从LINQ查询获取第一个结果-为什么当第一个成功时elementat(0)失败?

我有一个方法 Advess学生() 它查找同名学生,如果有同名学生,则从数据库返回现有学生,否则将创建一个新学生并将其添加到数据库中。

我很好奇为什么 se = students.First<StudentEntity>(); 成功时 se = students.ElementAt<StudentEntity>(0); 尝试从Linq查询获取第一个结果时失败。这两种方法不一样吗?

方法的完整代码如下所示。

public Student AddStudent(string name)
{
    using (SchoolEntities db = new SchoolEntities())
    {
        // find student with same name via LINQ
        var students = from s in db.StudentEntitySet
                       where s.name == name
                       select s;

        StudentEntity se = default(StudentEntity);

        // if student with the same name is already present, return 
        // that student
        if (students.Count<StudentEntity>() > 0)
        {
            // if i use ElementAt, if fails with a "LINQ to Entities does not
            // recognize the method 'StudentEntity ElementAt[StudentEntity]
            // (System.Linq.IQueryable`1[StudentEntity], Int32)' method, 
            // and this method cannot be translated into a store expression.", 
            // but not when I use First. Why?

            // se = students.ElementAt<StudentEntity>(0);
            se = students.First<StudentEntity>();
        }
        else
        {
            // passing 0 for first parameter (id) since it's represented by 
            // a BigInt IDENTITY field in the database so any value
            // doesn't matter.
            se = StudentEntity.CreateStudentEntity(0, name);
            db.AddToStudentEntitySet(se);
            db.SaveChanges();
        }

        // create a Student object from the Entity object
        return new Student(se);
    }
}

谢谢!