Question

The following method generates a random string with the given length.

When executed twice in a row, the same string is given. Is anybody able to shed any light on why this might be?

 public static string GenerateRandomString(int size, bool lowerCase)
        {
            StringBuilder builder = new StringBuilder();
            Random random = new Random();
            char ch;
            for (int i = 0; i < size; i++)
            {
                ch = Convert.ToChar(Convert.ToInt32(Math.Floor(26 * random.NextDouble() + 65)));
                builder.Append(ch);
            }
            if (lowerCase)
                return builder.ToString().ToLower();
            return builder.ToString();
        }

Answer 1

You are not using Random correctly.

When calling your GenerateRandomString in a tight loop, it will end up being seeded to the same value many times.

I suggest reading Random numbers from the C# in depth site, by Jon Skeet.

Here is the suggested way to get a Random instance:

using System;
using System.Threading;



public static class RandomProvider
{    
    private static int seed = Environment.TickCount;

    private static ThreadLocal<Random> randomWrapper = new ThreadLocal<Random>(() =>
        new Random(Interlocked.Increment(ref seed))
    );

    public static Random GetThreadRandom()
    {
        return randomWrapper.Value;
    }
}