vb.net combobox help

[nirajmchauhan]

Hello everyone,
I am having a vb.net 2008 form, in it I have added a combo-box, with autocomplete property and filled it with sql database,

It works fine,
but a user runs the program and types in combo box for auto-complete the he able type something which is not in database,
I want to avoid that,
he should be only able to write which is inside the database field, if he types something which is not in database and leaves the combo-box then an error message should be displayed,
I tried code in text changed event of combo box, but if I change the normal combo-box field then too it displays error
here is my code:

Private Sub ComboBox1_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles ComboBox1.TextChanged
        If ComboBox1.Text = DataSet1.Tables("bill").Columns("item").ToString Then

        Else
            MsgBox("error")
        End If
    End Sub

Any help will be appreciated