.NET Core 7迁移后RazorPage.Model.get返回空引用异常排查

将ASP.NET Core 3.1项目中的州与地方政府联动下拉框功能迁移至.NET Core 7后失效，抛出以下异常：

System.NullReferenceException: 'Object reference not set to an instance of an object
Microsoft.AspNetCore.Mvc.Razor.RazorPage.Model.get returned null.

SQL表无空列，已用JSON+Ajax实现交互，且已安装Microsoft.AspNetCore.Mvc.NewtonsoftJson包，相关代码如下：

模型类 #

public class BiodataVM
{
    public BiodataVM()
    {
        this.SteteCodes = new List<SelectListItem>();
        this.LocalCodes = new List<SelectListItem>();
    }
    public List<SelectListItem> SteteCodes { get; set; }
    public List<SelectListItem> LocalCodes { get; set; }

    public int LId { get; set; } 
    public int SId { get; set; } 
}

视图代码 #

<form asp-action="Create">
<select id="statecode" asp-for="SId" class="form-control">
      <option value="">--请选择--</option>
      @foreach (var item in ViewBag.Statelist)
      {
         <option value="@item.Value">@item.Text</option>
      }
 </select>
 <select id="localcode" asp-for="LId" asp-items="Model.LocalCodes" class="form-control">
    <option value="">--请选择--</option>
 </select>
</form>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>

<script type="text/javascript">
    $(function () {
        $("select").each(function () {
            if ($(this).find("option").length <= 1) {
                $(this).attr("disabled", "disabled");
            }
        });

        $("select").change(function () {
            var value = 0;
            if ($(this).val() != "") {
                value = $(this).val();
            }
            var id = $(this).attr("id");
            $.ajax({
                type: "POST",
                url: "/Biodata/AjaxMethod",
                data: { value: value, type: id },
                success: function (response) {
                    switch (id) {
                        case "statecode":
                            DisableDropDown("#localcode");
                            PopulateDropDown("#localcode", response.LocalCodes);
                            break;
                    }

                },
                failure: function (response) {
                    alert(response.responseText);
                },
                error: function (response) {
                    alert(response.responseText);
                }
            });
        });

        if ($("#statecode").val() != "" && $("#localcode").val() != "") {
            var message = "StateCode: " + $("#statecode option:selected").text();
            message += "\nLocalCode: " + $("#localcode option:selected").text();
            alert(message);
        }
    });

    function DisableDropDown(dropDownId) {
        $(dropDownId).attr("disabled", "disabled");
        $(dropDownId).empty().append('<option selected="selected" value="">--请选择--</option>');
    }

    function PopulateDropDown(dropDownId, list) {
        if (list != null && list.length > 0) {
            $(dropDownId).removeAttr("disabled");
            $.each(list, function () {
                $(dropDownId).append($("<option></option>").val(this['Value']).html(this['Text']));
            });
        }
    }
</script>

ActionResult代码 #

BiodataVM model = new BiodataVM
{
    SteteCodes = (from a in this._context.State
                 select new SelectListItem
                {
                    Value = a.SId.ToString(),
                    Text = a.State
                }).ToList(),
};
ViewBag.Statelist = model.SteteCodes;

JsonResult代码 #

public JsonResult AjaxMethod(string type, int value)
{
    BiodataVM model = new BiodataVM();
    switch (type)
    {
        case "statecode":
            model.LocalCodes = (from z in this._context.LocalGovt
                        where z.SId == value
                        select new SelectListItem
                        {
                            Value = z.LId.ToString(),
                            Text = z.LocalGovt
                        }).ToList();
            break;
    }
    return Json(model);
}

问题原因及解决办法 #

核心问题 #

视图中使用了Model.LocalCodes，但返回视图的Action里没有将实例化的BiodataVM模型传递给视图，导致Razor页面的Model属性为null，触发空引用异常。ASP.NET Core 3.1的兼容特性可能掩盖了这个问题，.NET 7对模型绑定的检查更严格，所以报错暴露了问题。

解决步骤 #

1. 修改返回视图的Action，传递模型

public IActionResult Create()
{
    BiodataVM model = new BiodataVM
    {
        SteteCodes = (from a in this._context.State
                     select new SelectListItem
                    {
                        Value = a.SId.ToString(),
                        Text = a.State
                    }).ToList(),
    };
    ViewBag.Statelist = model.SteteCodes;
    // 关键：将model传递给视图
    return View(model);
}

2. 可选优化：用模型属性替代ViewBag
   既然BiodataVM已定义SteteCodes，可以直接在视图中遍历模型属性，减少对ViewBag的依赖：

<select id="statecode" asp-for="SId" class="form-control">
    <option value="">--请选择--</option>
    @foreach (var item in Model.SteteCodes)
    {
        <option value="@item.Value">@item.Text</option>
    }
</select>

3. 验证NewtonsoftJson配置
   确保在Program.cs中正确配置NewtonsoftJson（若需替代默认的System.Text.Json）：

builder.Services.AddControllersWithViews()
    .AddNewtonsoftJson(options =>
    {
        options.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore;
    });

内容的提问来源于stack exchange，提问作者dausa