vb.net problem with recursive function inside while cycle - vb.net

Question

I try to create XtraDiagram (with DataTable) but this topic is related on DataTable construction.
Exscuse me if i ask you this problem. This is recursion problem. My scope is populate DataTable with all member of hierarchical structure (like for example file system).
In the last while i try to cycle for all children finded. In this section, i call recursive function for continue the analisys. The routine start ok and analisys until the last box. But after come back without analyze the other children… The sqldatareader lost his function? Because?
Thanks very much anticipated.
Public Function CreateDiagram(connection As SqlConnection, IDdoc2 As Integer, IDdoc1 As Integer) As Integer
'creation XtraDiagram
Dim cmd As SqlCommand = New SqlCommand()
Dim dr As SqlDataReader
Dim strItemName As String = String.Empty
'creation item
dt.Rows.Add(IDdoc2, strItemName, IDdoc1, "")
'extract how many children there are
cmd.Connection = connection
cmd.CommandType = CommandType.Text
cmd.CommandText = "SELECT IDdocumento_1 FROM tbRelazioniDocumenti WHERE
IDdocumento_2=" & IDdoc2
dr = cmd.ExecuteReader
If dr.HasRows Then
While dr.Read()
'for every children
Return CreateDiagram(connection, dr.Item("IDdocumento_1"), IDdoc2)
End While
Else
Return 0
End If
cmd.Dispose()
dr.Close()
End Function
Public Function CreateDiagram2(connection As SqlConnection, IDdoc2 As Integer, IDdoc1 As Integer) As Integer
Dim cmd As SqlCommand = New SqlCommand()
Dim dr As SqlDataReader
Dim strItemName As String = String.Empty
Dim arr() As Integer
ReDim arr(0)
'tento di estrapolare i figli
cmd.Connection = connection
cmd.CommandType = CommandType.Text
cmd.CommandText = "SELECT IDdocumento_1 FROM tbRelazioniDocumenti WHERE IDdocumento_2=" & IDdoc2
dr = cmd.ExecuteReader
If dr.HasRows Then
While dr.Read()
ReDim Preserve arr(UBound(arr) + 1)
arr(UBound(arr)) = dr.Item("IDdocumento_1")
End While
Else
Return 0
End If
cmd.Dispose()
dr.Close()
Dim i As Integer
For i = 1 To UBound(arr)
Return CreateDiagram2(connection, arr(i), IDdoc2)
Next i
End Function

Answer 1

When you close the datareader it also closes the connection. This means that when you move back up the stack the connection is now closed.
Additionally, you've got return CreateDiagram in your while loop. This will cause the method to execute CreateDiagram once and then return to the previous level.
I would suggest either using a try ... finally block around the creation item and the last if statement with the dispose and close in the finally or using blocks so that the command and reader are automatically disposed.
Next, I'd be inclined to put the results from datareader into a temporary table, and then iterate through the results recusively so that you don't run into trouble with your connection being closed while it's still being used.
Finally, is there any reason why you're not using a DataAdapter, and DataAdapter.fill(DataTable)? Once it has filled you could then use DataTable1.Load(DataTable2.CreateDataReader, upsert) to add the new rows to the existing datatable. See the official microsoft documentation here https://learn.microsoft.com/en-us/dotnet/api/system.data.datatable.load?view=netframework-4.8#System_Data_DataTable_Load_System_Data_IDataReader_System_Data_LoadOption_
Try this, I know I've modified a lot.
Public Function CreateDiagram(connection As SqlConnection, IDdoc2 As Integer, IDdoc1 As Integer) As Integer
'creation XtraDiagram
Dim cmd As SqlCommand = New SqlCommand()
'Dim dr As SqlDataReader
Dim strItemName As String = String.Empty
' Dim dt As DataTable
'creation item
dt.Rows.Add(IDdoc2, strItemName, IDdoc1, "")
Dim tmpdt As DataTable = dt.Clone
Dim cmdstr As String = String.Format("SELECT IDdocumento_1, {2} as strItemName, {1} as IDdoc1, '' as columnName3 FROM tbRelazioniDocumenti WHERE IDdocumento_2={0}", {IDdoc2, IDdoc1, strItemName})
'extract how many children there are
cmd.Connection = connection
cmd.CommandType = CommandType.Text
'cmd.CommandText = "SELECT IDdocumento_1 FROM tbRelazioniDocumenti WHERE IDdocumento_2=" & IDdoc2
cmd.commandtext = cmdstr
Dim da As New SqlClient.SqlDataAdapter(cmd)
da.Fill(tmpdt)
cmd.Dispose()
dt.Load(tmpdt.CreateDataReader, LoadOption.Upsert)
' dr = cmd.ExecuteReader
'If dr.HasRows Then
If tmpdt.Rows.Count > 0 Then
For Each dr As DataRow In tmpdt.Rows
'for every children
CreateDiagram(connection, dr.Item("IDdocumento_1"), IDdoc2)
Next
End If
Return 0
'dr.Close()
End Function
Or this which is your original with minimal changes
Public Function CreateDiagram(connection As SqlConnection, IDdoc2 As Integer, IDdoc1 As Integer) As Integer
'creation XtraDiagram
Dim cmd As SqlCommand = New SqlCommand()
Dim dr As SqlDataReader
Dim l As New List(Of Integer)
Dim strItemName As String = String.Empty
Try
'creation item
dt.Rows.Add(IDdoc2, strItemName, IDdoc1, "")
'extract how many children there are
cmd.Connection = connection
cmd.CommandType = CommandType.Text
cmd.CommandText = "SELECT IDdocumento_1 FROM tbRelazioniDocumenti WHERE
IDdocumento_2=" & IDdoc2
dr = cmd.ExecuteReader
If dr.HasRows Then
While dr.Read()
l.Add(dr.Item("IDdocumento_1"))
End While
Else
Return 0
End If
Finally
cmd.Dispose()
dr.Close()
End Try
For Each value In l
'for every children
CreateDiagram(connection, value, IDdoc2)
Next
End Function