Class For generating Random Code

Question

I wrote this class to generate a random code, it shouldn't create any two repetitive number. I want to know, in this code, how much it possible that we have collision?

 public string MyRandom()
 {
    Random r = new Random();
    int x = r.Next(1000);//Max range
    PersianCalendar pc = new PersianCalendar();
     string _myrandom = pc.GetMonth(DateTime.Now).ToString() + pc.GetDayOfMonth(DateTime.Now).ToString() + pc.GetHour(DateTime.Now).ToString() + pc.GetMinute(DateTime.Now).ToString() + pc.GetSecond(DateTime.Now).ToString() + pc.GetMilliseconds(DateTime.Now).ToString() + x.ToString();

    return _myrandom;
 }

Answer 1

If we do

List<string> s = new List<string>();

for(int i = 0; i < 100; i++)
{
    s.Add(MyRandom());
}

Why? When random class is created in a loop, you can get duplicate random numbers. More Info

Solution: Create Random instance outside loop.

Answer 2

It is safer to use more stronger methods such as RNGCryptoServiceProvider which you can find several samples of its usage. Generally, pure random generation is not possible is very high concurrency loads, but reduce its probability to a reasonable amount.

Answer 3

Well first, because you are using the date in the way that you are, then there is only one possible millisecond per year where the _myrandom string could possibly be repeated.

Within that millisecond each year, the odds of hitting a random string you have already generated are down to the variable x, and related directly to how many other random strings you have made in that millisecond (and the previous milliseconds of each previous year). If, for example, you were to have made 10 random strings last year in that millisecond and 4 the year before, the chances of duplication this year are 14:999, which is a chance of about 1.4%.

Of course, if you make 1000 strings per millisecond, the chance of duplication is 100%.

In fact, the chances of duplication are slightly higher, due to the fact that the C# Random number generator doesn't produce a true random number. It's a pseudo-random number, so the chances of a collision are higher.

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I see that you've written: "this generator is using for naming photos which uploaded in my application" in the comments above, which makes it unlikely that you'll be making 1000+ names per millisecond. Generally, whenever you are using a random number generator in this way, it's a good idea to just put in some error checking in 1 of two ways: 1. offensive or 2. defensive.

Offensive would be waiting for the error (two identical file names) to happen and then dealing with it (maybe running your random generator again until the error isn't thrown). Defensive would be to check all of the filenames in the file, or keep a list of them all, to check that your new filename isn't the same before you even try to save it. This is more watertight (because you aren't relying on an error to be thrown) but far more computationally expensive.