c# - 提高生成列表的性能

Question

我有一个包含 413 个对象的列表。现在，我正在根据要导出到 Excel 的这些对象创建一个新列表。

lstDailySummary = (List<DailySummary>)Session["Paging"];

List<ExportExcel> lstExportedExcel = lstDailySummary
    .Select(x => new ExportExcel
    {
        PropertyOne = x.ACInfo.RegNumber,
        PropertyTwo = db.MyTable.Find(x.NavProperty.subcategoryID).Text,
        PropertyThree = x.NavProperty.text,
        PropertyFour = (!string.IsNullOrWhiteSpace(x.Agcy.ToString())) ? x.codeAgcy.location : " ",
        PropertyFive = x.EventLocation,
        PropertySix = x.codeCounty.county,
        PropSeven = x.Flight,
        PropEight = x.FlightDay.ToString("MM/dd/yyyy HH:mm"),
        PropNine = x.IncidentNumber,
        PropTen = x.codeLocation.Location,
        PropEleven = x.Summary,
        PropTwelve = x.Total,
        PropThirteen = x.ATime
    })
    .ToList();

在 Debug模式下，使用 VS 2017，我发现这需要 47 到 52 秒，因此，执行时间不到一分钟。

在这种情况下，是否有比 .Select 更快的方法可以使用？

Answer 1

代码的问题很可能出现在对您在此处进行的数据库的 413 次调用(原始列表中的每个项目一次)中:

PropertyTwo = db.MyTable.Find(x.NavProperty.subcategoryID).Text

不要这样做，而是一次加载所有值并从内存中使用它们:

var distinctSubcategoryIds = lstDailySummary
    .Select(x => x.NavProperty.subcategoryID)
    .Distinct();

var dataForPropertyTwo = db.MyTable
    .Where(x => distinctSubcategoryIds.Contains(x.Id))
    .ToDictionary(x => x.Id, x => x.Text);

List<ExportExcel> lstExportedExcel = lstDailySummary.Select(x => new ExportExcel
{
    PropertyOne = x.ACInfo.RegNumber,
    PropertyTwo = dataForPropertyTwo[x.NavProperty.subcategoryID],
    PropertyThree = x.NavProperty.text,
    PropertyFour = (!string.IsNullOrWhiteSpace(x.Agcy.ToString())) ? x.codeAgcy.location : " ",
    PropertyFive = x.EventLocation,
    PropertySix = x.codeCounty.county,
    PropSeven = x.Flight,
    PropEight = x.FlightDay.ToString("MM/dd/yyyy HH:mm"),
    PropNine = x.IncidentNumber,
    PropTen = x.codeLocation.Location,
    PropEleven = x.Summary,
    PropTwelve = x.Total,
    PropThirteen = x.ATime
}).ToList();