Question

假设我有一个包含多个列的表（学生，老师，主题，标记），我想将这些列中的每一列与具有相同列的另一个表（使用sum（标记））进行比较。

我有以下方法，列名作为参数，然后在group by子句中使用。

public List<AllMarks> RIMarks(string filter)
{
  var MarkTable = from p in MainDB.classTable
                  let fil = filter
                  group p by new { fil } into g
                  select new AllMarks
                  {
                    Column = g.Key.fil,
                    Marks = g.Sum(f => f.Mark)
                  };

  List<AllMarks> lstRI = MarkTable.ToList();
  return lstRI;
}

public void Test()
{
  var filter = new string[] {"Student", "Teacher", "Subject"}
  foreach (f in filter) 
  {
    // Call RIMarks(f) and do something
  }
}

我的桌子中有许多不同的学生，但是使用这种方法，我得到的只是一行带有总和（第一个过滤标准的标记（所有学生）（这是学生）并且它实际上没有分组学生。

如何在linq groupby子句中使用局部变量？

更新：样本数据库：

Student    Teacher    Subject    Marks
stu1       teac1      sub1       23
stu1       teac1      sub1       45
stu2       teac2      sub2       34

Answer 1

您尝试按照您拥有的某个数据对象的特定属性的名称进行分组。您希望反映所需字段的属性，然后按该字段的值进行分组。您不能只提供名称，因为过滤器不起作用，它只会对整个集合进行分组。

在LinqPad中测试一下。

你所拥有的是在分组上总结的分数，不确定你是否想要平均值

你在这里所拥有的应该会让你获得平均值。

void Main()
{
    var mySchool = new List<School>{
    new School{Student = "Student A", Teacher = "Teacher A", Subject = "Math", Marks = 80},
    new School{Student = "Student B", Teacher = "Teacher A", Subject = "Math", Marks = 65},
    new School{Student = "Student C", Teacher = "Teacher A", Subject = "Math", Marks = 95},
    new School{Student = "Student A", Teacher = "Teacher B", Subject = "History", Marks = 80},
    new School{Student = "Student B", Teacher = "Teacher B", Subject = "History", Marks = 100},
    };

    GroupByFilter("Student", mySchool);
    GroupByFilter("Teacher", mySchool);
    GroupByFilter("Subject", mySchool);
}

public void GroupByFilter(string filter, List<School> school)
{
    PropertyInfo prop = typeof(School).GetProperties()
                                  .Where(x => x.Name == filter)
                                  .First();

    var grouping = from s in school
                   group s by new {filter = prop.GetValue(s)} into gr
                   select new {
                    Filter = gr.Key.filter,
                    Marks = gr.Sum(x => x.Marks)
                   };
    grouping.Dump(); // this is linqpad specific
}

// Define other methods and classes here
public class School{
    public string Student {get;set;}
    public string Teacher {get;set;}
    public string Subject {get;set;}
    public int Marks {get;set;}
}

结果

按学生分组

过滤标记

学生A 160

学生B 165

学生C 95

按教师分组过滤标记

老师A 240

老师B 180

按主题分组过滤标记

数学240

历史180

Answer 2

请尝试以下代码。

public List<AllMarks> RIMarks(string filter)
{
if (filter == "Student") {
 var MarkTable = from p in MainDB.classTable
                        group p by new { p.Student} into g
                        select new AllMarks
                             {
                                 Column = g.Key.fil,
                                 Marks = g.Sum(f => f.Mark)
                             };

    }
 else if (filter == "Teacher") {
   var MarkTable = from p in MainDB.classTable
                        group p by new { p.Teacher} into g
                        select new AllMarks
                             {
                                 Column = g.Key.fil,
                                 Marks = g.Sum(f => f.Mark)
                             };
}
 else if (filter == "Subject") {
    var MarkTable = from p in MainDB.classTable
                        group p by new { p.Subject} into g
                        select new AllMarks
                             {
                                 Column = g.Key.fil,
                                 Marks = g.Sum(f => f.Mark)
                             };
}
}

在linq group by子句中使用变量

2 个答案:

结果