You need to distinguish between already calculated number and not calculated numbers in the dictionary, which you currently don’t: you always recalculate the numbers.
if (n == 0)
{
// special case because fib(0) is 0
return dictionary[0];
}
else
{
int f = dictionary[n];
if (f == 0) {
// number wasn't calculated yet.
f = fibonacci(n-1) + fibonacci(n-2);
dictionary[n] = f;
}
return f;
}