c# - 表达式> predicate=null;在 where condition(query.Where(predicate)) 中抛出错误

Question

 Expression<Func<PartJoinTable, bool>> predicate = null;

 var query2 = query.Join(partJoinTableRepository.GetPartJoinQuery(),
                         "x.PartID", "PartID", "inner" + row + "", null);
 foreach (var item1 in query)
 {
     list.Add(new PartJoinTable
     {
          ProductTypeId = item1.x.ProductTypeId,
          PartId = item1.x.PartId
     });

  }
  predicate = PredicateBuilder.True(list.AsQueryable());


  if (typeid=="3")
  {
      if (row == 0)
      {
          predicate1 = predicate1.And(z => cpaclassids.Contains(z.x.ProductTypeId.ToString()));
      }
      else
      {
          predicate = predicate.And(x => cpaclassids.Contains(x.ProductTypeId.ToString()));
       }
    }
    row++;
}
int lst1 = query.Where(predicate).Select(x => x).Distinct().ToList().Count();

Cannot convert from 'System.Linq.Expressions.Expression>' to 'System.Func<, bool>'

我无法检索结果，因为当我声明变量 Expression<Func<PartJoinTable, bool>> predicate = null; 时显示上述错误

Answer 1

回答

Expression> predicate=null; throws error in where condition(query.Where(predicate))

是

if (predicate != null)
    query = query.Where(predicate);

int lst1 = query.Distinct().Count();

仅在 where 子句存在时添加它。在 .Count() 之前使用 .ToList() 可以将数据库中的每一行投影到内存中。