Crystal Report - Linq select 语句在字段中查找空值

Question

我正在尝试对 MyReports 文件夹中的自定义水晶报表使用 Linq 查询。但是，水晶报表引擎抱怨空值。

我已经设法将序列号留空，它可能为空，但 DateTime 对我来说更成问题。

我试过了：

InspectionDate = subpat.InspectionDate == null ? "" : subpat.InspectionDate,

但错误是：

Type of conditional expression cannot be determined because there is no implicit conversion between 'string' and 'System.DateTime?' 

如果为 null，我只想要一个空白结果。

我的查询：

        var today = DateTime.Now.Date;
        var tomorrow = today.AddDays(30);


        var dateresult = db.ClinicalINSs.GroupBy(d => d.ClinicalAssetID)
                   .SelectMany(g => g.OrderByDescending(d => d.NextInspectionDate)
                                     .Take(1));

        var ClinicalIDVM = (from s in dateresult.Where(q => q.NextInspectionDate <= tomorrow)
                           join co in db.ClinicalAssets on s.ClinicalAssetID equals co.ClinicalAssetID into AR
                           let subred = AR.OrderByDescending(subredASS => subredASS.ClinicalAssetID).FirstOrDefault()
                           join cp in db.ClinicalPATs on s.ClinicalAssetID equals cp.ClinicalAssetID into AP
                           let subpat = AP.OrderByDescending(SubPATASS => SubPATASS.ClinicalAssetID).FirstOrDefault()
                           orderby s.NextInspectionDate descending


                           select new ClinicalIDVM
                           {
                               ClinicalAssetID = s.ClinicalAssetID,
                               ProductName = subred.ProductName,
                               SerialNo = subred.SerialNo == null ? "" : subred.SerialNo,
                               InspectionDate = subpat.InspectionDate,
                               NextInspectionDate = s.NextInspectionDate

                           }).ToList();

Answer 1

你可以试试这个方法

InspectionDate = subpat.InspectionDate == null ? (DateTime?)null : subpat.InspectionDate

或者把InspectionDate的类型改成string，就可以了

InspectionDate = subpat.InspectionDate.HasValue 
? subpat.InspectionDate.Value.ToString("yyyy-MM-dd") : "";

更新

 InspectionDate = subpat.InspectionDate == null ? (object) "" : subpat.InspectionDate