如何检索所有视图状态列表

Question

我正在和几个学生一起保存 Viewstates：

 ViewState[currentStudent] = currentGradesList;

但现在我需要获取所有视图状态以获得所有成绩的平均值，我已经学会了用这样的字符串来做到这一点：

foreach (string str in ViewState.Keys) {....} 行得通。

但现在当我尝试

foreach(List<double> grades in ViewState.Keys) {....}

"grades" 保持为空，我得到错误：

Unable to cast object of type 'System.String' to type 'System.Collections.Generic.List`1[System.Double]'.

我猜它说键是字符串，但是我怎么才能得到所有列表呢？？

Answer 1

您正在遍历 foreach 循环中键的名称，而不是实际值。您使用 foreach 循环变量的值（现在是 ViewState 字典中的键的名称）从视图状态中获取值。

将您的 foreach 循环更改为类似这样的内容

foreach(var key in ViewState.keys)[
    var grades = ViewState[key] as List<double>;
    //LINQ has built in Average and Sum abilities on lists
    //I don't know what a CurrentStudentGrades looks like
    //but here is an example of using the built in average
    var studentAverage = grades.Average(x=>x.Grade);
    //do whatever else you are wanting to do
}

Answer 2

foreach (string str in ViewState.Keys)
{
    var grades = ViewState[str] as List<double>;
    if(grades != null)
    {
         var average = grades.Average();
    }
}

Answer 3

解决方法：我正在遍历视图状态键的名称，而不是值。所以现在我把它改成了

Foreach (string str in ViewState.Keys) 
{ //And then the value of "str"...
 List<double> templist = (List<double>)ViewState[str];

然后是我的代码的其余部分，这并不重要，但这里是为了获得每个学生的平均成绩，以及所有学生的平均成绩

foreach (double grade in templist)
{
currenttotal += grade;
}
currentaverage = currenttotal / templist.count;
AllAveragesList.add(currentaverage)

foreach (double average in AllAveragesList)
{
totalOfAll += average
}
averageOfAll = totalOfAll / AllAveragesList.count();`

如何检索所有视图状态列表

How to retrieve all viewstate lists

c#

asp.net

viewstate