EF 6：查询结果不能被多次枚举

Question

Is there any way to avoid "The result of a query cannot be enumerated more than once" exception without using ToList()?

Here is a simplified version of my code:

var entities = _db.Trades.Where(t => t.A > 10);
int totalCount = entities.Count();
entities = entities.Where(t => t.B > 10);
int totalCountAfterAdditionalFilter = entities.Count();

I cannot call ToList() due to performance considerations. I know that I could just generate one more IQueryable, but that seems wrong (I have more filters like this). Could I somehow preserve/duplicate the IQueryable after my first call of Count()?

Thanks!

Answer 1

No, you can't achieve your desired result like that. As an alternative you can, however, save your filters to variables and then compose them as needed:

 Func<Trade, bool> filter1 = t => t.A > 10;
 Func<Trade, bool> filter2 = t => t => t.B > 10;
 Func<Trade, bool> compositeFilter = t => filter1(t) && filter2(t);

 int totalCount = _db.Trades.Count(filter1);
 int totalCountAfterAdditionalFilter  = _db.Trades.Count(compositeFilter);

 //Need more?
 compositeFilter = t => compositeFilter(t) && t.C > 100;
 int totalAfterMoreFilters = _db.Trades.Count(compositeFilter);

Answer 2

may be:

Trades.Where(x => x.A > 10).Select(x => new { i = 1, j = x.B > 10 ? 1 : 0}).
    GroupBy(y => y.i).
    Select(g => new { c = g.Count(), = g.Sum(z => z.j)})

That's give you the 2 informations in one query.