tell user to press only +

i am trying to ask user to press only '+' key and if user gave a wrong input tell him that is not an operation; for example if user inputs numbers or any other keys, tell user "you have to input an operation" i have tried codes below but does not get fixed!

Console.WriteLine("press '+' to add\npress '-' to delete");

//these are the ways i have tried:
//1:

char add = '+';
if (Console.ReadKey() == add) ;

//2:
string input = Console.ReadLine();
char inputChar = Convert.ToChar(input); 

//if user give number the program shows error to me
if (inputChar == add) ;

//3:
if (Console.ReadKey().Key == ConsoleKey.+) ;

Solution 1:

You can try implementing do .. while loop:

  char op;

  do {
    Console.WriteLine("press '+' to add\npress '-' to delete");

    op = Console.ReadKey().KeyChar;
  }
  while (op != '+' && op != '-');

we keep asking user until valid operation (op) is provided (either '+' or '-')