`Model.plans.Count` 抛出类型为'System.NullReferenceException` 的异常

Question

我正在尝试将字符串对象列表显示为复选框。我有两个项目，ASP.NET Core MVC 应用程序项目和 ASP.NET Core Web API 都运行在 5.0 NET Core 上运行。在 MVC 项目中，我正在调用 Web API 项目，它将在其中创建一个新的对象列表。然后在我的 MVC 项目中我做了一个项目引用来调用视图中的控制器。当我运行应用程序时，MVC 将命中它调用 API 项目的行。在我的 API 控制器的 GetPlan() 方法中存储新创建的对象后，我尝试在我的 MVC 视图中将这些创建的对象显示为复选框。但是，当我这样做时，它说 Model.plans.Count 抛出了 System.NullReferenceException 类型的异常。然后进一步查看它，它只会告诉我：

System.NullReferenceException: 'Object reference not set to an instance of an object.'

Microsoft.AspNetCore.Mvc.Razor.RazorPage.Model.get returned null.

我不明白为什么我在已经创建了新对象的情况下返回 null。

MVC 项目：

Controller 我打电话给 API:

 public IActionResult SecIndex()
    {
        // call planapi
        var result = _planServiceClient.RetrievePlans().Result;
        _logger.Log(LogLevel.Information, "Reached the Second Page View....");
        return View();
    }

然后是我的 API 项目：

Model class for ViewPhoneNumberInput:

  public class ViewPhoneNumberInput
    {
        [Required(ErrorMessage = "You did not enter your phone number! Please enter your phone number!")]
        public String PhoneNumber { get; set; }
        public List<Plans> plans { get; set; }
    }

我的计划模型class：

    public class Plans
    {
        public int PlanId { get; set; }
        public string PlanName { get; set; }
    }

Controller 对于 api:

        [HttpGet]
        [Route("AvailablePlans")]
        public ActionResult GetPlan()
        {

            var _model = new ViewPhoneNumberInput();
            _model.plans = new List<Plans>();

            _model.plans.Add(new Plans { PlanId = 1, PlanName = "Internet" });
            _model.plans.Add(new Plans { PlanId = 2, PlanName = "TV & Streaming" });
            _model.plans.Add(new Plans { PlanId = 3, PlanName = "Mobile" });
            _model.plans.Add(new Plans { PlanId = 4, PlanName = "Home Security" });
            _model.plans.Add(new Plans { PlanId = 5, PlanName = "Home Phone" });


            var plansAvailable = JsonSerializer.Serialize(_model.plans);
            _logger.Log(LogLevel.Information, "Response-Body: {@Response-Body}", plansAvailable);


            return Ok(_model.plans);
        }

上面 _model.plans 将保存创建的新对象的值，所以我去我的 MVC 视图项目调用它，这样我就可以显示它，但这就是它说 Model.plans.Count 引发了类型为“System.NullReferenceException”的异常。

View 对于 MVC 项目：

@model Plan.API.Models.ViewPhoneNumberInput


<h2>Second page</h2>




@using (Html.BeginForm())
{

    for (int i = 0; i < Model.plans.Count; i++)
    {
        <input type="checkbox" id="@Model.plans[i].PlanId" name="Model.plans[i].PlanId" />
        <label for="Model.plans[i].PlanId"> @Model.plans[i].PlanName </label> <br />
    }


    @Html.TextBoxFor(r => r.PhoneNumber)
    <input id="Button" type="submit" value="Next" />

    <p>  @Html.ValidationMessageFor(r => r.PhoneNumber) </p>

}

为什么它告诉我这个而不是实际保存来自 API 控制器的新创建的值？

在实施下面的建议时，我能够看到 PhoneNumber 的值，但对于 Plans，它显示为空（当鼠标悬停在 post SecIndex 中的 phoneNumberInput 上时）。

我的检索计划():

        public async Task<List<Plan.API.Models.Plans>> RetrievePlans()
        {
            var response = await _httpClient.GetAsync("api/plans/AvailablePlans");
            response.EnsureSuccessStatusCode();

            var result = await response.Content.ReadAsStringAsync();

           var json = JsonConvert.DeserializeObject<List<Plan.API.Models.Plans>>(result);

            return json;
        }

我的更新 SecIndex() with Post SecIndex():

        public IActionResult SecIndex()
        {
            // call planapi
            var result = _planServiceClient.RetrievePlans().Result;
            _logger.Log(LogLevel.Information, "Reached the Second Page View....");
            return View(new Plan.API.Models.ViewPhoneNumberInput { plans = result });//So that you don't need to change the model in the view and the parameter type in the HttpPost Action.
        }

        [HttpPost]
        public IActionResult SecIndex(Plan.API.Models.ViewPhoneNumberInput phoneNumberInput)
        {
            try
            {

                if (ModelState.IsValid)
                {
                    _logger.Log(LogLevel.Information, "User successfully entered their phone number.");
                    return RedirectToAction("FinalIndex", "Final");
                }
                else
                {
                    _logger.Log(LogLevel.Error, "User missed to enter their phone number");
                    throw new Exception("User did not enter a phone number in the textfield!");
                }
             
