Dependency injection in ASP.NET Core 2 throws exception

Quoting documentation

Services Available in Startup

ASP.NET Core dependency injection provides application services during
an application’s startup. You can request these services by including
the appropriate interface as a parameter on your Startup class’s
constructor or one of its Configure or ConfigureServices methods.

Looking at each method in the Startup class in the order in which
they are called, the following services may be requested as
parameters:

• In the constructor: IHostingEnvironment, ILoggerFactory
• In the ConfigureServices method: IServiceCollection
• In the Configure method: IApplicationBuilder, IHostingEnvironment,
  ILoggerFactory, IApplicationLifetime

You are trying to resolve services that are not available during startup,

...CommunicatorContext dbContext, ILdapService ldapService) {

which will give you the errors you are getting. If you need access to the implementations then you need to do one of the following:

1. Modify the ConfigureServices method and access them there from the service collection. i.e.

   public IServiceProvider ConfigureServices(IServiceCollection services) {
       services.AddDbContext<CommunicatorContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
       services.AddCookieAuthentication();
       services.Configure<LdapConfig>(Configuration.GetSection("Ldap"));
       services.AddScoped<ILdapService, LdapService>();
       services.AddMvc();
   
       // Build the intermediate service provider
       var serviceProvider = services.BuildServiceProvider();
   
       //resolve implementations
       var dbContext = serviceProvider.GetService<CommunicatorContext>();
       var ldapService = serviceProvider.GetService<ILdapService>();
       DbInitializer.Initialize(dbContext, ldapService);
   
       //return the provider
       return serviceProvider();
   }
   

2. Modify the ConfigureServices method to return IServiceProvider, Configure method to take a IServiceProvider and then resolve your dependencies there. i.e.

   public IServiceProvider ConfigureServices(IServiceCollection services) {
       services.AddDbContext<CommunicatorContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
       services.AddCookieAuthentication();
       services.Configure<LdapConfig>(Configuration.GetSection("Ldap"));
       services.AddScoped<ILdapService, LdapService>();
       services.AddMvc();
   
       // Build the intermediate service provider then return it
       return services.BuildServiceProvider();
   }
   
   public void Configure(IApplicationBuilder app, IHostingEnvironment env, 
                         ILoggerFactory loggerFactory, IServiceProvider serviceProvider) {
   
       //...Other code removed for brevity
   
       app.UseMvc();
   
       //resolve dependencies
       var dbContext = serviceProvider.GetService<CommunicatorContext>();
       var ldapService = serviceProvider.GetService<ILdapService>();
       DbInitializer.Initialize(dbContext, ldapService);
   }