c# - WebAPI POST 方法抛出空异常

Question

我创建了一个 WebAPI 项目，它在使用 DELETE 和 GET 方法时工作正常，但是当我尝试 POST 或 PUT 时，我收到了

{"Value cannot be null.\r\nParameter name: entity"} on line 84, db.Reviews.Add(review)

我正在使用 Postman 进行测试，我也尝试过使用 XMLHttpRequest，但这两种方法都适用于 GET/DELETE，但在 PUT/POST 上失败。

var update1 = new XMLHttpRequest();
var params = "ReviewID=123";
update1.open("POST", "http://localhost:49681/Api/Reviews/", true);
update1.setRequestHeader("Content-type", "application/json");
update1.send(params);

评论 Controller

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.Entity;
using System.Data.Entity.Infrastructure;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Threading.Tasks;
using System.Web.Http;
using System.Web.Http.Description;
using ReviewApp.Models;

namespace ReviewApp.Controllers
{
    public class ReviewsController : ApiController
    {
        private CheetahEntities db = new CheetahEntities();

        // GET: api/Reviews
        public IQueryable<Review> GetReviews()
        {
            return db.Reviews;
        }

        // GET: api/Reviews/5
        [ResponseType(typeof(Review))]
        public async Task<IHttpActionResult> GetReview(string id)
        {
            Review review = await db.Reviews.FindAsync(id);
            if (review == null)
            {
                return NotFound();
            }

            return Ok(review);
        }

        // PUT: api/Reviews/5
        [ResponseType(typeof(void))]
        public async Task<IHttpActionResult> PutReview(string id, Review review)
        {
            if (!ModelState.IsValid)
            {
                return BadRequest(ModelState);
            }

            if (id != review.ReviewID)
            {
                return BadRequest();
            }

            db.Entry(review).State = EntityState.Modified;

            try
            {
                await db.SaveChangesAsync();
            }
            catch (DbUpdateConcurrencyException)
            {
                if (!ReviewExists(id))
                {
                    return NotFound();
                }
                else
                {
                    throw;
                }
            }

            return StatusCode(HttpStatusCode.NoContent);
        }

        // POST: api/Reviews
        [ResponseType(typeof(Review))]
        public async Task<IHttpActionResult> PostReview(Review review)
        {
            /*
            if (!ModelState.IsValid)
            {
                return BadRequest(ModelState);
            }
            */
            db.Reviews.Add(review);

            try
            {
                await db.SaveChangesAsync();
            }
            catch (DbUpdateException)
            {
                if (ReviewExists(review.ReviewID))
                {
                    return Conflict();
                }
                else
                {
                    throw;
                }
            }

            return CreatedAtRoute("DefaultApi", new { id = review.ReviewID }, review);
        }

        // DELETE: api/Reviews/5
        [ResponseType(typeof(Review))]
        public async Task<IHttpActionResult> DeleteReview(string id)
        {
            Review review = await db.Reviews.FindAsync(id);
            if (review == null)
            {
                return NotFound();
            }

            db.Reviews.Remove(review);
            await db.SaveChangesAsync();

            return Ok(review);
        }

        protected override void Dispose(bool disposing)
        {
            if (disposing)
            {
                db.Dispose();
            }
            base.Dispose(disposing);
        }

        private bool ReviewExists(string id)
        {
            return db.Reviews.Count(e => e.ReviewID == id) > 0;
        }
    }
}

回顾

//------------------------------------------------------------------------------
// <auto-generated>
//     This code was generated from a template.
//
//     Manual changes to this file may cause unexpected behavior in your application.
//     Manual changes to this file will be overwritten if the code is regenerated.
// </auto-generated>
//------------------------------------------------------------------------------

namespace ReviewApp.Models
{
    using System;
    using System.Collections.Generic;

    public partial class Review
    {
        public string ReviewID { get; set; }
        public string ReviewStatus { get; set; }
        public Nullable<System.DateTime> Date { get; set; }
        public string Account { get; set; }
    }
}

猎豹实体

//------------------------------------------------------------------------------
// <auto-generated>
//     This code was generated from a template.
//
//     Manual changes to this file may cause unexpected behavior in your application.
//     Manual changes to this file will be overwritten if the code is regenerated.
// </auto-generated>
//------------------------------------------------------------------------------

namespace ReviewApp.Models
{
    using System;
    using System.Data.Entity;
    using System.Data.Entity.Infrastructure;

    public partial class CheetahEntities : DbContext
    {
        public CheetahEntities()
            : base("name=CheetahEntities")
        {
        }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            throw new UnintentionalCodeFirstException();
        }

        public virtual DbSet<Review> Reviews { get; set; }
    }
}

Answer 1

在请求中，您的内容类型为 application/json，但发送您的 params 时带有 ReviewId=123，这不是 json。如果您将 Content-Type 作为 application/json 发送，params 应该是正确格式化的 JSON 对象。

var update1 = new XMLHttpRequest();
var params = JSON.stringify({ ReviewID : 123 }); //JSON payload
update1.open("POST", "http://localhost:49681/Api/Reviews/", true);
update1.setRequestHeader("Content-type", "application/json");
update1.send(params);

在您的 PostReview 方法中，如果 POST 请求是未正确发送，但在尝试将其添加到数据库之前没有检查变量。您注释掉了对模型的验证检查。

// POST: api/Reviews
[ResponseType(typeof(Review))]
public async Task<IHttpActionResult> PostReview(Review review) {

    if (review == null) ModelState.AddModelError("", "invalid data");

    if (!ModelState.IsValid) {
        return BadRequest(ModelState);
    }

    db.Reviews.Add(review);

    try {
        await db.SaveChangesAsync();
    } catch (DbUpdateException) {
        if (ReviewExists(review.ReviewID)) {
            return Conflict();
        } else {
            throw;
        }
    }

    return CreatedAtRoute("DefaultApi", new { id = review.ReviewID }, review);
}