使用 LINQ 聚合数据表中的行

Question

我有一个这样的DataTable

ProductId CountThisWeek   CountLastWeek
        1          10.0            15.0         
        1          20.0             5.0

        2           5.0            10.0
        2        DBNull            15.0
        2          10.0          DBNull

        3        DBNull            15.0
        3        DBNull            15.0

我需要通过“压缩”（按 productId 求和）我的初始 DataTable 来获得一个新的 DataTable（或者，如果不是，至少是一个匿名数据结构，如字典），如下所示：

ProductId CountThisWeek   CountLastWeek
        1          30.0            20.0         
        2          15.0            25.0
        3        DBNull            30.0

规则很简单：DBNull + Value = Value，DBNull + DBNull = DBNull

我尝试实现这个聚合，像这样：

(from row in table.AsEnumerable()
group row by row.Field<int>("ProductId") into g
select g).ToDictionary(
g => g.Key, 
g => new {
   CountThisWeek = g.Sum(r => row.Field<decimal>("CountThisWeek")),
   CountLastWeek = g.Sum(r => row.Field<decimal>("CountLastWeek"))
}

但是这个 Sum 没有考虑到我的 DBNull 规则...

我不能用可为空的小数来做吗？

g.Sum(r => r.Field<decimal?>("CountThisWeek")),

所以，也许我需要做一些类似的事情

g.Aggregate((summRow, nextRow) => {
                        DataRow dr = summRow.Table.NewRow();
                        decimal? sum = summRow.Field<decimal?>("CountThisWeek");
                        decimal? next = nextRow.Field<decimal?>("CountThisWeek");
                        decimal? result = (!sum.HasValue && !next.HasValue) ? 
                            (decimal?)null : sum ?? 0 + next ?? 0;
                        dr["CountThisWeek"] = result;
                        return dr;
                    }),

这是实现我尝试的最佳方式吗？

我想在聚合函数中获得decimal? 而不是DataRow...

Answer 1

你可以试试

(from row in table.AsEnumerable()
group r by r.Field<int>("ProductId") into g
select g).ToDictionary(
g => g.Key, 
g => new {
   CountThisWeek = g.All(r => r.IsNull("CountThisWeek")) ?
                      null : 
                      (decimal?)g.Sum(r => r.Field<decimal>("CountThisWeek")),
   CountLastWeek = g.All(r => r.IsNull("CountLastWeek")) ? 
                       null : 
                       (decimal?)g.Sum(r => r.Field<decimal>("CountLastWeek"))
}

Answer 2

你可以试试Aggregate function with Accumulator这样

g.Aggregate((decimal?)null,(summRow, nextRow) => 
                    nextRow.IsNull("CountThisWeek")?
                        summRow:
                        ((summRow??0) + nextRow.Field<decimal>("CountThisWeek"))
                ),