linq - 如果过滤器参数为空，则 linq 到实体然后选择所有实体？

Question

我使用 ID 过滤实体，但如果 ID 为空，那么我想返回所有实体。因此，在下面的代码中，如果竞争者 ID 为空 id，则希望返回所有简短的竞争者。

var competitors =
                NeptuneUnitOfWork.Briefs.FindWhere(b => b.ID == briefID)
                                 .Select(b => b.BriefCompetitors.Where(b=>b.ID == competitorID)).ToList();

Answer 1

通常你这样做：

IQueryable<Brief> briefs = NeptuneUnitOfWork.Briefs.Where(b => b.ID == briefID);

if (competitorID != null)
{
    competitors = briefs.Select(b => b.BriefCompetitors.Where(b=>b.ID == competitorID));
}
else
{
    competitors = briefs.Select(b => b.BriefCompetitors);
}

从技术上讲，您可以将问题留给 SQL Server：

// Note the || clause
competitors = briefs.Where(b => b.ID == briefID)
                    .Select(b => b.BriefCompetitors.Where(b=>b.ID == competitorID || competitorID == null));

Answer 2

只是对@xanatos 帖子的一个小改动

// Note the || clause
competitors = briefs.Where(b => b.ID == briefID && (competitorID == null || b.ID == competitorID));