com.CommandText = "ALTER DATABASE [" + Application.StartupPath + "\DB\mydb.mdf" + "] SET Single_User WITH Rollback Immediate;"
              + "ALTER DATABASE [" + Application.StartupPath + "\DB\mydb.mdf" + "] SET Multi_User;" + "USE [master]; RESTORE DATABASE [mydb] FROM DISK = N'" + filename + "' WITH FILE = 1, NOUNLOAD, REPLACE, STATS = 10";

con2.ConnectionString = @"Data Source=.SQLEXPRESS;AttachDbFilename=" + Application.StartupPath + @"DBmydb.mdf;Integrated Security=True;MultipleActiveResultSets=True;Connect Timeout=30;User Instance=true;";