Error while doing restore Database Application Windows Forms

I'm creating the database backup, but I can not restore it. It shows the following error:

Thisismycodetoperformtherestoreofthebanktomyapplication.

try{var_with1=openFileDialog1;_with1.Filter=("DB Backup File|*.bak;");
            _with1.FilterIndex = 4;
            //Clear the file name
            openFileDialog1.FileName = "";

            if (openFileDialog1.ShowDialog() == DialogResult.OK)
            {
                Cursor = Cursors.WaitCursor;
                timer2.Enabled = true;
                SqlConnection.ClearAllPools();
                cg.con = new SqlConnection(cn.DBconn);
                cg.con.Open();
                string cb = "USE Master ALTER DATABASE [" + Application.StartupPath + "\SISTEMA_CCA.bak] SET Single_User WITH Rollback Immediate Restore database [" + Application.StartupPath + "\SISTEMA_CCA.bak] FROM disk='" + openFileDialog1.FileName + "' WITH REPLACE ALTER DATABASE [" + Application.StartupPath + "\SISTEMA_CCA.bak] SET Multi_User ";
                cg.cmd = new SqlCommand(cb);
                cg.cmd.Connection = cg.con;
                cg.cmd.ExecuteReader();
                cg.con.Close();
                st1 = LB_Usuario.Text;
                st2 = "Restauração realizada com sucesso";
                cf.LogFunc(st1, System.DateTime.Now, st2);
                MessageBox.Show("Operação concluída com sucesso", "Restuaração do banco de dados", MessageBoxButtons.OK, MessageBoxIcon.Information);
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
        }

Thank you so much!