“Not allowed to change the 'ConnectionString' property.” error - database

Question

While i was adding data to access database, i got error:
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
TB2.Clear()
NameofG.Clear()
NunberofG.Clear()
UnitofG.Clear()
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
pro = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\Piyawat\Desktop\PPCC\Stock.accdb;"
connstring = pro
myconnection.ConnectionString = connstring
If myconnection.State = ConnectionState.Closed Then
myconnection.Open()
End If
command = "insert into Stock([InvoiceID],[Type],[Item],[Amout],[Unit]) Value ('" & TB2.Text & "','" & CB1.Text & "','" & NameofG.Text & "','" & NunberofG.Text & "','" & UnitofG.Text & "')"
Dim cmd As OleDbCommand = New OleDbCommand(command, myconnection)
cmd.Parameters.Add(New OleDbParameter("InvoiceID", CType(TB2.Text, String)))
cmd.Parameters.Add(New OleDbParameter("Type", CType(CB1.Text, String)))
cmd.Parameters.Add(New OleDbParameter("Item", CType(NameofG.Text, String)))
cmd.Parameters.Add(New OleDbParameter("Amout", CType(NunberofG.Text, String)))
cmd.Parameters.Add(New OleDbParameter("Unit", CType(UnitofG.Text, String)))
MsgBox("Record Save")
Try
cmd.ExecuteNonQuery()
cmd.Dispose()
TB2.Clear()
CB1.Text.DefaultIfEmpty
NameofG.Clear()
NunberofG.Clear()
UnitofG.Clear()
Catch ex As Exception
MsgBox("Mistake")
End Try
End Sub
How to fix the error? Please help. Thanks

Answer 1

I would AWAYS ensure that after you use the connection, you close it.
however, you can get .net to automatics do this for you.
Also, go into project->settings and add your connection string there - that way you NEVER have to place or have connection strings in code - (bad idea).
Hence, your code should and would look like this:
Dim strSQL As String = "INSERT INTO Stock([InvoiceID],[Type],[Item],[Amout],[Unit]) " &
"VALUES (#TB2, #CB1, #NameofG, #NunberofG, #UnitofG)"
Using cmd As New OleDbCommand(strSQL, New OleDbConnection(My.Settings.Test44))
cmd.Parameters.Add("#TB2", OleDbType.VarWChar).Value = TB2.Text
cmd.Parameters.Add("#CB1", OleDbType.VarWChar).Value = CB1.Text
cmd.Parameters.Add("NameofG", OleDbType.VarWChar).Value = NameofG.Text
cmd.Parameters.Add("#NumberofG", OleDbType.Integer).Value = NunberofG.Text
cmd.Parameters.Add("#UnitofG", OleDbType.Integer).Value = UnitOfG.Text
cmd.Connection.Open()
Try
cmd.ExecuteNonQuery()
MsgBox("Record Save")
Catch ex As Exception
MsgBox("Error = " & ex.Message)
End Try
End Using
note the following:
No messy string concentation.
No messy quotes for strings, non quotes for numbers.
clean easy to read.
use parmaters - safe.
And with a using block, we do NOT have to dispose or close the conneciton - it is automatic for you. So ALWAYS assume it is closed.
note how we don't have a seperate conneciton object - don't need one.
note how the command object is REALLY nice.
The command object has:
a connection object - you don't need to create a new one
command text
a reader!!! - you can use command object - no need to load + create a reader

Answer 2

I moved the database code to a separate method.
Starting with the user interface code, you first want to validate the input. I had to guess at the datatypes because your code made them all look like strings. I hope this is not true.
It seems you got mixed up with Parameters. I am very glad that you tried to used them but they don't appear in your sql string so they are not being used. You don't want to concatenate strings with user input because it risks sql injection.
The # sign preceding the parameter names is just a convention that makes it easy to recognize.
You need to keep database objects local to the method where they are used. That way they can be closed and disposed with Using...End Using blocks as quickly as possible.
Private ConStr As String = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\Piyawat\Desktop\PPCC\Stock.accdb;" '"Your connection string"
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
'Validate input
Dim ID As Integer
Dim Amt As Decimal
Dim Unt As Integer
Dim message = "Please enter a number for the"
If Not Integer.TryParse(TB2.Text, ID) Then
MessageBox.Show(message & " Invoice ID.")
Return
End If
If Not Decimal.TryParse(NunberofG.Text, Amt) Then
MessageBox.Show(message & " Amount.")
Return
End If
If Not Integer.TryParse(UnitofG.Text, Unt) Then
MessageBox.Show(message & " Unit")
Return
End If
'Call the insert method
Try
InsertInvoice(ID, CB1.Text, NameofG.Text, Amt, )
Catch ex As Exception
MsgBox(ex.Message)
Return
End Try
MessageBox.Show("Record Saved")
TB2.Clear()
CB1.Text.DefaultIfEmpty
NameofG.Clear()
NunberofG.Clear()
UnitofG.Clear()
End Sub
Private Sub InsertInvoice(InvoiceID As Integer, Type As String, Item As String, Amount As Decimal, Unit As Integer)
Dim Command = "insert into Stock([InvoiceID],[Type],[Item],[Amout],[Unit]) Value (#InvoiceID,#Type,#Item,#Amount,#Unit)"
Using myconnection As New OleDbConnection(ConStr),
cmd As New OleDbCommand(Command, myconnection)
With cmd.Parameters
.Add("#InvoiceID", OleDbType.Integer).Value = InvoiceID
.Add("#Type", OleDbType.VarChar).Value = Type
.Add("#Item", OleDbType.VarChar).Value = Item
.Add("#Amout", OleDbType.Decimal).Value = Amount
.Add("#Unit", OleDbType.Integer).Value = Unit
End With
End Using
End Sub