function to generate random numbers, without repeating digits

The code for this article applies to C#

Reading blogs, I found this post (in italian), where Marco wants to write a function to generate random numbers with the following rules:

• 5 digits long
• result should be returned in a string
• all the digits should be different (on each result you can't have two of the same digit)

The function he came up with was the following:

private string GetRandom()
{
     Random rn = new Random();
     string resultnum=string.Empty;
     do
         {
              string a = rn.Next(0, 9).ToString();
              if (resultnum.Contains(a)!=true)
              resultnum = resultnum + a;
         }
      while (resultnum.Length<5);
      return resultnum;
}

I felt curious to see what areas I could improve, and I started writing a function to get the same result, but in a more optimized way

A couple things that jump out to my mind are:

1. string concatenation
2. the loop and comparing each time to see if the digit is in the result already

So I wanted to write a function that: would avoid concatenation and would execute the loop exactly 5 times, this is the result:

static char[] allNumbers = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9' };
Random r = new Random();
public string GetRandom2() {
  
   char[] result = new char[5];
   int max = 9, pos=0;
   for (int i=1; i<6; i++) {
       pos = r.Next(0, max);
       result[i - 1] = allNumbers[pos];
   //*** swap positions
       allNumbers[pos] ^= allNumbers[max];
       allNumbers[max] ^= allNumbers[pos];
       allNumbers[pos] ^= allNumbers[max--];
   }
   return new string(result);
}

The technique I used was:

• Have a predefined array with all possible characters
• I have a variable max that I use to call the method Random.Next(0, max)
• the variable is decremented on each iteration
• I swap the digit from the result to the last position, with this I leave this digit out of the posibilities for the next call

The performance gain from these changes was minimal (3 ms per 1000 calls), then I thought about moving the Random variable declaration outside the function, so it could be reused between calls, this gave me the performance gain I was looking for, calling the function 10,000 times takes ~114ms using the old method and only ~6ms using the new one, that's where the problem was, the rest was pretty much insignificant =o(

I'm sure someone else can write a faster version, but I accomplished my goal =o)