Get services from scope with IWebHost

GetServiceFromScope.cs

// ASP Net Core 2.2.2
// Get registered services from WebHost instance.
// In case the IServiceCollection or ApplicationBuilder is not available in the current context.
// The method can be used in separate assembly 

pulbic void TestMethod() 
{
    using (IServiceScope scope = this.webHost.Services.GetRequiredService<IServiceScopeFactory>().CreateScope())
    {
        RoleManager<IdentityRole> roleManager = scope.ServiceProvider.GetRequiredService<RoleManager<IdentityRole>>();
    }
}

// Source: https://github.com/aspnet/Mvc/issues/4783#issuecomment-381978090

For RoleManager<IdentityRole>, the AddRoles() method should be used:

services.AddDefaultIdentity<IdentityUser>()
      .AddRoles<IdentityRole>()