InvalidOperationException while running ASP.NET Core MVC Web App

Question

I'm new to learning ASP.NET Core MVC C#. I tried learning from a Udemy course, but they were using an old version believe it was 2.0. But I'm trying to build a note taking web application. I keep receiving an Unhandled exception occurred while processing the request. Followed by InvalidOperationException: Unable to resolve service for type 'Notely_Application.Repository.INoteRepository' while attempting to activate 'Notely_Application.Controllers.HomeController'.

Here is the source code for both:

Notely_Application.Repository.INoteRepository

using System;
using System.Collections.Generic;
using Notely_Application.Models;

namespace Notely_Application.Repository
{
    public interface INoteRepository
    {
        NotesModel FindNoteById(Guid id);
        IEnumerable<NotesModel> GetAllNotes();
        void SaveNote(NotesModel notesModel);
        void DeleteNote(NotesModel notesModel);
    }
}

Notely_Application.Controllers.HomeController

using System;
using System.Diagnostics;
using System.Linq;
using Microsoft.AspNetCore.Mvc;
using Notely_Application.Models;
using Notely_Application.Repository;

namespace Notely_Application.Controllers
{
    public class HomeController : Controller
    {
        private readonly INoteRepository _noteRepository;

        public HomeController(INoteRepository noteRepository)
        {
            _noteRepository = noteRepository;
        }

        public IActionResult Index()
        {
            var notes = _noteRepository.GetAllNotes().Where(n => n.IsDeleted == false);
            return View(notes);
        }

        public IActionResult NoteDetail(Guid id)
        {
            var note = _noteRepository.FindNoteById(id);
            return View(note);
        }

        [HttpGet]
        public IActionResult NoteEditor(Guid id = default)
        {
            if(id != Guid.Empty)
            {
                var note = _noteRepository.FindNoteById(id);

                return View(note);
            }
            return View();
        }

        [HttpPost]
        public IActionResult NoteEditor(NotesModel notesModel)
        {
            if (ModelState.IsValid)
            {
                var date = DateTime.Now;
                if (notesModel != null && notesModel.Id == Guid.Empty)
                {
                    notesModel.Id = Guid.NewGuid();
                    notesModel.CreatedDate = date;
                    notesModel.LastModified = date;

                    _noteRepository.SaveNote(notesModel);
                }
                else
                {
                    var note = _noteRepository.FindNoteById(notesModel.Id);
                    note.LastModified = date;
                    note.Subject = notesModel.Subject;
                    note.Detail = notesModel.Detail;
                }
                return RedirectToAction("Index");
            }
            else
            {
                return View();
            }
        }

        public IActionResult DeleteNote(Guid id)
        {
            var note = _noteRepository.FindNoteById(id);

            note.IsDeleted = true;

            return RedirectToAction("Index");
        }

        public IActionResult Privacy()
        {
            return View();
        }

        [ResponseCache(Duration = 0, Location = ResponseCacheLocation.None, NoStore = true)]
        public IActionResult Error()
        {
            return View(new ErrorViewModel { RequestId = Activity.Current?.Id ?? HttpContext.TraceIdentifier });
        }
    }
}

Answer 1

You need an implementation of INoteRepository, do you have that? And you have to add the dependency injection mapping in Startup.cs in the ConfigureServices() method, a line like similar

services.AddTransient<INoteRepository, ClassThatImplementsINoteRepository>();

Answer 2

First of all I suggest you to use

try
{
    // my code here
}
catch (Exception ex)
{
    // my error here -> ex       
}

and then read exactly what error is inside ex