值不能为 null 参数名称：即使 IEnumerable 不为 null 时也是如此

Question

我有以下型号：

public class Student
{
    public int Id { get; set; }
    [Display(Name = "First Name")]
    public string FirstName { get; set; }
    [Display(Name = "Last Name")]
    public string LastName { get; set; }
    public ICollection<WorkExperience> WorkExperiences { get; set; }
    public ICollection<Education> Educations { get; set; }
    public ICollection<Skill> Skills { get; set; }
}

public class Employer
{
    public int Id { get; set; }
    [Display(Name = "First Name")]
    public string FirstName { get; set; }
    [Display(Name = "Last Name")]
    public string LastName { get; set; }
    public ICollection<WorkExperience> WorkExperiences { get; set; }
    public ICollection<Education> Educations { get; set; }
    public ICollection<Skill> Skills { get; set; }
}

public class WorkExperience
{
    public int Id { get; set; }
    [Display(Name = "Company Name")]
    public string CompanyName { get; set; }
    [Display(Name = "Job Title")]
    public string JobTitle { get; set; }
    [Display(Name = "Experience (in years)")]
    public string Experience { get; set; }
    [Display(Description = "Enter City, Country for e.g. San Francisco, California")]
    public string Location { get; set; }
    public virtual ICollection<Employer> Employers { get; set; }
    [Display(Name = "Set as Current Company")]
    public virtual ICollection<Student> Students { get; set; }
}

我希望每个学生和雇主都有多种工作经验。因此，学生可以拥有任意数量的经验，甚至雇主也可以拥有任意数量的经验。我面临的问题是，当它到达工作体验代码时，它会说：

值不能为空。参数名称：来源

但是，在传递模型 Student 的 IEnumerable 后，我在视图中获取了所有值

下面是视图代码

@model IEnumerable<OpenOpportunity.Models.Student>
@foreach (var student in Model)
{
    <div class="row clearfix">
        <div class="col-xs-12">
            <h1>
            @student.FirstName @student.LastName
            </h1>
        </div>
    </div>

    var count = 0;
    if (@student.WorkExperiences == null)
    {
        <small>No current position & company set</small>
        <a id="currentDetails" href="@Url.Action("EditCurrentDetails", "Main", new { area = "Students", id = student.Id })">
        <i class="fa fa-edit"></i>
        </a>
    }
    else if (@student.WorkExperiences.Count() > 0)
    {

        foreach (var work in student.WorkExperiences)
        {
            if (work.IsCurrentCompany == true)
            {
                <small>@work.JobTitle,@work.CompanyName</small>
                <a id="currentDetails" href="@Url.Action("EditCurrentDetails", "Main", new { area = "Students", id = student.Id })">
                <i class="fa fa-edit"></i>
                </a>
            }
            else
            {
                count++;
            }
        }
    }
    if (@student.WorkExperiences.Count() == count)
    {
        <small>No current position & company set</small>
        <a id="currentDetails" href="@Url.Action("EditCurrentDetails", "Main", new { area = "Students", id = student.Id })">
        <i class="fa fa-edit"></i>
        </a>
    }
}

这是控制器代码：

var userID = db.Users.Find(User.Identity.GetUserId());
                var getStudentDetails = (from x in db.Students.Include("WorkExperiences")
                                         where x.Id == userID.Student.Id
                                         select x).ToList();
                return View(getStudentDetails);

另外我想告诉你，雇主视图工作正常，并且是具有相同代码的视图。关系没有正确定义吗？还是别的什么？

Answer 1

查看您的代码，这是您的流程的简化示例：

var count = 0;
if (@student.WorkExperiences == null)
{
    <small>No current position & company set</small>
    ...
}
else if (@student.WorkExperiences.Count() > 0)
{
    foreach (var work in student.WorkExperiences)
    ...
}
if (@student.WorkExperiences.Count() == count)
{
    <small>No current position & company set</small>
    ...
}

看到问题了吗？如果WorkExperiences 集合为空，那么您将在最后一个if 语句中对其调用.Count()，而之前没有进行空检查。

我认为最后一个 if 语句是一些糟糕的剩余部分，您应该简单地删除它，以便您的流程变成这样：

var count = 0;
if (student.WorkExperiences == null || !student.WorkExperiences.Any())
{
    <small>No current position & company set</small>
    ...
}
else
{
    foreach (var work in student.WorkExperiences)
    ...
}