Question

I dont know why i am getting the following exception how can I cast into grouping Row ? What does grouping row means

Answer 1

InvoiceHeaderCollection is a collection of groups, the groups is something like:

      Key               Value
 invoice account1      datarow1 //the value is a `DataRow`
                       datarow2
                       datarow4
 invoice account2      datarow3
 invoice account3      datarow5 

So I think you need:

foreach(var g in InvoiceHeaderCollection)
{
    DataRow[] rows = g.ToArray();   //to get all the DataRows in the group
    //maybe another loop through the rows
}