Contoso大学项目：InvalidCastException：无法将类型为“ System.String”的对象转换为类型为“ System.Int32”的

如何解决Contoso大学项目：InvalidCastException：无法将类型为“ System.String”的对象转换为类型为“ System.Int32”的

我正在.net核心中的Contoso大学项目中，当我从导航菜单中选择“学生”以查看学生列表时，然后在页面上的“详细信息”链接下以每个人的名字命名学生，我收到以下错误：

InvalidCastException: Unable to cast object of type 'System.String' to type 'system.int32'.

它说此行上的StudentController.cs错误：

+
37            var student = await _context.Students

有关该问题可能是什么的任何建议都会很有帮助。

这是我的控制器代码：

// StudentsController.cs

// GET: Students/Details/5
        public async Task<IActionResult> Details(int? id)
        {
            if (id == null)
            {
                return NotFound();
            }

            var student = await _context.Students
                .Include(s => s.Enrollments)
                    .ThenInclude(e => e.Course)
                .AsNoTracking()
                .FirstOrDefaultAsync(m => m.ID == id);
            if (student == null)
            {
                return NotFound();
            }

            return View(student);
        }

// SchoolContext.cs

    namespace ContosoUniversity.Data
    {
        public class SchoolContext : DbContext
        {
            public SchoolContext(DbContextOptions<SchoolContext> options) : base(options)
            {
            }
    
            public DbSet<Course> Courses { get; set; }
            public DbSet<Enrollment> Enrollments { get; set; }
            public DbSet<Student> Students { get; set; }
    
            protected override void OnModelCreating(ModelBuilder modelBuilder)
            {
                modelBuilder.Entity<Course>().ToTable("Course");
                modelBuilder.Entity<Enrollment>().ToTable("Enrollment");
                modelBuilder.Entity<Student>().ToTable("Student");
            }
        }
    }

// Student.cs（模型）

    namespace ContosoUniversity.Models
    {
        public class Student
        {
            public int ID { get; set; }
            public string LastName { get; set; }
            public string FirstMidName { get; set; }
            public DateTime EnrollmentDate { get; set; }
            public ICollection<Enrollment> Enrollments { get; set; }
        }
    }

// Index.cshtml（列表视图）

    @foreach (var item in Model) {
            <tr>
                <td>
                    @Html.displayFor(modelItem => item.LastName)
                </td>
                <td>
                    @Html.displayFor(modelItem => item.FirstMidName)
                </td>
                <td>
                    @Html.displayFor(modelItem => item.EnrollmentDate)
                </td>
                <td>
                    <a asp-action="Edit" asp-route-id="@item.ID">Edit</a> |
                    <a asp-action="Details" asp-route-id="@item.ID">Details</a> |
                    <a asp-action="Delete" asp-route-id="@item.ID">Delete</a>
                </td>
            </tr>
    }

// Enrollment.cs

    namespace ContosoUniversity.Models
    {
   }
        public class Enrollment
        {
            public int EnrollmentID { get; set; }
    
            public int CourseID { get; set; }
    
            public int StudentID { get; set; } // foreign key
    
    // The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.
            public string Grade { get; set; } // nullable
    // The annotation for nullable reference types should only be used in code within a '#nullable' annotations context.
    
            public Course Course { get; set; } // foreign key
    
            public Student Student { get; set; }
    
        }
    }

我的数据结构：

dbo.Course:
CourseId (PK,int,not null)
Title (varchar(255),not null)
Credits (int,null)

dbo.Enrollment
EnrollmentId (PK,not null)
StudentID( int,null)
CourseID (int,null)
Grade (varchar(255),null)

Student
Id(PK,not null)
FirstMidName(varchar(255),null)
LastName(varchar(255),null)
EnrollmentDate(datetime,null)

解决方法

看这行：

var student = await _context.Students
         .Include(s => s.Enrollments)
         .ThenInclude(e => e.Course)
         .AsNoTracking()
         .FirstOrDefaultAsync(m => m.ID == id);

您传递的是可为null的int吗？

public async Task<IActionResult> Details(int? id)

在该类中，它只是int

   public int ID { get; set; }

尝试将可为空（int？）强制转换为类似（int）的

 var student = await _context.Students
            .Include(s => s.Enrollments)
            .ThenInclude(e => e.Course)
            .AsNoTracking()
            .FirstOrDefaultAsync(m => m.ID == (int)id);