Question

I want to generate a 9 digit unique random string. Currently I am using

Guid.NewGuid().ToString().Replace("-","").Substring(0,9)

But I am afraid it would have a collision very soon. Is there any better way for this or would this be OK?

Answer 1

If you take a sub string of a GUID you are not guaranteed randomness uniqueness at all.

See my answer to a older SO question to fulfill your randomness requirement. Here is the basic code to do it.

public static string CreateRandomString(int length)
{
    length -= 12; //12 digits are the counter
    if (length <= 0)
        throw new ArgumentOutOfRangeException("length");
    long count = System.Threading.Interlocked.Increment(ref counter);
    Byte[] randomBytes = new Byte[length * 3 / 4];
    RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider();
    rng.GetBytes(randomBytes);

    byte[] buf = new byte[8];
    buf[0] = (byte)count;
    buf[1] = (byte)(count >> 8);
    buf[2] = (byte)(count >> 16);
    buf[3] = (byte)(count >> 24);
    buf[4] = (byte)(count >> 32);
    buf[5] = (byte)(count >> 40);
    buf[6] = (byte)(count >> 48);
    buf[7] = (byte)(count >> 56);
    return Convert.ToBase64String(buf) + Convert.ToBase64String(randomBytes);
}

it gives you 12 digits of counting to prevent collisions and any additional digits you want of randomness. you can modify the code as you want to make shorter than 12 digit strings.