Question

将此转换为左连接的首选方法是什么？这是Linq to Objects（不是SQL）。

var result = 
(
   from e in entries
   from r in results
   where r.PropertyId == e.PropertyId &&
         e.ValueAsString.Equals(r.Value, StringComparison.InvariantCultureIgnoreCase)
   select new
   {
      Result = r,
      Entry = e
  }
)
.ToList();

问题：可读性与效率之间的权衡。应该尽量避免ToLowerInvariant（）？

Answer 1

你写的是cross join，而不是left join

var result = 
(
   from e in entries
   join r in results
   let ev=e.ValueAsString.ToLower()
   let rv=r.Value.ToLower()
   on new{e.PropertyId,ev} equals new {r.PropertyId,rv } into lg
   from r in lg.DefaultIfEmpty()
   select new
   {
      Result = r,
      Entry = e
  }
)
.ToList();

Answer 2

对于左连接，您需要使用DefaultIfEmpty

var result = 
    (
       from e in entries
       from r in results.Where(x => x.PropertyId == e.PropertyId)
                        .Where(x => e.ValueAsString.Equals(x.Value, StringComparison.InvariantCultureIgnoreCase))
                        .DefaultIfEmpty()
       select new
       {
          Result = r,
          Entry = e
      }
    )
    .ToList();

如何使用多个字段编写Linq左连接，其中一个不区分大小写

2 个答案: