simple if statement

simple_if_ statement.htm

Short-circuit operator, The short-circuit AND operator is && and the short-circuit OR operator is | |, and short-circuit versions evaluates the second operand only when necessary.

warning CS0219 for unused variables

    // csc  simple_f.cs
using System;
public class simple_if
{
public simple_if(){ Console.WriteLine("Constructor is evoked");}
       ~simple_if(){ Console.WriteLine("Destructor is evoked");}
  public int size(int n)
  {
  
  string sz = "something" ; 
  if(n == 0) 
  { 
  Console.WriteLine("You entered : " + n + " you are out ");
  } // terminate loop when i is positive 
  ;  
  if (n >= 1 && n <= 9)
   { Console.WriteLine(" You Entered " + n + " that is below 9"); }
  if (n >= 10 && n <= 20 ) 
  { Console.WriteLine(" You Entered " + + n + " that is below 20");}   
 	  return n;
  }

}

class Test
{
    static void Main()
    {
        int n = 0;
        simple_if ts = new simple_if();
        Console.WriteLine("enter a number from ");
        Console.Write("Please enter your selection: ");
        string s = Console.ReadLine();
        n= int.Parse(s);
	ts.size(n);
    }
}

It is just right, but not error proof. You should also note that I had a variable never used and compiler sends an warning CS0219

Console.ReadLine has a problem, if you don't enter and hit the carriage return it will throw an error.
How to create your own custom error with try and catch, click here

with if condition if you don't provide specific, compiler will bypass or will not enter if statement when statement is false.