c# - Double not working when more than one decimal? -

hey absolute beginner in c#. tried make simple maths question asks multiplication. used "double" variable , when answer in less than/equals 1 decimal says answer correct when answer more 1 decimal wrong if correct how resolve this? thanks

using system; namespace mathsquiestion {     class mainclass {         public static void main (string[] args) {             double n1 = 1.1;             double n2 = 1.1;             double answer;             console.writeline ("what " + n1 + " times " + n2);              answer = convert.todouble (console.readline ());             if (answer == n1 * n2) {                 console.writeline ("well done!");                 console.readkey ();             }             if (answer != n1 * n2) {                 console.writeline ("you have practice more!");                 console.writeline ("<<press space terminate>>");                 console.readkey ();             }         }     } } 

the problem due rounding errors.

it's not possible accurately represent floating point numbers type double - one's 1.1 or 2.3. means when multiply them 1.2099999 (for example) rather 1.21.

you're doing equality test user's answer (1.21) against calculated value , fail.

if switch using type decimal these problems should solve - @ least small numbers using here.

the other solution test difference between 2 numbers less small amount (0.000001 example):

if (math.abs(answer - n1 * n2) < 0.000001) {     console.writeline ("well done!");     console.readkey (); }