You declared int ch; but in switch case you are comparing options char. change case as
switch(ch)
{
case 1:
area=l*b;
cout<<"Area= "<<area;
break;
case 2:
peri=2*(l+b);
cout<<"Perimeter= "<<peri;
break;
case 3:
diag=sqrt((l*l)+(b*b));
cout<<"Diagonal= "<<diag;
break;
case 4:
cout<<"Exiting....";
exit(0);
default:
cout<<"Wrong choice!!!!";
cout<<"Enter a valid choice";
break;
}