C# 值不能为null参数名称：source，即使IEnumerable处于非null状态

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; }
}

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另外，我想告诉你，雇主视图工作正常&是具有相同代码的视图。这些关系是否没有正确定义？还是别的什么？

看看你的代码，这里有一个简化的流程示例：

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>
    ...
}

---

哪行代码引发该异常？您的代码在视图中有语法错误。if语句中的

@

符号需要如下：

@if（…）

不是

如果（@…

@CodingYoshi，它不是语法错误，它需要是

如果（…）

-

@if（…）

会抛出异常（学生面前的

@

只是毫无意义）@Stephen最后一个if循环的闭括号}抛出错误。不，它没有。但您甚至没有显示相关代码。你有一个

count++

和

if（@student.WorkExperiences.Count（）==Count）

但是你甚至没有在哪里为

Count

初始化一个变量，并且代码没有编译。这是不可能重现你的问题。谢谢，但现在的代码是工作的，我仍然不明白是什么使它工作，因为它是相同的代码，因为以前。我注意到的唯一一件事是，在视图中调试时，如果您在集合中获得的第一个键值为null，那么它将抛出，即使您没有在循环中的该点访问该值。但现在我将简化我的循环，使其看起来整洁。

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);

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>
    ...
}

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