Question

我在C＃中遇到以下异常：

InvalidCastException：无法从类型转换＆＃39; System.Collections.Generic.List`1 [MyProj.Data.Entities.Student]＆＃39;至键入＆＃39; MyProj.Data.Access.IStudent＆＃39;。

我的项目编译成功。我有两个对象定义如下：

public interface IStudent
{
    long ID { get; set; }
    string SystemCode { get; set; }
    string FirstName { get; set; }
    string LastName { get; set; }
    DateTime DateOfBirth { get; set; } 
}
public class Student : IStudent
{
    public long ID { get; set; }
    public string SystemCode { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public DateTime DateOfBirth { get; set; } 
}

public interface ICourseRequest
{
    long ID { get; set; }
    long StudentID { get; set; }
    int CourseID { get; set; }
    int? FacultyID { get; set; }
    DateTime StartDate { get; set; }
    DateTime CreationDate { get; set; }
    IStudent Student { get; set; }
}

public class CourseRequest : ICourseRequest
{
    public long ID { get; set; }
    public long StudentID { get; set; }
    public int CourseID { get; set; }
    public int? FacultyID { get; set; }
    public DateTime StartDate { get; set; }
    public DateTime CreationDate { get; set; }
    public IStudent Student { get; set; }
}

我正在使用SQLtoLinq来检索我的数据（来自SQL Server），如下所示：

IList<ICourseRequest> result = _database.course_requests.Select(a => new CourseRequest
{        
    CreationDate = a.crsreq_create_date,
    StartDate = a.crsreq_start,
    FacultyID = a.crsreq_faculty,
    ID = a.crsreq_id,
    CourseID = a.crsreq_crs,
    StudentID = a.crsreq_student,
    Student =  _database.students.Where(p => p.stud_id == a.crsreq_student)
                                 .Select(s => new Student
                                  {
                                      ID = p.stud_id,
                                      DateOfBirth = s.stud_dob,
                                      FirstName = s.stud_fname,
                                      LastName = s.stud_lname,
                                      SystemCode = s.stud_sys_code
                                  }).SingleOrDefault(),
}).ToList<ICourseRequest>();

为什么在项目成功编译时会在运行时抛出此强制转换异常？

Answer 1

这非常奇怪。在整个项目中（我和其他对象）一直在这样查询，只是这给了一个错误。

我通过将其转换为IStudent来修复它。我不知道为什么简单的SingleOrDefault不起作用。任何人都可以解释原因吗？

这就是修正它的原因：

IList<ICourseRequest> result = _database.course_requests.Select(a => new CourseRequest
{        
    CreationDate = a.crsreq_create_date,
    StartDate = a.crsreq_start,
    FacultyID = a.crsreq_faculty,
    ID = a.crsreq_id,
    CourseID = a.crsreq_crs,
    StudentID = a.crsreq_student,
    Student =  _database.students.Where(p => p.stud_id == a.crsreq_student)
                                 .Select(s => new Student
                                  {
                                      ID = p.stud_id,
                                      DateOfBirth = s.stud_dob,
                                      FirstName = s.stud_fname,
                                      LastName = s.stud_lname,
                                      SystemCode = s.stud_sys_code
                                  }).SingleOrDefault() as IStudent, //<-- This is what fixed it
}).ToList<ICourseRequest>();

获得一个奇怪的InvalidCastException＆＃39;在ASP.NET MVC中

1 个答案: