DepartmentNamePageModel和DepartmentNameSL正常工作-我的RentalPropertyNameSL可以编译但无法运行

Question

我已使用Contoso University教程作为我的项目的基础-我已经进行了四次检查，以确保没有错别字错误（事实可以编译）

但是当我单击“新建”时，出现上述错误。

断点

        RentalPropertyNameSL = new 
SelectList(propertyQuery.AsNoTracking(),
            "RentalPropertyID", 
"PropertyName", selectedRentalProperty);

在F10之后，RentalPropertyNameSL为空

Name    Value   Type
▶   RentalPropertyNameSL     
{Microsoft.AspNetCore.Mvc.Rendering.SelectList} Microsoft.AspNetCore.Mvc.Rendering.SelectList

显示正确，但引发此错误

NullReferenceException: Object reference 
not set to an instance of an object.
Microsoft.AspNetCore.Mvc.Rendering.MultiSelectList.GetListItemsWithValueField()

Stack  Query  Cookies  Headers
    NullReferenceException: Object reference 
    not set to an instance of an object.
  Microsoft.AspNetCore.Mvc.Rendering.MultiSelectList.GetListItemsWithValueField()
Microsoft.AspNetCore.Mvc.Rendering.MultiSelectList.GetListItems()
Microsoft.AspNetCore.Mvc.Rendering.MultiSelectList.GetEnumerator()
System.Collections.Generic.List<T>.AddEnumerable(IEnumerable<T> enumerable)
System.Linq.Enumerable.ToList<TSource>(IEnumerable<TSource> source)
Microsoft.AspNetCore.Mvc.ViewFeatures.DefaultHtmlGenerator.GenerateGroupsAndOptions(string optionLabel, IEnumerable<SelectListItem> selectList, ICollection<string> currentValues)
Microsoft.AspNetCore.Mvc.ViewFeatures.DefaultHtmlGenerator.GenerateSelect(ViewContext viewContext, ModelExplorer modelExplorer, string optionLabel, string expression, IEnumerable<SelectListItem> selectList, ICollection<string> currentValues, bool allowMultiple, object htmlAttributes)
Microsoft.AspNetCore.Mvc.TagHelpers.SelectTagHelper.Process(TagHelperContext context, TagHelperOutput output)
Microsoft.AspNetCore.Razor.TagHelpers.TagHelper.ProcessAsync(TagHelperContext context, TagHelperOutput output)
Microsoft.AspNetCore.Razor.Runtime.TagHelpers.TagHelperRunner.RunAsync(TagHelperExecutionContext executionContext)
Rent2.Pages.Tenants.Pages_Tenants_Create.<ExecuteAsync>b__21_0() in Create.cshtml
<button class="expandCollapseButton" data-frameid="frame11" style="cursor:pointer;float:left;height:16px;width:16px;font-size:10px;background-color:#eeeeee;padding:0px;border-width:0px;border-style:initial;border-color:initial;margin:0px;">+</button>

asp-items="@Model.RentalPropertyNameSL">
Microsoft.AspNetCore.Razor.Runtime.TagHelpers.TagHelperExecutionContext.GetChildContentAsync(bool useCachedResult, HtmlEncoder encoder)
Microsoft.AspNetCore.Mvc.TagHelpers.RenderAtEndOfFormTagHelper.ProcessAsync(TagHelperContext context, TagHelperOutput output)
Microsoft.AspNetCore.Razor.Runtime.TagHelpers.TagHelperRunner.RunAsync(TagHelperExecutionContext executionContext)
Rent2.Pages.Tenants.Pages_Tenants_Create.ExecuteAsync() in Create.cshtml
<button class="expandCollapseButton" data-frameid="frame15" style="cursor:pointer;float:left;height:16px;width:16px;font-size:10px;background-color:#eeeeee;padding:0px;border-width:0px;border-style:initial;border-color:initial;margin:0px;">+</button>

ViewData["Title"] = "Create Course";
Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageCoreAsync(IRazorPage page, ViewContext context)
Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageAsync(IRazorPage page, ViewContext context, bool invokeViewStarts)
using Rent2.Data;
using System.Linq;
using Microsoft.AspNetCore.Mvc.RazorPages;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.EntityFrameworkCore;

namespace Rent2.Pages.Tenants
{
    public class RentalPropertyNamePageModel : PageModel
    {
        public SelectList RentalPropertyNameSL { get; set; }

        public void PopulateRentalPropertyDropDownList(RentContext _context,
            object selectedRentalProperty = null)
        {
            var propertyQuery = from d in _context.RentalProperties
                                  orderby d.PropertyName
                                  select d;

            RentalPropertyNameSL = new SelectList(propertyQuery.AsNoTracking(),
                "RentalPropertyID", "PropertyName", selectedRentalProperty);
        }
    }
}


