如果没有找到项，LINQ表达式是否可以使用左连接或默认为空?

本文关键字：连接 默认 可以使 LINQ 是否 表达式 如果没有 | 更新日期: 2023-09-27 18:14:02

我试图追踪在我继承的web应用程序中的一些现有代码崩溃。我把它追溯到一个LINQ表达式:

var projInfo = (from v in context.TPM_PROJECTVERSION
                join p in context.TPM_PROJECT on v.PROJECTID equals p.PROJECTID
                join pto in context.TPM_USER on v.TPM_USER1.USERID equals pto.USERID
                join pt in context.TPM_PROJECTTYPES on p.PROJECTTYPEID equals pt.PROJECTTYPEID
                where v.PROJECTID == projectId && v.VERSIONID == versionId
                select new
                {
                    ProjectName = v.NAME,
                    ProjectType = pt.SHORTNAME,
                    ProjectDesc = v.DESCRIPTION,
                    BusinessLaunchData = p.BUSINESSLAUNCHDATE,
                    BusinessSponsor = (v.TPM_USER3.FIRSTNAME + " " + v.TPM_USER3.LASTNAME),
                    PrimaryTrainingOwner = (pto.FIRSTNAME + " " + pto.LASTNAME)
                }).First();

错误是"Sequence contains no elements"，我跟踪到这一行:

join pto in context.TPM_USER on v.TPM_USER1.USERID equals pto.USERID

如果USER1为空，则不返回该行。相反，我需要PrimaryTrainingOwner只是null或空白。基本上，我需要做一个LEFT JOIN，并在设置PrimaryTrainerOwner时检查这一点。有没有办法更新这个LINQ表达式来处理这种情况?谢谢!

如果没有找到项，LINQ表达式是否可以使用左连接或默认为空?

我想我找到解决办法了。我很感谢任何关于这是否是最好的方法或语法或其他方面的评论。谢谢!

var projInfo = (from v in context.TPM_PROJECTVERSION
                join p in context.TPM_PROJECT on v.PROJECTID equals p.PROJECTID
                join pto in context.TPM_USER on v.TPM_USER1.USERID equals pto.USERID into primaryowner
                from subpto in primaryowner.DefaultIfEmpty()
                join pt in context.TPM_PROJECTTYPES on p.PROJECTTYPEID equals pt.PROJECTTYPEID
                where v.PROJECTID == projectId && v.VERSIONID == versionId
                select new
                {
                    ProjectName = v.NAME,
                    ProjectType = pt.SHORTNAME,
                    ProjectDesc = v.DESCRIPTION,
                    BusinessLaunchData = p.BUSINESSLAUNCHDATE,
                    BusinessSponsor = (v.TPM_USER3.FIRSTNAME + " " + v.TPM_USER3.LASTNAME),
                    PrimaryTrainingOwner = subpto == null ? String.Empty : (subpto.FIRSTNAME + " " + subpto.LASTNAME)
                }).First();
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