查询语法有效，但lambda语法不适用于linq select语句

Question

This code doesn't work:

return this.Context.StockTakeFacts.Select(stf => ((stf.StockTakeId == stocktakeid) && (stf.FactKindId == ((int)kind)))).ToList<IStockTakeFact>();

This statement does:

        var f = from stf in this.Context.StockTakeFacts
                where (stf.StockTakeId == stocktakeid) && (stf.FactKindId == ((int)kind))
                select stf;
        return f.ToList<IStockTakeFact>();

What is the difference?? The first complains that IQueryable does not have a toList method so I gather I've written the first statement incorrectly.

Answer 1

You need to use a Where call in order to filter elements (not Select )

return this.Context.StockTakeFacts
  .Where(stf => ((stf.StockTakeId == stocktakeid) && (stf.FactKindId == ((int)kind))))
  .ToList<IStockTakeFact>();

When using explicit LINQ API queries the select item is implicit. It can be made explicit with a call to Select but it's not necessary (unless you map the values in some way)

Answer 2

You have to use Where() in order to be able filtering by the predicate:

return this.Context.StockTakeFacts
                    .Where(stf => stf.StockTakeId == stocktakeid 
                                  && stf.FactKindId == (int)kind)
                    .ToList<IStockTakeFact>();