Question

I came across a strange problem today that i could understand the reason. Take the following console program.

internal class Program
    {
        private static void Main(string[] args)
        {
            string s1 = GenerateRandomCode(8);
            string s2 = GenerateRandomCode(8);
            string s3 = GenerateRandomCode(8);
        }

        public static string GenerateRandomCode(int length)
        {
            string charPool = "ABCDEFGOPQRSTUVWXY1234567890ZabcdefghijklmHIJKLMNnopqrstuvwxyz";
            StringBuilder rs = new StringBuilder();
            Random random = new Random();

            for (int i = 0; i < length; i++)
            {
                rs.Append(charPool[(int)(random.NextDouble() * charPool.Length)]);
            }

            return rs.ToString();
        }
    }

If i put a breakpoint at the of the program and run the program the values of s1, s2, s3 are all the equal. Now if i put a breakpoint at s2 for example the value returned will be different.

Seems like some kind of concurrency issue? What’s going on?

Thanks

Answer 1

Random number generators are in fact not fully random: given the same seed value, multiple instances will generate the same random sequence.

Couldn’t say it better than the MSDN documentation for the Random constructor

“The default seed value is derived from the system clock and has
finite resolution. As a result, different Random objects that are
created in close succession by a call to the default constructor will
have identical default seed values and, therefore, will produce
identical sets of random numbers. This problem can be avoided by using
a single Random object to generate all random numbers. You can also
work around it by modifying the seed value returned by the system
clock and then explicitly providing this new seed value to the
Random(Int32) constructor. For more information, see the Random(Int32)
constructor.”

So, in your case you need to keep the Random instance as a class-level field or a function parameter, and instantiate it only once.