@page
@model Rent2.Pages.Tenants.CreateModel

@{
    ViewData["Title"] = "Create Course";
}

<h2>Create</h2>

<h4>Tenant</h4>
<hr />
<div class="row">
    <div class="col-md-4">
        <form method="post">
            <div asp-validation-summary="ModelOnly" class="text-danger"></div>
            <div class="form-group">
                <label asp-for="Tenant.LastName" class="control-label"></label>
                <input asp-for="Tenant.LastName" class="form-control" />
                <span asp-validation-for="Tenant.LastName" class="text-danger"></span>
            </div>
            <div class="form-group">
                <label asp-for="Tenant.FirstMidName" class="control-label"></label>
                <input asp-for="Tenant.FirstMidName" class="form-control" />
                <span asp-validation-for="Tenant.FirstMidName" class="text-danger"></span>
            </div>
            <div class="form-group">
                <label asp-for="Tenant.StartDate" class="control-label"></label>
                <input asp-for="Tenant.StartDate" class="form-control" />
                <span asp-validation-for="Tenant.StartDate" class="text-danger"></span>
            </div>
            <div class="form-group">
                <label asp-for="Tenant.VacatingDate" class="control-label"></label>
                <input asp-for="Tenant.VacatingDate" class="form-control" />
                <span asp-validation-for="Tenant.VacatingDate" class="text-danger"></span>
            </div>
            <div class="form-group">
                <label asp-for="Tenant.MobileNumber" class="control-label"></label>
                <input asp-for="Tenant.MobileNumber" class="form-control" />
                <span asp-validation-for="Tenant.MobileNumber" class="text-danger"></span>
            </div>
            <div class="form-group">
                <label asp-for="Tenant.RentalProperty" class="control-label"></label>
                <select asp-for="Tenant.RentalPropertyID" class="form-control"
                        asp-items="@Model.RentalPropertyNameSL">
                    <option value="">-- Select Rental Property --</option>
                </select>

                <span asp-validation-for="Tenant.RentalPropertyID" class="text-danger"></span>
            </div>
            <div class="form-group">
                <input type="submit" value="Create" class="btn btn-default" />
            </div>
        </form>
    </div>
</div>

<div>
    <a asp-page="Index">Back to List</a>
</div>

@section Scripts {
    @{await Html.RenderPartialAsync("_ValidationScriptsPartial");}
}
Here's Contoso's two code pages 

using ContosoUniversity.Data;
using Microsoft.AspNetCore.Mvc.RazorPages;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.EntityFrameworkCore;
using System.Linq;

namespace ContosoUniversity.Pages.Courses
{
    public class DepartmentNamePageModel : PageModel
    {
        public SelectList DepartmentNameSL { get; set; }

        public void PopulateDepartmentsDropDownList(SchoolContext _context,
            object selectedDepartment = null)
        {
            var departmentsQuery = from d in _context.Departments
                                   orderby d.Name // Sort by name.
                                   select d;

            DepartmentNameSL = new SelectList(departmentsQuery.AsNoTracking(),
                        "DepartmentID", "Name", selectedDepartment);
        }
    }
}

@page
@model ContosoUniversity.Pages.Courses.CreateModel
@{
    ViewData["Title"] = "Create Course";
}
<h2>Create</h2>
<h4>Course</h4>
<hr />
<div class="row">
    <div class="col-md-4">
        <form method="post">
            <div asp-validation-summary="ModelOnly" class="text-danger"></div>
            <div class="form-group">
                <label asp-for="Course.CourseID" class="control-label"></label>
                <input asp-for="Course.CourseID" class="form-control" />
                <span asp-validation-for="Course.CourseID" class="text-danger"></span>
            </div>
            <div class="form-group">
                <label asp-for="Course.Title" class="control-label"></label>
                <input asp-for="Course.Title" class="form-control" />
                <span asp-validation-for="Course.Title" class="text-danger"></span>
            </div>
            <div class="form-group">
                <label asp-for="Course.Credits" class="control-label"></label>
                <input asp-for="Course.Credits" class="form-control" />
                <span asp-validation-for="Course.Credits" class="text-danger"></span>
            </div>
            <div class="form-group">
                <label asp-for="Course.Department" class="control-label"></label>
                <select asp-for="Course.DepartmentID" class="form-control"
                        asp-items="@Model.DepartmentNameSL">
                    <option value="">-- Select Department --</option>
                </select>
                <span asp-validation-for="Course.DepartmentID" class="text-danger" />
            </div>
            <div class="form-group">
                <input type="submit" value="Create" class="btn btn-default" />
            </div>
        </form>
    </div>
</div>
<div>
    <a asp-page="Index">Back to List</a>
</div>
@section Scripts {
    @{await Html.RenderPartialAsync("_ValidationScriptsPartial");}
}

    enter code here