为c#的linq查询中的多个连接获取错误

Question

下面指定的查询在编译时显示错误。它说名称wfmilestoneprojectrel不在equals右侧的作用域中。考虑交换equals两端的表达式。我该如何解决这个问题？

请帮帮我。

var test = 
   (from wfmilestonedefinition in _context.WF_MILESTONE_DEFINITION
    join wfmilestoneprojectrel in _context.WF_MILESTONE_PROJECT_REL
        on wfmilestonedefinition.MILESTONE_ID equals wfmilestoneprojectrel.MILESTONE_ID
    join workflowrecord in _context.WORKFLOW_RECORD
        on wfmilestoneprojectrel.PROJECT_ID equals workflowrecord.PROJECT_ID
    join workflowmilestone in _context.WORKFLOW_MILESTONE
        on
        new
            {
                wfmilestoneprojectrel.MILESTONE_PROJECT_REL_ID,
                workflowrecord.WF_ID
            } equals
        new
            {
                MILESTONE_PROJECT_REL_ID = wfmilestoneprojectrel.MILESTONE_PROJECT_REL_ID,
                workflowmilestone.WF_ID
            }
    select workflowmilestone).ToList();

下面给出了运行良好的相关Sql查询: SELECT dbo.WF_MILESTONE_DEFINITION.MILESTONE_ID，dbo.WF_MILESTONE_DEFINITION.MILESTONE_NAME，dbo.WF_MILESTONE_PROJECT_REL.PROJECT_ID，dbo.WORKFLOW_RECORD.WF_ID，dbo.WORKFLOW_MILESTONE.MILESTONE_E_DATE FROM dbo.WF_MILESTONE_DEFINITION INNER JOIN dbo.WF_MILESTONE_PROJECT_REL ON dbo.WF_MILESTONE_DEFINITION.MILESTONE_ID = dbo.WF_MILESTONE_PROJECT_REL.MILESTONE_ID INNER JOIN dbo.WORKFLOW_RECORD ON dbo.WF_MILESTONE_PROJECT_REL.PROJECT_ID = dbo.WORKFLOW_RECORD.PROJECT_ID INNER JOINdbo.WF_MILESTONE_PROJECT_REL.MILESTONE_PROJECT_REL_ID = dbo.WF_MILESTONE_PROJECT_REL.MILESTONE_PROJECT_REL_ID和dbo.WORKFLOW_RECORD.WF_ID = dbo.WORKFLOW_MILESTONE.WF_ID上的dbo.WORKFLOW_MILESTONE

Answer 1

看看将在幕后调用的Join方法的定义：

Join<TOuter, TInner, TKey, TResult>(IEnumerable<TOuter>, 
                                    IEnumerable<TInner>, 
                                    Expression<Func<TOuter, TKey>>, 
                                    Expression<Func<TInner, TKey>>, 
                                    Expression<Func<TOuter, TInner, TResult>>)

之所以会出现此错误，是因为在第二个join子句中变量wfmilestoneprojectrel不可用，因为向该委托传递了变量workflowmilestone (TInner)：

    new 
    {
      //here is no varialbe with name wfmilestoneprojectrel
      MILESTONE_PROJECT_REL_ID = wfmilestoneprojectrel.MILESTONE_PROJECT_REL_ID,
      workflowmilestone.WF_ID
    }

为什么不直接写：

join workflowmilestone in _context.WORKFLOW_MILESTONE
    on workflowrecord.WF_ID equals workflowmilestone.WF_ID

也许我误解了什么..。