Der Vorgang ist aufgrund des aktuellen Zustands des Objekts ungültig.

cs = "Data Source=" & DbDataSource & ";" & "User Id=" & DbUserId & ";" & _
  "Password=" & DbPassword & ";"
            Dim oradb As String = cs
 
            Dim conn As New OracleConnection(oradb)
            conn.Open()
 
            Dim cmd As New OracleCommand
            cmd.Connection = conn
            cmd.CommandText = "select * from tbl_login where tbl_login.mkz =" & _
              "'" & txt_user.Text & "'"
            cmd.CommandType = CommandType.Text
            Dim dr As OracleDataReader = cmd.ExecuteReader()
            dr.Read()