c# - Entered wrong value. goto Start; -

edit. based on answers received realized asking more know @ stage in learning , such, have decided forget question until have learnt , understand more. not want overstep , confuse myself.

just newbie looking guidance.

from previous question asked here, introduced goto start; in switch statement.

i started using , had me thinking wanted program go start in event user entered invalid input. example, in calculator below, if user enters other number, how program disregard input , prompt user again enter number? maybe display "invalid input. please try again" user.

right now, if number entered, console crashes , throws me error.

thanks in advance assistance

      int num1, num2, output;         string  op;          console.write("\n\n");         console.writeline("calculator\n");         console.writeline("=============");         console.write("\n\n");          start:          console.write("please enter first number:");         num1 = convert.toint32(console.readline());           console.write("please enter second number: ");         num2 = convert.toint32(console.readline());          operator:          console.writeline("please select operator: ");         console.writeline("\naddition : +");         console.writeline("multiplication: *");         console.writeline("division: /");         console.writeline("subtraction: -");         console.write("enter operator: ");         op = console.readline();             switch (op)         {             case "+":                 output = num1 + num2;                 console.writeline("{0} added {1} = {2}", num1, num2, output);                 break;              case "*":                 output = num1 * num2;                 console.writeline("{0} multiplied {1} = {2}", num1, num2, output);                 break;              case "/":                 if (num2 == 0)                 {                     console.writeline("cannot divide zero. please try again");                     goto start;                  }                 else                 {                     output = num1 / num2;                     console.writeline("{0} divided {1} = {2}", num1, num2, output);                     break;                 }              case "-":                 output = num1 - num2;                 console.writeline("{0} minus{1} = {2}", num1, num2, output);                 break;              default:                 console.writeline("you entered invalid operator. please try again\n");                 goto operator;                                }         console.writeline("\npress enter continue....");        console.readline(); 

please, whatever do, not use goto. ever. maybe except in switch fall down other case, should rare. makes code unreadable, confusing , hard maintain. please see spaghetti code.

that said, should int32.tryparse instead of convert if not sure if input valid:

int num1; {     console.write("please enter first number:"); } while (!int32.tryparse(console.readline(), out num1)); 

and change switch (to avoid goto):

bool inputok = false; while (!inputok) {     console.writeline("please select operator: ");     //...     op = console.readline();      inputok = true;     switch (op)     {         //...         default:             console.writeline("you entered invalid operator. please try again\n");             inputok = false;             break;      } }