            }
            catch(Exception ex)
            {
                _logger.LogError(ex, "Empty textfield!");
                return View(phoneNumberInput);
            }
        }

SecIndex.cshtml

@model Plan.API.Models.ViewPhoneNumberInput

<h2>Second page</h2>


@using (Html.BeginForm())
{

    for (int i = 0; i < Model.plans.Count; i++)
    {
        <input type="checkbox" id="@Model.plans[i].PlanId" />
        <input hidden value="@Model.plans[i].PlanId" />
        <input hidden value="@Model.plans[i].PlanName" />

        <label for="Model.plans[i].PlanId"> @Model.plans[i].PlanName </label>
        <br />
    }


    @Html.TextBoxFor(r => r.PhoneNumber)
    <input id="Button" type="submit" value="Next" />

    <p>  @Html.ValidationMessageFor(r => r.PhoneNumber) </p>

}
<script>
    $('form').submit(function () {
        var count = 0;
        $('input[type=checkbox]').each(function () {
            if (this.checked) {
                this.nextElementSibling.name = "plans["+count+"].PlanId";
                this.nextElementSibling.nextElementSibling.name = "plans[" + count + "].PlanName";
                count++;
            }
        });
    });
</script>

我的 API GetPlan 控制器：

        [HttpGet]
        [Route("AvailablePlans")]
        public List<Plans> GetPlan()
        {

            var _model = new ViewPhoneNumberInput();
            _model.plans = new List<Plans>();

            _model.plans.Add(new Plans { PlanId = 1, PlanName = "Internet" });
            _model.plans.Add(new Plans { PlanId = 2, PlanName = "TV & Streaming" });
            _model.plans.Add(new Plans { PlanId = 3, PlanName = "Mobile" });
            _model.plans.Add(new Plans { PlanId = 4, PlanName = "Home Security" });
            _model.plans.Add(new Plans { PlanId = 5, PlanName = "Home Phone" });


            var plansAvailable = JsonSerializer.Serialize(_model.plans);
            _logger.Log(LogLevel.Information, "Response-Body: {@Response-Body}", plansAvailable);


            return _model.plans;
        }

Answer 1

@MarCo 问题是你没有通过你的模式

return View(result);

Answer 2

这是一个工作演示：

api方法：

[HttpGet]
        [Route("AvailablePlans")]
        public List<Plan.API.Models.Plans> GetPlan()
        {

            var _model = new ViewPhoneNumberInput();
            _model.plans = new List<Plans>();

            _model.plans.Add(new Plans { PlanId = 1, PlanName = "Internet" });
            _model.plans.Add(new Plans { PlanId = 2, PlanName = "TV & Streaming" });
            _model.plans.Add(new Plans { PlanId = 3, PlanName = "Mobile" });
            _model.plans.Add(new Plans { PlanId = 4, PlanName = "Home Security" });
            _model.plans.Add(new Plans { PlanId = 5, PlanName = "Home Phone" });


            var plansAvailable = JsonSerializer.Serialize(_model.plans);
            _logger.Log(LogLevel.Information, "Response-Body: {@Response-Body}", plansAvailable);


            return _model.plans;
        }

控制器动作：

public IActionResult SecIndex()
    {
        // call planapi
        var result = _planServiceClient.RetrievePlans().Result;
        _logger.Log(LogLevel.Information, "Reached the Second Page View....");
        return View(new ViewPhoneNumberInput {  plans=result});//So that you don't need to change the model in the view and the parameter type in the HttpPost Action.
    }

查看：

@model Plan.API.Models.ViewPhoneNumberInput


<h2>Second page</h2>
@using (Html.BeginForm())
{

    for (int i = 0; i < Model.plans.Count; i++)
    {
        <input type="checkbox" id="@Model.plans[i].PlanId" />
        <input hidden value="@Model.plans[i].PlanId"/>
        <input hidden value="@Model.plans[i].PlanName" />

        <label for="Model.plans[i].PlanId"> @Model.plans[i].PlanName </label>
        <br />
    }


    @Html.TextBoxFor(r => r.PhoneNumber)
    <input id="Button" type="submit" value="Next" />

    <p>  @Html.ValidationMessageFor(r => r.PhoneNumber) </p>

}
<script>
    $('form').submit(function () {
        var count = 0;
        $('input[type=checkbox]').each(function () {
            if (this.checked) {
                this.nextElementSibling.name = "plans["+count+"].PlanId";
                this.nextElementSibling.nextElementSibling.name = "plans[" + count + "].PlanName";
                count++;
            }
        });
    });
    </script>

结果：

`Model.plans.Count` 抛出类型为'System.NullReferenceException` 的异常

`Model.plans.Count` threw an exception of type 'System.NullReferenceException`

c#

asp.net

asp.net-mvc

asp.net-core-mvc

asp.net-core-webapi