Linq - Grouping with sum calculation

Question

Linq noob here. How do I correct this?

                var result = (from row in rulestable.AsEnumerable()
                         let sup = row.Field

        

Answer 1

Inside the lambda, grp (which I've renamed r for row to distinguish from the group itself) is going to be a DataRow. The compiler doesn't know what datatype grp[string] returns, since it just returns an object. Try this:

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("Supplier")
              let amo = row.Field("Amount")
              let adv = row.Field("Advance")
              let bal = row.Field("Balance")
              let vat = row.Field("VatRate")
              group row by new { sup, vat } into grp
              select new
              {
                  Supplier = grp.Key.sup,
                  Amount   = grp.Sum(r => r.Field("Amount")),
                  Advance  = grp.Sum(r => r.Field("Advance")),
                  Balance  = grp.Sum(r => r.Field("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

Using the input you supplied, this gives the results you expected above.