Question

我有一个任务是创建一个类Student并创建一个方法，只有当他们的名字在使用LINQ查询运算符的字母顺序排在他们的姓氏之前时才从数组中选择Students。我写了Student类：

public class Student
{
     private string firstName;
     private string familyName;
     private int age;

    public Student(string firstName, string familyName, int age)
        : this()
    {
        this.firstName = firstName;
        this.familyName = familyName;
        this.age = age;
    }

    public Student()
    {
        firstName = "Dancho";
        familyName = "Mastikata";
        age = 24;
        this.firstName = firstName;
        this.familyName = familyName;
        this.age = age;
    }

    public string FirstName
    {
        get { return firstName; }
        set { firstName = value; }
    }

    public string FamilyName
    {
        get { return familyName; }
        set { familyName = value; }
    }

    public int Age
    {
        get { return age; }
        set { age = value; }
    }
}

然后我编码了这个：

public class StudentsManipulations
{
    static void Main()
    {
        Student[] students = new Student[6]{new Student("Georgi", "Milanov", 21      ),
                                            new Student("Ilko", "Pirgov", 30         ),
                                            new Student("Neboisha", "Yelenkovich", 34),
                                            new Student("Dimitar", "Berbatov", 32    ),
                                            new Student(                             ),
                                            new Student("Nikolai", "Bodurov", 24     )};

    }

    public static List<Student> StudentSortByFirstName(Student[] students)
    {
        List<Student> sortedStudents = from student in students 
             where student.FirstName.CompareTo(student.FamilyName) < 0 
             select student;

        return sortedStudents;
    } 
}

不幸的是，关键字where有一个错误，我无法完全理解：

无法隐式转换类型 'System.Collections.Generic.IEnumerable'到 'System.Collections.Generic.List'。显式转换存在（你错过了演员？）。

你能帮我理解究竟出了什么问题吗？

Answer 1

使用ToList()方法枚举结果并将其放入列表中：

var query = from student in students
            where student.FirstName.CompareTo(student.FamilyName) < 0
            select student;

List<Student> sortedStudents = query.ToList();

或者在一个声明中：

List<Student> sortedStudents = (from student in students
                                where student.FirstName.CompareTo(student.FamilyName) < 0
                                select student).ToList();

Answer 2

您的错误意味着您的流利Linq将返回IEnumerable<Student>，而不是您期望的List<Student>。您需要额外.ToList()来电，或者您可以使用IEnumerable<Student>。

Answer 3

您只需将linq表达式转换为列表：

List<Student> sortedStudents = (from student in students 
where student.FirstName.CompareTo(student.FamilyName) < 0 select student).ToList();

Answer 4

此查询，

from student in students 
where student.FirstName.CompareTo(student.FamilyName) < 0 
select student

返回IEnumerable<Student>。

如果您想要List<Student>，则应在结果ToList()上致电IEnumerable<Student>：

List<Student> sortedStudents = 
    (from student in students 
     where student.FirstName.CompareTo(student.FamilyName) < 0 
     select student)
    .ToList();

请注意，作为一般规则，LINQ查询在枚举结果时进行评估，而不是之前（这称为“延迟执行”或“延迟加载”）。但是，调用ToList()会立即执行查询，枚举结果并填充列表。

Answer 5

您应该在声明的末尾添加.ToList()。

这会将您的Enumerable转换为List

Answer 6

将.ToList()添加到您的查询中：

List<Student> sortedStudents = (from student in students where student.FirstName.CompareTo(student.FamilyName) < 0 select student).Tolist();

无法隐式转换类型错误

6 个答案: