用户未处理 ASP.Net 核心 NullReferenceException [重复]

Question

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我几乎是 ASP.net Core 开发的新手。我正在构建一个使用 MySql 作为我的数据库的示例 Web api。

我的书模型：

public class Book
    {   
        [Key]
        public long Id { get; set; }
        [Required]
        public string Name { get; set; }
        [Required]
        public string Author { get; set; }
        public DateTime CreatedOn { get; set; } = DateTime.UtcNow;

        public Book()
        {

        }
    }

    public class WebAPIDataContext : DbContext
    {
        public WebAPIDataContext(DbContextOptions<WebAPIDataContext> options)
            : base(options)
        {
        }
        public DbSet<Book> Books { get; set; }
    }

书库：

public class BookRepository: IBookRepository
    {
        private readonly WebAPIDataContext _db;

        public BookRepository(WebAPIDataContext db) {
            _db = db;
        }

        public Book CreateBook(Book book) {
            _db.Books.Add(book);
            _db.SaveChanges();
            return book;
        }

        public void DeleteBook(long id)
        {
            Book book = GetBook(id);
            if (book != null) {
                _db.Books.Remove(book);
                _db.SaveChanges();
            }
        }

        public List<Book> GetAllBooks()
        {
            return _db.Books.AsNoTracking().ToList();
        }

        public Book GetBook(long id)
        {
            return _db.Books.FirstOrDefault(o => o.Id == id);
        }

        public void UpdateBook(long id, Book book)
        {
            throw new NotImplementedException();
        }
    }

    public interface IBookRepository
    {
        List<Book> GetAllBooks();
        Book GetBook(long id);
        Book CreateBook(Book book);
        void UpdateBook(long id, Book book);
        void DeleteBook(long id);
    }

和 Book 控制器：

[Route("api/[controller]")]
    public class BooksController : Controller
    {
        private readonly IBookRepository _books;

        [HttpGet("")]
        public IActionResult GetAllBooks()
        {
            List<Book> books = _books.GetAllBooks();
            return Ok(books);
        }

        [HttpGet("{id}")]
        public IActionResult GetBook(long id)
        {
            Book book = _books.GetBook(id);
            if (book == null)
            {
                return NotFound();
            }
            return Ok(book);
        }

        [HttpPost]
        public IActionResult CreateBook([FromBody] Book book)
        {
            if (ModelState.IsValid == false)
            {
                return BadRequest(ModelState);
            }

            Book createdBook= _books.CreateBook(book);

            return CreatedAtAction(
                nameof(GetBook), new { id = createdBook.Id }, createdBook);
        }

        [HttpPut("{id}")]
        public IActionResult UpdateBook(long id, [FromBody] Book book)
        {
            if (ModelState.IsValid == false)
            {
                return BadRequest(ModelState);
            }

            try
            {
                _books.UpdateBook(id, book);
                return Ok();
            }
            catch (EntityNotFoundException<Book>)
            {
                return NotFound();
            }
        }

        [HttpDelete("{id}")]
        public IActionResult DeleteBook(long id)
        {
            _books.DeleteBook(id);
            return Ok();
        }
    }

当我从 Swagger 发出 POST 请求以创建一本新书时，它给了我NullReferenceException 错误。我做错了什么？

Answer 1

要使第一个案例起作用，您需要拥有

services.AddScoped<IBookRepository, BookRepository>();

在ConfigureServices方法中位于startup.cs

public class BooksController : Controller
{
   public BooksController(IBookRepository books){
       _books = books;
   }
   ...

现在假设您已将 IBookRepository 连接到启动类中的 Asp.NET Core 容器中。

如果没有那么这就是你所缺少的！

public class BooksController : Controller
{
   public BooksController(){
       _books = new BookRepository();
   }

如果您在此之前从未使用过依赖注入，我建议您阅读它，也可以使用 IOC（控制反转）。