transaction in ms access and vb.net - vb.net

Question

I am trying the below code but not getting executed.
Private Sub btnsave_Click(sender As Object, e As EventArgs) Handles btnsave.Click
Using con As New OleDbConnection(connectionString)
Dim tra As OleDbTransaction = Nothing
Try
con.Open()
cmd.Transaction = tra
tra = con.BeginTransaction
Dim sqlstr As String = "insert into category(cname,comment) values('" + txtcategory.Text + "','" + txtcomment.Text + "')"
cmd = New OleDb.OleDbCommand(sqlstr, con)
cmd.ExecuteNonQuery()
Dim sql As String = "UPDATE tblInvoices SET avail = 1 WHERE (cname = txtcategory.Text)"
cmd = New OleDb.OleDbCommand(sqlstr, con)
cmd.ExecuteNonQuery()
tra.Commit()
Catch ex As Exception
MsgBox(ex.Message)
Try : tra.Rollback() : Catch : End Try
End Try
End Using
End Sub
I don't understand the transactions.

Answer 1

The command need to know about the existence of a Transaction. But you assign the Transaction instance before the opening the connection and then ask the connection to start the transaction.
In this way the command has a null reference for the transaction and not the good instance created by the connection. Also, when you create again the command, there is no transaction associated with it.
Better use the OleDbCommand constructor that takes a Transaction as third parameter
Private Sub btnsave_Click(sender As Object, e As EventArgs) Handles btnsave.Click
Using con As New OleDbConnection(connectionString)
Dim tra As OleDbTransaction = Nothing
Try
con.Open()
tra = con.BeginTransaction
Dim sqlstr As String = "insert into category(cname,comment)" &
" values(#cat, #com)"
cmd = New OleDb.OleDbCommand(sqlstr, con, tra)
cmd.Parameters.Add("#cat", OleDbType.VarWChar).Value = txtcategory.Text
cmd.Parameters.Add("#com", OleDbType.VarWChar).Value = txtcomment.Text
cmd.ExecuteNonQuery()
Dim sql As String = "UPDATE tblInvoices SET avail = 1 " &
"WHERE cname = #cat"
cmd = New OleDb.OleDbCommand(sqlstr, con, tra)
cmd.Parameters.Add("#car", OleDbType.VarWChar).Value = txtcategory.Text
cmd.ExecuteNonQuery()
tra.Commit()
Catch ex As Exception
MsgBox(ex.Message)
tra.Rollback()
End Try
End Using
End
I have also changed your code to use a more safe approach to your queries. Instead of using a string concatenation use ALWAYS a parameterized query. In this way you are safe from Sql Injection, you don't have problems with parsing texts and your queries are more readable.