Random() generating repeated numbers

Question

I am developing a small application to perform XML import, which searches the items present in the note and performs the search for them in DB company. When the item is not found, the link is done manually and this information is saved in a table ItemFornecXItemEmpresa and, next time, the program will find the item.

My problem is in the generation of ObjID, which is generated based on the day and time of insertion and plus 5 random numbers:

for (int i = 0; i < itens.Count; i++)
{
    if (itens[i].Novo == true)
    {
        Random rand = new Random();
        string ano = String.Format("{0:yy}", DateTime.Now);
        string mes = String.Format("{0:MM}", DateTime.Now);
        string dia = String.Format("{0:dd}", DateTime.Now);
        string hora = String.Format("{0:hh}", DateTime.Now);
        string mili = String.Format("{0:ff}", DateTime.Now);
        string random = rand.Next(10000,99999).ToString();

    string id = ano + mes + dia + hora + mili + random;
    [...]
    }
}

And every insertion I make I have that kind of ID generated:

150325094512431
1503250946*44723*
1503250947*44723*
150325094929997
150325095062289
1503250951*94581*
1503250952*94581*
201513031638315
201516031131187
201516031136284
201516031137201

I observe several IDs repeated. There is some logic to this?

Answer 1

Yes, there is. Your code is initiating the random seed every time inside the loop. So it will repeat itself. If you want different values you should start the seed only once, that is, you should remove the initialization of the loop variable. So (I took the opportunity to simplify):

var rand = new Random();
for (int i = 0; i < itens.Count; i++) {
    if (itens[i].Novo) 
        var id = $"{DateTime.Now:yy}{DateTime.Now:MM}{DateTime.Now:dd}{DateTime.Now:hh}{DateTime.Now:ff}{rand.Next(10000, 99999)}";
        [...]
    }
}

I put in the Github for future reference.

There’s an important detail, I think I’d better keep the DateTime.Now in a variable and use it. It may be rare to happen but there will be some case that the mount will be inconsistent. In an extreme case it can give a difference of one year. If the operation starts in the last second of the year, almost to turn, then you can take the current year in the first printing, when it will take the month already turned, then it considers January because the year is already another, only it was printed the year before, look at the shit nonsense that gives. It could happen with month, day, hour, minute or even the error of 1 second. And the fact of using a random makes it highly unlikely, but possible to give a conflict, so every concept seems wrong.

If you want it to be unique and chronological maybe you should use a more precise time going beyond the second, or do sequence taking care to have some form of exclusivity in obtaining the sequential number. If you only want a random name and no matter the chronology then you should use the OS API or a standard library that provides a guaranteed unique name.

To better understand and see examples of use read carefully all documentation from Random().

See an example demonstrating the problem:

using System;
using static System.Console;
                    
public class Program {
    public static void Main() {
        var rand = new Random();
        for (int i = 0; i < 10; i++) {
            WriteLine(rand.Next(10000,99999));
        }
        WriteLine();
        for (int i = 0; i < 10; i++) {
            rand = new Random();
            WriteLine(rand.Next(10000,99999));
        }
    }
}

Behold working in the ideone. And in the .NET Fiddle. Also put on the Github for future reference.