demo.cs

public void ConfigureServices(IServiceCollection services) 
{
  services.AddDbContext<AppDbContext>(options => 
options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")
);

services
  .AddMvc()
  .AddJsonOptions(
     options => options.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore
  )
 SetCompatibilityVersion(CompatibilityVersion.Version_2_2);

 services.AddScoped<Seeder>();
}

public void Configure(IApplicationBuilder app, IHostingEnvironment env, Seeder seeder)
{
  var post = File.ReadAllText(@"wwwroot/post.json");
  var comment = File.ReadAllText(@"wwwroot/comment.json");
  seeder.Seedit(post, comment);
}
 

public class Seeder
    {
        private readonly AppDbContext _context;

        public Seeder(AppDbContext context)
        {
            _context = context;

        }

        public void Seedit(string postData, string commentData)
        {
            JsonSerializerSettings settings = new JsonSerializerSettings
            {
                ContractResolver = new PrivateSetterContractResolver()

            };

            List<Post> posts = JsonConvert.DeserializeObject<List<Post>>(postData, settings);
            List<Comment> comments = JsonConvert.DeserializeObject<List<Comment>>(commentData, settings);

            if (!_context.Posts.Any())
            {
                _context.AddRange(posts);
                _context.SaveChanges();
            }

            if (!_context.Comments.Any())
            {
                _context.AddRange(comments);
                _context.SaveChanges();
            }
        }
    }