Question

我在这一行收到InvalidCastException：

List<Student> studentsWithSameSurname =
    (List<Student>)_studentsList
        .Where(i => i.LastName == textBoxLastName.Text.Trim())
        .GroupBy(j => j.FamilyID);

_studentsList是List<Student>，所以我不知道为什么（List<Student>）是无效的演员。

对于那些想要更多上下文的人来说，这是一个Winforms应用程序，其中我将自定义类的通用列表序列化为json文件，然后在我想处理数据时反序列化它，并使用LINQ查询它。

_studentsList以这种方式定义：

List<Student> _studentsList;

学生班是：

public class Student
{
    public int StudentID { get; set; }
    public int FamilyID { get; set; }
    public bool EnrolledInAYttFM { get; set; }
    public DateTime DateEnrolledOrHiatusAYttFM { get; set; }
    public bool GivesBibleReading { get; set; }
    public bool PresentsICRVBS { get; set; }
    public bool IsHouseholder { get; set; }
    public bool IsMale { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string EmailAddr { get; set; }
    public DateTime WeekOfLastAssignment { get; set; }
    public int RecommendedNextTalkTypeID { get; set; }
    public int NextCounselPoint { get; set; }
}

我正在努力做的事情背后的逻辑是：

如果添加了学生，并且他们的姓氏已经存在于通用列表中，则需要确定该名称的第二个实例是否是与前一个实例相同的成员。更具体地说，如果添加“史密斯”，并且他们是第一个具有该姓氏的人，他们将获得下一个可用的familyID;否则（如果已经输入该姓氏）必须确定他们是否属于已经进入的其中一个家庭，或者他们是否是新家庭的第一个成员：

Student surNameCandidateRecord = _studentsList.SingleOrDefault(s => s.LastName == textBoxLastName.Text);
if (null == surNameCandidateRecord)
{
    student.FamilyID = _studentsList.Max(x => x.FamilyID) + 1;
}
else // at least one family with that surname exists already
{
    student.FamilyID = GetFamilyID();
}

GetFamilyID（）方法是抛出此异常的方法，在这里它是完整的：

private int GetFamilyID()
{
    List<KeyValuePair<int, string>> familyIDsAndNames;

    // InvalidCastException on the line below
    List<Student> studentsWithSameSurname = (List<Student>)_studentsList.Where(i => i.LastName == textBoxLastName.Text.Trim()).GroupBy(j => j.FamilyID);
    // </ InvalidCastException on the line above

    familyIDsAndNames = new List<KeyValuePair<int, string>>();
    foreach (Student s in studentsWithSameSurname)
    {
        string fullNameWithFamilyIdPrepended = String.Format("({0}). {1} {2}", s.FamilyID, s.FirstName, s.LastName);
        familyIDsAndNames.Add(new KeyValuePair<int, string>(s.FamilyID, fullNameWithFamilyIdPrepended));
    }
    ChooseFamilyForm cff = new ChooseFamilyForm(familyIDsAndNames);
    cff.ShowDialog();
    return AYttFMConstsAndUtils.familyIDSelected;
}

在抛出InvalidCastException的时候，通用列表的内容是（名称和电子邮件地址已被更改以保护垃圾邮件）：

[{"StudentID":1,"FamilyID":1,"EnrolledInAYttFM":true,"DateEnrolledOrHiatusAYttFM":"2016-02-12T21:52:55.9560088-08:00","GivesBibleReading":false,"PresentsICRVBS":true,"IsHouseholder":true,"IsMale":false,"FirstName":"Carly","LastName":"Shannon","EmailAddr":"carlyjeannette@att.net","WeekOfLastAssignment":"2015-08-12T21:52:50.6959113-07:00","RecommendedNextTalkTypeID":3,"NextCounselPoint":16},{"StudentID":2,"FamilyID":2,"EnrolledInAYttFM":true,"DateEnrolledOrHiatusAYttFM":"0001-01-01T00:00:00","GivesBibleReading":false,"PresentsICRVBS":true,"IsHouseholder":true,"IsMale":false,"FirstName":"Jennifer","LastName":"Volando","EmailAddr":"jenni_v@ymail.com","WeekOfLastAssignment":"0001-01-01T00:00:00","RecommendedNextTalkTypeID":0,"NextCounselPoint":0}]

...我正在尝试添加第二个姓“Volando”的人（因此将代码路由到GetFamilyID（）方法）。

我的LINQ查询可能有误，但我不知道以哪种方式。

更新

这是我成功使用的，基于Tordek的回答：

List<Student> studentsWithSameSurname = (List<Student>)_studentsList
        .Where(i => i.LastName == textBoxLastName.Text.Trim())
        .OrderBy(j => j.FamilyID)
        .ToList();

我将尽快给予dicho奖励;自从达到10K后，我将做一个卡内基并且“放弃”所有点。

Answer 1

在这一行

List<Student> studentsWithSameSurname =
    (List<Student>)_studentsList
        .Where(i => i.LastName == textBoxLastName.Text.Trim())
        .GroupBy(j => j.FamilyID);

你没有施放_studentsList;你正在施展整个表达。换句话说，这相当于

    (List<Student>)(_studentsList
        .Where(i => i.LastName == textBoxLastName.Text.Trim())
        .GroupBy(j => j.FamilyID));

而不是（如你所料）

    ((List<Student>)_studentsList)
        .Where(i => i.LastName == textBoxLastName.Text.Trim())
        .GroupBy(j => j.FamilyID);

如果要获取Linq查询的结果列表，则必须使用.ToList（）方法：

     studentsList
        .Where(i => i.LastName == textBoxLastName.Text.Trim())
        .GroupBy(j => j.FamilyID)
        .ToList();

但是，您的结果不是List<Student>，而是新类型，因为GroupBy方法会创建一个新的学生列表类型。 GroupBy表达式为IEnumerable<IGrouping<T, U>>的os的结果，其中T是您的分组变量，U是元素＆＃39;类型;在您的情况下，如果IEnumerable<IGrouping<int, Student>>的类型为FamilyID，则为int。

为什么这个LINQ查询会抛出InvalidCastException？

更新

1 个答案: