backup restor database sql express

OpenFileDialog1.FileName = database
                        OpenFileDialog1.ShowDialog()
                        Filename = OpenFileDialog1.FileName
                        strQuery = "ALTER DATABASE [c:\Didar\didar.mdf] SET SINGLE_USER WITH ROLLBACK IMMEDIATE"
                        strQuery = "RESTORE DATABASE [c:\Didar\didar.mdf] FROM DISK=N'" & Filename & "' WITH NOFORMAT, NOINIT, NAME = 'x', SKIP, NOREWIND, NOUNLOAD, STATS = 10"

                        strQuery = "ALTER DATABASE [c:\Didar\didar.mdf] SET MULTI_USER"
                        Dim cmd As SqlCommand
                        cmd = New SqlCommand(strQuery, con)
                        cmd.ExecuteNonQuery()