Oracle INSERT on VB.net Null reference exception - vb.net

Question

Please help i am a newbie on oracle database i am used on using mysql and i really don't have any idea how can i debug my code. Here is my code.
Imports System.Diagnostics
Imports Oracle.DataAccess.Client
Imports Oracle.DataAccess.Types
Public Class mainMenu
Dim conn As New OracleConnection
Private cmd As OracleCommand
Private da As OracleDataAdapter
Private cb As OracleCommandBuilder
Private ds As DataSet
Private Sub mainMenu_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
AddHandler txtProductName.TextChanged, AddressOf ValidateInputs
AddHandler txtQty.TextChanged, AddressOf ValidateInputs
AddHandler txtPrice.TextChanged, AddressOf ValidateInputs
conn.ConnectionString = "User Id=antonina" & _
";Password=antonina" & _
";Data Source=xe"
Try
conn.Open()
Catch ex As Exception
MessageBox.Show(ex.Message.ToString())
Finally
conn.Dispose()
End Try
End Sub
Private Sub ValidateInputs(ByVal Sender As Object, ByVal e As EventArgs)
Button1.Enabled = Not (txtProductName.Text = String.Empty OrElse txtQty.Text = String.Empty OrElse txtPrice.Text = String.Empty)
End Sub
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim productName As String
Dim qty As Integer
Dim price As Integer
Dim cat As String
productName = txtProductName.Text
qty = Integer.Parse(txtQty.Text)
price = Integer.Parse(txtPrice.Text)
cat = cmbCat.Text
conn = New OracleConnection("User Id=antonina;Password=antonina;Data Source=xe")
da = New OracleDataAdapter()
Try
da.InsertCommand = New OracleCommand("INSERT INTO INVENTORY(INVENTORY_PRODUCTNAME,INVENTORY_CAT,INVENTORY_QTY,INVENTORY_PRICE) VALUES ('" & productName & "'," & qty & "," & price & ",'" & cat & "')", conn)
Debug.WriteLine("Success")
Catch ex As Exception
MsgBox(ex.Message)
End Try
End Sub
End Class
When i click my add button it says object reference not set to an instance of an object. I know i might just have a stupid syntax error. Please help me.
I am sure that all my variables have a value. also i am using oracle 10g

Answer 1

I have never seen that somebody uses a OracleDataAdapter in order to insert anything to database, usually OracleDataAdapter is used to retrieve data from database, i.e. a SELECT ... command or a function call which gets a RefCursor.
Perhaps InsertCommand, resp. UpdateCommand and DeleteCommand are used to modify data of OracleDataAdapter after it has been filled from your database - I do not know, I never used it. These command are called with method OracleDataAdapter.Update(da)
Anyway, usually you would do it like this:
Dim cmd As OracleCommand
cmd = New OracleCommand("INSERT INTO INVENTORY (INVENTORY_PRODUCTNAME,INVENTORY_QTY,INVENTORY_PRICE,INVENTORY_CAT) VALUES (:productName, :qty, :price, :cat)", conn)
cmd.CommandType = CommandType.Text
cmd.Parameters.Add("productName", OracleDbType.Varchar2, ParameterDirection.Input).Value = productName
cmd.Parameters.Add("qty", OracleDbType.Int32, ParameterDirection.Input).Value = qty
cmd.Parameters.Add("price", OracleDbType.Int32, ParameterDirection.Input).Value = price
cmd.Parameters.Add("cat", OracleDbType.Varchar2, ParameterDirection.Input).Value = cat
cmd.ExecuteNonQuery()
Note the order of columns, see your code:
INVENTORY_PRODUCTNAME, INVENTORY_CAT, INVENTORY_QTY, INVENTORY_PRICE
('" & productName & "'," & qty & "," & price & ",'" & cat & "')"

Answer 2

I assume da is for DataAdapter. You need first instantiate your dataadapter by call its constructor.
conn = new OracleConnection(yourConnectionString)
da = New OracleDataAdapter()
da.InsertCommand = New OracleCommand("INSERT INTO INVENTORY(INVENTORY_PRODUCTNAME,INVENTORY_CAT,INVENTORY_QTY,INVENTORY_PRICE) VALUES ('" & productName & "'," & qty & "," & price & ",'" & cat & "')", conn)
also, you missing '()' after VALUES statement in sql syntax.
Other things may help is you can move your parsing statement into try-catch element to avoid error.
hope this help.