C# 如果项为'，LINQ表达式是否可以使用左连接或默认为null；没有找到？

我正在试图追踪我继承的web应用程序中某些现有代码的崩溃。。我将其追溯到一个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”（序列不包含任何元素），我已经跟踪到这一行：

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

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如果USER1为null，则不会返回该行。相反，我需要PrimaryTrainingOwner为null或空白。基本上，我需要做一个左连接，并在设置PrimaryTrainerOwner时检查它。是否有方法更新此LINQ表达式以处理此情况？谢谢

我想我找到了解决办法。如果您能就这是否是最好的方法、语法或其他方面发表意见，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();

---

完全正确。您确实可以选择重用

pto

而不是

subto

，因为原始

pto

值超出范围，如果您认为这有助于可读性的话。