如果未找到项目，LINQ 表达式可以使用 LEFT JOIN 或默认为 null 吗？

Question

我正在尝试追踪我继承的 Web 应用程序中某些现有代码中的崩溃。我已将其追溯到单个 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“序列不包含元素”，我已将其追踪到此line：

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

如果 USER1 为空，则不会返回该行。相反，我需要 PrimaryTrainingOwner 为 null 或空白。基本上，我需要执行 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！

Answer 1

我想我找到了解决方案。对于这是否是最好的方法或语法或其他什么的任何评论，我将不胜感激。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();