Random String Generator Returning Same String

user_136271 0
Viewed 309369

I've developed a random string generator but it's not behaving quite as I'm hoping. My goal is to be able to run this twice and generate two distinct four character random strings. However, it just generates one four character random string twice.

Here's the code and an example of its output:

private string RandomString(int size)
{
    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);
    }

    return builder.ToString();
}

// get 1st random string 
string Rand1 = RandomString(4);

// get 2nd random string 
string Rand2 = RandomString(4);

// create full rand string
string docNum = Rand1 + "-" + Rand2;

...and the output looks like this: UNTE-UNTE ...but it should look something like this UNTE-FWNU

How can I ensure two distinctly random strings?

random c#
30 Answers

You're making the Random instance in the method, which causes it to return the same values when called in quick succession. I would do something like this:

private static Random random = new Random((int)DateTime.Now.Ticks);//thanks to McAden
private string RandomString(int size)
    {
        StringBuilder builder = new StringBuilder();
        char ch;
        for (int i = 0; i < size; i++)
        {
            ch = Convert.ToChar(Convert.ToInt32(Math.Floor(26 * random.NextDouble() + 65)));                 
            builder.Append(ch);
        }

        return builder.ToString();
    }

// get 1st random string 
string Rand1 = RandomString(4);

// get 2nd random string 
string Rand2 = RandomString(4);

// creat full rand string
string docNum = Rand1 + "-" + Rand2;

(modified version of your code)

user_1170690

You're instantiating the Random object inside your method.

The Random object is seeded from the system clock, which means that if you call your method several times in quick succession it'll use the same seed each time, which means that it'll generate the same sequence of random numbers, which means that you'll get the same string.

To solve the problem, move your Random instance outside of the method itself (and while you're at it you could get rid of that crazy sequence of calls to Convert and Floor and NextDouble):

private readonly Random _rng = new Random();
private const string _chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

private string RandomString(int size)
{
    char[] buffer = new char[size];

    for (int i = 0; i < size; i++)
    {
        buffer[i] = _chars[_rng.Next(_chars.Length)];
    }
    return new string(buffer);
}
Luke_558470

This is because each new instance of Random is generating the same numbers from being called so fast. Do not keep creating a new instance, just call next() and declare your random class outside of your method.

Mike_Leffard_364570

You should have one class-level Random object initiated once in the constructor and reused on each call (this continues the same sequence of pseudo-random numbers). The parameterless constructor already seeds the generator with Environment.TickCount internally.

user_1331430

I added the option to choose the length using the Ranvir solution

public static string GenerateRandomString(int length)
    {
        {
            string randomString= string.Empty;

            while (randomString.Length <= length)
            {
                randomString+= Path.GetRandomFileName();
                randomString= randomString.Replace(".", string.Empty);
            }

            return randomString.Substring(0, length);
        }
    }
user_12960870
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