如果没有找到item，LINQ表达式可以使用LEFT JOIN或默认为null吗？
如果没有找到item，LINQ表达式可以使用LEFT JOIN或默认为null吗？
提问于 2011-11-11 07:23:13
我正在尝试跟踪我继承的web应用程序中的一些现有代码中的崩溃。我已经将其追溯到一个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"，我已经追踪到这一行：
join pto in context.TPM_USER on v.TPM_USER1.USERID equals pto.USERID如果USER1为null，则不返回该行。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时进行检查。有没有办法更新这个LINQ表达式来处理这种情况？谢谢!
回答 1
Stack Overflow用户
回答已采纳
发布于 2011-11-11 08:00:49
我想我找到了一个解决方案。我非常感谢任何关于这是不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();页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/8087825
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