.net - How to stop adding numbers that occur twice in C# -

so trying find answer question:

if list natural numbers below 10 multiples of 3 or 5, 3, 5, 6 , 9. sum of these multiples 23. find sum of multiples of 3 or 5 below 1000.

i using c# , have pretty idea of do, code keeps counting numbers occur twice (e.g. 15, 30) , know quickest/easiest way counteract that. have found far has been in different language sorry if seems relatively easy you. have far:

static void main(string[] args)     {         var result1 = 0;         var result2 = 0;         var result3 = 0;         var uniqueints3 = new list<int>();         (var = 0; < 1000; += 3)         {             uniqueints3.add(i);             result1 += i;         }         var uniqueints5 = new list<int>();         (var o = 0; o < 1000; o += 5)         {             uniqueints5.add(o);             result2 += o;         }         result3 += result1 + result2;         console.writeline(result3);         console.readline();     } 

i love if explain me not sure @ point.

not efficient way, should work

var sum = 0;  for(int i=0;i<1000;i++) {    if(i%3==0||i%5==0) //checks if multiple of 3 or 5       sum+=i; // sums when it's multiple of 3 or 5 } 

it ommits situations multiple of 3 , 5. takes each number once.

one line linq way:

var sum = enumerable.range(3, 1000).sum(x => (x % 3 == 0 || x % 5 == 0) ? x : 0); 

fastest mathematical approach version:

var result = sumdivisbleby(3,999)+sumdivisbleby(5,999)-sumdivisbleby(15,999);  private int sumdivisbleby(int n, int p) {     return n*(p/n)*((p/n)+1)/2; } 

it calculates sum of numbers divisible 3 , 5 substracts sum of numbers divisible 15. explanation: http://www.wikihow.com/sum-the-integers-from-1-to-n