Open DataReader associated with this Connection which must be closed first - mysql

Question

I am trying to make a POS (Point of Sales) Application and I got this error. "There is already an open DataReader associated with this Connection which must be closed first."
Below are my codes:
Database using MySQL
If txt_notr.Text = "" Or txt_kodep.Text = "" Or txt_item.Text = "" Or txt_gt.Text = "" Or txt_bayar.Text = "" Then
MsgBox("Data belum lengkap...!!!")
Exit Sub
Else
'Simpan ke tabel penjualan
db.Close()
db.Open()
Call Koneksi()
Dim simpan1 As String = "Insert Into tb_penjualan values('" & txt_notr.Text & "','" & Format(Now, "yyyy-MM-dd") & "','" & txt_kodep.Text & "','" & txt_item.Text & "','" & txt_gt.Text & "','" & txt_bayar.Text & "')"
cmd = New MySqlCommand(simpan1, db)
cmd.ExecuteNonQuery()
db.Close()
db.Open()
'Simpan ke tabel detail penjualan
For baris As Integer = 0 To DGV.Rows.Count - 2
Dim simpandet As String = "Insert into tb_detjual values('" & txt_notr.Text & "','" & DGV.Rows(baris).Cells(0).Value & "','" & DGV.Rows(baris).Cells(3).Value & "','" & DGV.Rows(baris).Cells(4).Value & "','" & DGV.Rows(baris).Cells(5).Value & "')"
cmd = New MySqlCommand(simpandet, db)
cmd.ExecuteNonQuery()
db.Close()
db.Open()
cmd = New MySqlCommand("Select * from tb_stok where id_obat = '" & DGV.Rows(baris).Cells(0).Value & "'", db)
dr = cmd.ExecuteReader
dr.Read()
If dr.HasRows Then
Dim kurangstok As String = "Update tb_stok set stok = '" & dr.Item("stok") - DGV.Rows(baris).Cells(4).Value & "' where id_obat = '" & DGV.Rows(baris).Cells(0).Value & "'"
cmd = New MySqlCommand(kurangstok, db)
cmd.ExecuteNonQuery() 'The Error shows here...
End If
Next
Call hapustemp()
Call bersih()
Call notrans()
End If
db.Close()

Answer 1

It's counter-intuitive, but ADO.Net providers use a feature called Connection Pooling, such that you really are better off creating a new connection object in most cases for individual calls to a database, rather than trying to keep a single database connection in your class to re-use. The code below shows the correct way to re-use the connection object: create a new connection for the method, and use it for the duration of the method. But then let the connection be collected when the method completes.
I noticed that you also have some seriously insecure code. Before doing any more work with databases, you should read up on Sql Injection. This is a HUGE issue. You should not be writing database code professionally if you don't know about it, and the correct way to avoid the problem.
Finally, the code uses some conventions that originated with older VBScript/VB6 and are no longer appropriate.
The code below solves all of those issues, and should be much faster by avoiding the need to run a SELECT from the database for each gridview row:
If String.IsNullOrWhiteSpace(txt_notr.Text) OrElse String.IsNullOrWhiteSpace(txt_kodep.Text) OrElse String.IsNullOrWhiteSpace(txt_item.Text) OrElse String.IsNullOrWhiteSpace(txt_gt.Text) OrElse String.IsNullOrWhiteSpace(txt_bayar.Text_ Then
MsgBox("Data belum lengkap...!!!")
Exit Sub
End If
'No need for an "Else". The "Exit Sub" takes care of it.
'Simpan ke tabel penjualan
'Note that I was able to let the database set the time stamp
Dim sql As String = "Insert Into tb_penjualan values(#notr, current_timestamp, #kodep, #item, #gt, #bayar);"
'The "Using" keyword will guarantee the connection closes, even if an exception is thrown
Using cn As New MySqlConnection(" connection string here "), _
cmd As New MySqlCommand(sql, cn)
'Use parameter placeholders rather than string concatenation. This avoids a SERIOUS security issue.
' I have to guess parameter types/lengths, but you should use actual types/lengths that match your database
cmd.Parameters.Add("#notr", MySqlDbType.Int32).Value = CInt(txt_notr.Text)
cmd.Parameters.Add("#kodep", MySqlDbType.VarChar, 20).Value = txt_kodep.Text
cmd.Parameters.Add("#item", MySqlDbType.VarString, 1000).Value = txt_item.Text
cmd.Parameters.Add("#gt", MySqlDbType.VarChar, 50).Value = txt_gt.Text
cmd.Parameters.Add("#bayar", MySqlDbType.VarChar, 50).Value = txt_bayar.Text
cn.Open()
cmd.ExecuteNonQuery()
'Two sql statements in a single call to the database.
'This is MUCH better than the Insert/Select/Update process you were using
sql = "INSERT INTO tb_detjual VALUES (#notr, #c0, #c3, #c4, #c5);" & _
"UPDATE tb_stok SET stok = stok - #c4 WHERE id_obat = #c0;"
'See how I was able to re-use the same parameters in the query.
cmd.Parameters.Clear()
cmd.CommandText = sql
cmd.Parameters.Add("#notr", MySqlDbType.Int32).Value = CInt(txt_notr.Text)
cmd.Parameters.Add("#c0", MySqlDbType.VarChar, 50)
cmd.Parameters.Add("#c3", MySqlDbType.VarChar, 50)
cmd.Parameters.Add("#c4", MySqlDbType.VarChar, 50)
cmd.Parameters.Add("#c5", MySqlDbType.VarChar, 50)
'Simpan ke tabel detail penjualan
For baris As Integer = 0 To DGV.Rows.Count - 2
'I'm able to re-use the same parameters for each loop
cmd.Parameters("#c0").Value = DGV.Rows(baris).Cells(0).Value
cmd.Parameters("#c3").Value = DGV.Rows(baris).Cells(3).Value
cmd.Parameters("#c4").Value = DGV.Rows(baris).Cells(4).Value
cmd.Parameters("#c5").Value = DGV.Rows(baris).Cells(5).Value
cmd.ExecuteNonQuery()
Next
End Using
hapustemp()
bersih()
notrans()