Question

我和几个学生一起保存了这样的Viewstates：

 ViewState[currentStudent] = currentGradesList;

但是现在我需要让所有观察者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) {....}并且有效。

但现在我尝试

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

＆＃34;成绩＆＃34;保持为空，我收到错误：

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

我想它说Keys是字符串，但我怎么会得到所有列表？

Answer 1

您循环遍历foreach循环中的键名，而不是实际值。您可以使用foreach循环变量的值（现在是ViewState字典中的键的名称）从viewstate获取值。

将你的foreach循环改为这样的

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

解决方案：我循环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];

然后我的其余代码实际上并不重要，但这里是为了获得每个学生的平均成绩，以及所有学生在一起

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

3 个答案: