Linq - 分组计算总计

Question

Linq noob here。我该如何纠正？

   var result = (from row in rulestable.AsEnumerable() 
         let sup = row.Field<int>("Supplier") 
         let amo = row.Field<decimal>("Amount") 
         let adv = row.Field<decimal>("Advance") 
         let bal = row.Field<decimal>("Balance") 
         let vat = row.Field<decimal>("VatRate") 
         group row by new { sup, vat } into grp 
         select new 
         { 
          Supplier = grp.Key.sup, 
          Amount = grp.Sum(grp => grp["amo"]), 
          Advance = grp.Sum(grp => grp["adv"]), 
          Balance = grp.Sum(grp => grp["bal"]), 
          VatRate = grp.Key.vat 
         }).ToList(); 

我想将我的datatable“rulestable”分组在2列Supplier和VatRate上。结果的金额，提前和结余必须设置为分组的任何列的总和。

我得到一个错误 - “类型参数无法从grp.Sum的用法推断”。但是，我认为这个表达式是错误的 - 我认为它会对结果列进行求和，而不是像我想要的那样对源列进行求和。

实施例：

输入数据表：
供应商，数量，超前，平衡，VatRate
S1，10，5,5，1
S1，10，5,5，1

预期输出：
S1，20，10，10，1

---

什么是最好的方法来做到这一点？

Answer 1

在lambda里面，grp（我已经更名为r，以便与组自身区分）将会是DataRow。编译器不知道什么数据类型grp [string]返回，因为它只返回一个object。试试这个：

DataTable rulestable = new DataTable(); 
rulestable.Columns.Add("Supplier"); 
rulestable.Columns.Add("Amount", typeof(decimal)); 
rulestable.Columns.Add("Advance", typeof(decimal)); 
rulestable.Columns.Add("Balance", typeof(decimal)); 
rulestable.Columns.Add("VatRate", typeof(decimal)); 
rulestable.Rows.Add("S1", 10M, 5M, 5M, 1M); 
rulestable.Rows.Add("S1", 10M, 5M, 5M, 1M); 

var result = (from row in rulestable.AsEnumerable() 
       let sup = row.Field<string>("Supplier") 
       let amo = row.Field<decimal>("Amount") 
       let adv = row.Field<decimal>("Advance") 
       let bal = row.Field<decimal>("Balance") 
       let vat = row.Field<decimal>("VatRate") 
       group row by new { sup, vat } into grp 
       select new 
       { 
        Supplier = grp.Key.sup, 
        Amount = grp.Sum(r => r.Field<decimal>("Amount")), 
        Advance = grp.Sum(r => r.Field<decimal>("Advance")), 
        Balance = grp.Sum(r => r.Field<decimal>("Balance")), 
        VatRate = grp.Key.vat 
       }).ToList(); 

 
Input: 
Supplier | Amount | Advance | Balance | VatRate 
----------------------------------------------- 
    S1 | 10 | 5 | 5 | 1 
----------------------------------------------- 
    S1 | 10 | 5 | 5 | 1 

Result: 
Supplier | Amount | Advance | Balance | VatRate 
----------------------------------------------- 
    S1 | 20 | 10 | 10 | 1 

使用你提供的输入，这给你上述预期的效果。

Answer 2

当我运行这个查询时，我在Visual Studio中失败了。在更多的搜索发现以下，并在我的系统中运行良好。

var result = (from row in rulestable.AsEnumerable() 
      group row by new {sup= row.Field<string>("Supplier"), 
           vat=row.Field<decimal>("VatRate")} into grp = Group 
      select new With 
      {.Supplier = sup, 
       .Amount = grp.Sum(r => r.Field<decimal>("Amount")), 
       .Advance = grp.Sum(r => r.Field<decimal>("Advance")), 
       .Balance = grp.Sum(r => r.Field<decimal>("Balance")), 
       .VatRate = vat 
       } 
      ).ToList();