cs = string.Format("Data Source={0};User" & _
  "Id={1};Password={2}",DbDataSource,DbUserId,DbPassword)
Using conn As New OracleConnection(cs)
  conn.Open()
 
  Dim cmd As New OracleCommand("select * from tbl_login where tbl_login.mkz" & _
    "=:MKZ", conn)
  cmd.Parameters.AddWithValue(":MKZ", txt_user.Text)
  Dim dr As OracleDataReader = cmd.ExecuteReader()
  ....
 
End Using