Performance issue with generation of random unique numbers

Question

I have a situation where by I need to create tens of thousands of unique numbers. However these numbers must be 9 digits and cannot contain any 0\'s. My current approach is

Answer 1

Looking at the solutions already posted, mine seems fairly basic. But, it works, and generates 1million values in approximate 1s (10 million in 11s).

public static void generateIdentifiers(int quantity)
{
    HashSet<int> uniqueIdentifiers = new HashSet<int>();

    while (uniqueIdentifiers.Count < quantity)
    {
        int value = random.Next(111111111, 999999999);
        if (!value.ToString().Contains('0') && !uniqueIdentifiers.Contains(value))
            uniqueIdentifiers.Add(value);
    }
}

Answer 2

As others have mentioned, use a HashSet<T> instead of a List<T>.
Furthermore, using StringBuilder instead of simple string operations will gain you another 25%. If you can use numbers instead of strings, you win, because it only takes a third or fourth of the time.

var quantity = 400000;
var uniqueIdentifiers = new HashSet<int>();
while (uniqueIdentifiers.Count < quantity)
{
    int i=0;
    i = i*10 + random.Next(1,10);
    i = i*10 + random.Next(1,10);
    i = i*10 + random.Next(1,10);
    i = i*10 + random.Next(1,10);
    i = i*10 + random.Next(1,10);
    i = i*10 + random.Next(1,10);
    i = i*10 + random.Next(1,10);
    i = i*10 + random.Next(1,10);
    i = i*10 + random.Next(1,10);
    uniqueIdentifiers.Add(i);
}

It takes about 270 ms on my machine for 400,000 numbers and about 700 for 1,000,000. And this even without any parallelism. Because of the use of a HashSet<T> instead of a List<T>, this algorithm runs in O(n), i.e. the duration will grow linear. 10,000,000 values therefore take about 7 seconds.

Answer 3

Meybe this will bee faster:

        //we can generate first number wich in 9 base system will be between 88888888 - 888888888 
        //we can't start from zero becouse it will couse the great amount of 1 digit at begining

        int randNumber = random.Next((int)Math.Pow(9, 8) - 1, (int)Math.Pow(9, 9));


        //no we change our number to 9 base, but we add 1 to each digit in our number
        StringBuilder builder = new StringBuilder();

        for (int i=(int)Math.Pow(9,8); i>0;i= i/9)
        {
            builder.Append(randNumber / i +1);
            randNumber = randNumber % i;
        }

        id = builder.ToString();

Answer 4

I think first thing would be to use StringBuilder, instead of concatenation - you'll be pleasantly surprised. Antoher thing - use a more efficient data structure, for example HashSet<> or HashTable.

If you could drop the quite odd requirement not to have zero's - then you could of course use just one random operation, and then format your resulting number the way you want.