InvalidCastException when trying to Sum datatable rows with LINQ - vb.net

Question

Hi i am trying to sum all my datatable values to one row. but i retrieve a InvalidCastException:
Failed to convert an object of
typeWhereSelectEnumerableIterator2[System.Linq.IGrouping2[System.Object,System.Data.DataRow],VB$AnonymousType_0`4[System.Object,System.Double,System.Decimal,System.Decimal]]
to type System.Data.DataTable.
SQL Datatypes:
NAME_AGE string
LON money
sal_tjformon money
sal_sjuklon money
Private Function GroupByName(dataTable As DataTable) As DataTable
Dim result = dataTable.AsEnumerable().GroupBy(
Function(row) row.Item("NAME_AGE")).Select(Function(group) New With {
.Grp = group.Key,
.LON = group.Sum(Function(r) Decimal.Parse(r.Item("LON"))),
.sal_tjformon = group.Sum(Function(r) Decimal.Parse(r.Item("sal_tjformon"))),
.sal_sjuklon = group.Sum(Function(r) Decimal.Parse(r.Item("sal_sjuklon")))
})
Return result

Answer 1

The LINQ statement returns an IEnumerable(Of <anonymous_type>). There are two problems with this. First of all, your function returns a DataTable, which your object definitely is not. Secondly of all, you can't return an anonymous type from a function call.
If you want to return the select result, you have to create an explicit type (a class) and return the IEnumerable(Of MyType), like in the code below. I strongly advice to set an explicit type to the Grp property (like String?).
Class GroupNameAgeResult
Public Property Grp As Object
Public Property LON As Decimal
Public Property sal_tjformon As Decimal
Public Property sal_sjuklon As Decimal
End Class
Private Function GroupByName(dataTable As DataTable) As IEnumerable(Of GroupNameAgeResult)
Dim result = dataTable.AsEnumerable().GroupBy(Function(row) row.Item("NAME_AGE")) _
.Select(Function(grp) New GroupNameAgeResult() With
{.Grp = grp.Key,
.LON = grp.Sum(Function(r) Decimal.Parse(r.Item("LON").ToString)),
.sal_tjformon = grp.Sum(Function(r) Decimal.Parse(r.Item("sal_tjformon").ToString)),
.sal_sjuklon = grp.Sum(Function(r) Decimal.Parse(r.Item("sal_sjuklon").ToString))})
Return result
End Function
If you want to return a DataTable, you can define this, loop over the groups and add a row. You can return afterwards the result. See example code below.
Private Function GroupByName(dataTable As DataTable) As DataTable
Dim result As New DataTable()
result.Columns.Add("Grp", GetType(Object))
result.Columns.Add("LON", GetType(Decimal))
result.Columns.Add("sal_tjformon", GetType(Decimal))
result.Columns.Add("sal_sjuklon", GetType(Decimal))
For Each grp In dataTable.AsEnumerable().GroupBy(Function(row) row.Item("NAME_AGE"))
Dim row As DataRow = result.NewRow()
row.Item("Grp") = grp.Key
row.Item("LON") = grp.Sum(Function(r) Decimal.Parse(r.Item("LON").ToString))
row.Item("sal_tjformon") = grp.Sum(Function(r) Decimal.Parse(r.Item("sal_tjformon").ToString))
row.Item("sal_sjuklon") = grp.Sum(Function(r) Decimal.Parse(r.Item("sal_sjuklon").ToString))
result.Rows.Add(row)
Next
Return result
End Function
Last but not least. I strongly advice you to turn on "Option strict" (you can set this in the project properties -> Compile). You'll notice many more (possible) errors with your code (even the small function from this question).