store class:

public class MyUserStore : IdentityUser
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

And my user role class:

public class MyUserRole : IdentityRole
{
    public string Description { get; set; }
}

My DbContext:

public class ApplicationDbContext : IdentityDbContext<MyUserStore,MyUserRole,string>
{
    public ApplicationDbContext(DbContextOptions<ApplicationDbContext> 
      options): base(options) { }
}

My ConfigureServices method in Startup.cs:

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<CookiePolicyOptions>(options =>
    {
        // This lambda determines whether user consent for non-essential cookies is needed for a given request.
        options.CheckConsentNeeded = context => true;
        options.MinimumSameSitePolicy = SameSiteMode.None;
    });

    services.AddDbContext<ApplicationDbContext>(options =>
        options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));

    //services.AddDefaultIdentity<IdentityUser>()
    //    .AddEntityFrameworkStores<ApplicationDbContext>();

    services.AddIdentity<MyUserStore, MyUserRole>(cfg => {
        cfg.User.RequireUniqueEmail = true;
    }).AddEntityFrameworkStores<ApplicationDbContext>();
    services.AddTransient<Seeder>();
    services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
}

i want to understand why this is happening and what is the best practice.

This usually happens in the _LoginPartial.cshtml or _ManageNav.cshtml razor view. Eg.

@inject UserManager<IdentityUser> userManager

Must be changed to

@inject UserManager<MyUserStore> userManager

The same applies to SignInManager.

When registering your own MyUserStore (bad name, should be MyUser) for the AspNetCore Identity, the UserManager<> type will be registered to the ServiceCollection as UserManager<MyUserStore>.

Whenever you want to resolve the UserManager<>, specify the identity user model registered in your startup as the type parameter. Which would be UserManager<MyUserStore> in your specific case:

Or like-wise, when resolving it inside other classes, as may be the case in your Seeder service. The call stack of your exception should give you a hint of where this is happening.

In _LoginPartial.cshtml, replace

@using Microsoft.AspNetCore.Identity
@inject SignInManager<IdentityUser> SignInManager
@inject UserManager<IndentityUser> UserManager

with

@using Microsoft.AspNetCore.Identity
@inject SignInManager<MyUserStore> SignInManager
@inject UserManager<MyUserStore> UserManager

Notice the difference, IdentityUser vs MyUserStore

Had same issue with core 2. One more area where you need to check is the file _ManageNav.cshtml. Try updating the line

@inject SignInManager<IdentityUser> SignInManager

with

@inject SignInManager<YOURCUSTOMMODEL> SignInManager

On the _ManageNav.cshtml file make sure you have the following:

@using PROJECTNAME.Models
@using Microsoft.AspNetCore.Identity
@inject SignInManager<ApplicationUser> SignInManager

Please change PROJECTNAME to the name of your project.

Got same error in a partial view. Turns out if you are injecting sign in manager , you will have to register it in Configure Services (in Startup.cs/ in Program.cs in .Net 6)

In my case added this in ConfigureServices function in Startup.cs

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

add below code in your Startup.cs or IdentityHostingStartup.cs file

 services.AddDefaultIdentity<ApplicationUser>()
                    .AddEntityFrameworkStores<ApplicationDbContext>().AddDefaultTokenProviders().AddDefaultUI();