c# - Using Cache in Collatz Sequence implementing issues -

i have created 2 lists, 1 number , 1 chain count. if number in number list, take it's chain count , add current chain count. issue right here program stucks , it's not returning me anything. problem it's solved, takes me 4.7 seconds find number under 1 million has longest sequence , chain count want optimize using cache memory.

static list<long> cache_number = new list<long>();     static list<long> cache_chains = new list<long>();      static long collatzchain(long num) {         int chain = 1;         while (num != 1) {             if(cache_number.contains(num)){                 long x = chain + cache_chains[cache_number.indexof(num)];                 return x;             }             if (num % 2 == 0) {                 num = num / 2;             }             else {                 num = 3 * num + 1;             }             chain++;              }         cache_number.add(num);         cache_chains.add(chain);         return chain;     }      static void main(string[] args){          stopwatch time = new stopwatch();         time.start();          list<long> chain = new list<long>();         list<long> numbers = new list<long>();           (int = 13; < 1000000; i++) {             numbers.add(i);             chain.add(collatzchain(i));         }          time.stop();         long elapsed = time.elapsedmilliseconds;         console.writeline("the longest chain made number {0} chain of {1} elements", numbers[chain.indexof(chain.max())], chain.max());         console.write("time elapsed: {0} ms", elapsed);         console.readkey();     }