Question

所以我有一个函数可以从Web服务获取学生列表，还可以查询localdb中的所有学生。数据放在两个不同的列表中。所以我想查看localdb列表中是否已经存在新学生。如果是，请更新它，如果没有，则添加它。我无法让它工作。我试图使用LINQ执行此操作，但我似乎无法使其正常工作。我的LINQ技能充其量只是业余。

        public async Task GetStudents()
    {
        String controllerName = "Students";

        List<Students> newStudentData = await RunGetAsync<Students>(controllerName);

        // get all the service types that already exists in the localStudent Db
        List<Students> currentStudentData = db.Studentss.ToList();

        foreach (Students existingStudents in currentStudentData)
        {
            foreach (Students newStudents in newStudentData)
            {
                IEnumerable<Students> selectStudents = from student in newStudentData  // check if Students exist in the database
                                                             where student.Id == existingStudents.Id
                                                             select student;

                if (selectStudents == null) // didn't find it, then add it
                {
                    db.Students.Add(newStudents);
                }
                if (selectStudents != null) // found it , then update the informations
                {
                    Students updatedStudents = new Students();
                    foreach (var field in selectStudents)
                    {
                        updatedStudents.FName = field.FName;
                        updatedStudents.LName = field.LName;
                        updatedStudents.ZipCode = field.ZipCode;
                        updatedStudents.AccessCode = field.AccessCode;
                    }
                    db.Entry(updatedStudents).State = System.Data.Entity.EntityState.Modified;
                }
            }

        }
        db.SaveChanges();
    }

非常感谢你的帮助。

Answer 1

您的循环次数超出了您的需求：

foreach (Students newStudents in newStudentData)
{
     var student = currentStudentData.FirstOrDefault(s => s.Id == newStudents.Id);

     if(student == null)
     {
          //add
     }
     else
     {
         //update
     }
}

使用FirstOrDefault，您可以查看它是否存在并同时获取对它的引用，如果存在的话。

Answer 2

您可以使用Intersect和Except，如下所示：

        //Find students that already exist to update
        var updateStudents = currentStudentData.Intersect(newStudentData);

        //Find new students to add
        var addStudents = newStudentData.Except(currentStudentData);

迭代c＃中的两个列表

2 个答案: