在原来的查询，这些电话以Enumerable.Join使用幕后一个哈希表，使事情很快。如果切换到。凡，你没有得到这些哈希的好处。您可以使用明确散列来获得同样的效果。

In the original query, those calls to Enumerable.Join are using a hashtable behind the scenes to make things fast. If you switch to .Where, you don't get those hash benefits. You can use hashing explicitly to get the same effect.

ILookup<int, string> deskNameLookup = tradingDesks.Values
  .ToLookup(
    d => d.Id,
    d => d.Name
  );

ILookup<int, ProductInfo> infoLookup = ProductInfos.Values
  .ToLookup(p.RiskBookId);

foreach(b in books.Values)
{
  foreach(dName in deskNameLookup[b.TradingDeskId])
  {
    foreach(p in infoLookup[b.Id].Concat(infoLookup[0]))
    {
      var x = new {p, Book = b.Name, TradingDeskName = dName};
    }
  }
}