如何使用 C# 在列表中使用 GroupBy 和 Select？

Question

我有一个脚本，它读取 XML 页面将一些内容存储到一个列表中。

获得列表后，我想按名称对结果进行分组。

这是我所做的

DashboardViewModel vm = new DashboardViewModel();

using (var client = new WebClient())
{
    string result = client.DownloadString(url);

    XDocument doc = XDocument.Parse(result);

    foreach (XElement row in doc.Descendants("row"))
    {
        var ticket = new Ticket();

        ticket.company_name = row.Element("company_name").Value;
        ticket.team_name = row.Element("team_name").Value;
        ticket.Age = Parser.decimalVal(row.Element("Age").Value);
        ticket.resource_list = row.Element("resource_list").Value;

        ticket.Closed_Flag = Parser.boolVal(row.Element("Closed_Flag").Value);
        ticket.Date_Entered_UTC = Parser.dateTimeVal(row.Element("Date_Entered_UTC").Value);
        ticket.Date_Closed_UTC = Parser.dateTimeVal(row.Element("Date_Closed_UTC").Value);


        tickets.Add(ticket);

    }

}

vm.summaryByTeam = tickets.GroupBy(x => x.team_name)
                  .Select(x =>
                      new DashboardTicketTotal()
                  {
                      category = x.Key,
                      totalTickets = x.Count(),
                      totalOpenTickets = x.Where(c => c.Closed_Flag == false).Count(),
                      averageOpenDaysFormatted = String.Format("{0:0.##}", x.Average(c => c.Age)),
                      averageDaysToCloseFormatted = String.Format("{0:0.##}", x.Where(c => c.Closed_Flag == true).Average(c => c.Age)),
                      totalClosedInPast30Days = x.Where(c => c.Closed_Flag == true && c.Date_Closed_UTC > now.AddDays(-30)).Count()
                  }).ToList();

这是我的 DashboardViewModel 类

public class DashboardViewModel
{

    public int totalOpen { get; set; }
    public int totalClosed { get; set; }
    public int over30DaysAge { get; set; }
    public decimal? totalAge { get; set; }

    public int totalTickets { get; set; }

    public decimal? averageAge { get; set; }
    public string averageAgeFormatted { get; set; }
    public decimal? averageOpenAge { get; set; }
    public decimal? totalOpenAge { get; set; }
    public int totalClosedIn30Days { get; set; }

    public int totalOpenTicketsOver30Day { get; set; }
    public int totalCloseTicketsOver30Day { get; set; }


    public int totalTicketOver30DaysOld { get; set; }
    public int createdInPast30Days { get; set; }
    public int age0To15 { get; set; }
    public int age16To30 { get; set; }
    public int age31To60 { get; set; }
    public int age61To90 { get; set; }
    public int age91Plus { get; set; }

    public List<DashboardTicketTotal> summaryByTeam { get; set; }

    public List<DashboardTicketTotal> summaryByCenter { get; set; }

    public DashboardViewModel()
    {
        over30DaysAge = 0;
        totalAge = 0;
        totalTickets = 0;
        createdInPast30Days = 0;
        averageOpenAge = 0;
        summaryByTeam = new List<DashboardTicketTotal>();
        summaryByCenter = new List<DashboardTicketTotal>();
    }
}

然而行

 .Select(x =>
                          new DashboardTicketTotal()

给我一​​个错误

 Sequence contains no elements
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidOperationException: Sequence contains no elements




[InvalidOperationException: Sequence contains no elements]
   System.Linq.Enumerable.Average(IEnumerable`1 source) +2578534
   System.Linq.Enumerable.Average(IEnumerable`1 source, Func`2 selector) +58
   ConnectWiseReports.Controllers.<>c__DisplayClass30.<DashBoard>b__13(IGrouping`2 x) in c:....ReportsController.cs:102
   System.Linq.WhereSelectEnumerableIterator`2.MoveNext() +157
   System.Collections.Generic.List`1..ctor(IEnumerable`1 collection) +381
   System.Linq.Enumerable.ToList(IEnumerable`1 source) +58
   ConnectWiseReports.Controllers.ConnectWiseReportsController.DashBoard() in c:...ReportsController.cs:100
   lambda_method(Closure , ControllerBase , Object[] ) +62
   System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase controller, Object[] parameters) +14
   System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) +156
   System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +27
   System.Web.Mvc.Async.AsyncControllerActionInvoker.<BeginInvokeSynchronousActionMethod>b__39(IAsyncResult asyncResult, ActionInvocation innerInvokeState) +22
   System.Web.Mvc.Async.WrappedAsyncResult`2.CallEndDelegate(IAsyncResult asyncResult) +29
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult) +32
   System.Web.Mvc.Async.AsyncInvocationWithFilters.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3d() +50
   System.Web.Mvc.Async.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f() +225
   System.Web.Mvc.Async.<>c__DisplayClass33.<BeginInvokeActionMethodWithFilters>b__32(IAsyncResult asyncResult) +10
   System.Web.Mvc.Async.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult) +10
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethodWithFilters(IAsyncResult asyncResult) +34
   System.Web.Mvc.Async.<>c__DisplayClass2b.<BeginInvokeAction>b__1c() +26
   System.Web.Mvc.Async.<>c__DisplayClass21.<BeginInvokeAction>b__1e(IAsyncResult asyncResult) +100
   System.Web.Mvc.Async.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult) +10
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +27
   System.Web.Mvc.Controller.<BeginExecuteCore>b__1d(IAsyncResult asyncResult, ExecuteCoreState innerState) +13
   System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +36
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +54
   System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +39
   System.Web.Mvc.Controller.<BeginExecute>b__15(IAsyncResult asyncResult, Controller controller) +12
   System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +28
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +54
   System.Web.Mvc.Controller.EndExecute(IAsyncResult asyncResult) +29
   System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.EndExecute(IAsyncResult asyncResult) +10
   System.Web.Mvc.MvcHandler.<BeginProcessRequest>b__5(IAsyncResult asyncResult, ProcessRequestState innerState) +21
   System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +36
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +54
   System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +31
   System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +9
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +9651516
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155

Answer 1

代码在以下行失败：

averageDaysToCloseFormatted = String.Format("{0:0.##}",
    x.Where(c => c.Closed_Flag == true)
     .Average(c => c.Age))

当没有集合Closed_Flag的对象时，则序列为空，无法计算Average。解决问题的一种方法是将Age 转换为decimal?（使其可以为空），那么可以为空的值的空序列的平均值将为null。然后我们使用空合并运算符 (??) 将得到的空平均值替换为 0。

averageDaysToCloseFormatted = String.Format("{0:0.##}",
    x.Where(c => c.Closed_Flag == true)
     .Average(c => (decimal?)c.Age) ?? 0)