要实现ASP.Net Core的多对多CRUD操作，需要以下步骤：

1. 创建数据库模型和关联表。首先，创建两个实体类（例如Student和Course），分别表示学生和课程。然后，创建一个关联表（例如StudentCourse），用于存储学生和课程之间的关系。

public class Student
{
    public int StudentId { get; set; }
    public string Name { get; set; }
    // other properties
    public ICollection StudentCourses { get; set; }
}

public class Course
{
    public int CourseId { get; set; }
    public string Name { get; set; }
    // other properties
    public ICollection StudentCourses { get; set; }
}

public class StudentCourse
{
    public int StudentId { get; set; }
    public Student Student { get; set; }
    public int CourseId { get; set; }
    public Course Course { get; set; }
}

2. 配置数据库上下文。在DbContext类中添加DbSet属性，以便访问数据库表。

public class ApplicationDbContext : DbContext
{
    public DbSet Students { get; set; }
    public DbSet Courses { get; set; }
    public DbSet StudentCourses { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity().HasKey(sc => new { sc.StudentId, sc.CourseId });

        modelBuilder.Entity()
            .HasOne(sc => sc.Student)
            .WithMany(s => s.StudentCourses)
            .HasForeignKey(sc => sc.StudentId);

        modelBuilder.Entity()
            .HasOne(sc => sc.Course)
            .WithMany(c => c.StudentCourses)
            .HasForeignKey(sc => sc.CourseId);
    }
}

3. 实现CRUD操作。在控制器类中，注入数据库上下文，然后使用上下文进行CRUD操作。

public class StudentController : ControllerBase
{
    private readonly ApplicationDbContext _context;

    public StudentController(ApplicationDbContext context)
    {
        _context = context;
    }

    // GET: api/students
    [HttpGet]
    public async Task>> GetStudents()
    {
        return await _context.Students.ToListAsync();
    }

    // POST: api/students
    [HttpPost]
    public async Task> CreateStudent(Student student)
    {
        _context.Students.Add(student);
        await _context.SaveChangesAsync();

        return CreatedAtAction(nameof(GetStudent), new { id = student.StudentId }, student);
    }

    // GET: api/students/{id}
    [HttpGet("{id}")]
    public async Task> GetStudent(int id)
    {
        var student = await _context.Students.FindAsync(id);

        if (student == null)
        {
            return NotFound();
        }

        return student;
    }

    // PUT: api/students/{id}
    [HttpPut("{id}")]
    public async Task UpdateStudent(int id, Student student)
    {
        if (id != student.StudentId)
        {
            return BadRequest();
        }

        _context.Entry(student).State = EntityState.Modified;

        try
        {
            await _context.SaveChangesAsync();
        }
        catch (DbUpdateConcurrencyException)
        {
            if (!StudentExists(id))
            {
                return NotFound();
            }
            else
            {
                throw;
            }
        }

        return NoContent();
    }

    // DELETE: api/students/{id}
    [HttpDelete("{id}")]
    public async Task DeleteStudent(int id)
    {
        var student = await _context.Students.FindAsync(id);

        if (student == null)
        {
            return NotFound();
        }

        _context.Students.Remove(student);
        await _context.SaveChangesAsync();

        return NoContent();
    }

    private bool StudentExists(int id)
    {
        return _context.Students.Any(e => e.StudentId == id);
    }
}

这是一个简单的示例，演示了如何使用ASP.Net Core进行多对多关系的CRUD操作。你可以根据自己的需求对代码进行扩展和修改。