asp.net - 随机字符串生成器在多次调用中创建相同的字符串

Question

我已经构建了一个随机字符串生成器，但我遇到了一个问题，如果我在 Page_Load 方法中多次调用该函数，该函数将返回相同的字符串两次。

这是代码

Public Class CustomStrings
    ''' <summary>'
    ''' Generates a Random String'
    ''' </summary>'
    ''' <param name="n">number of characters the method should generate</param>'
    ''' <param name="UseSpecial">should the method include special characters? IE: # ,$, !, etc.</param>'
    ''' <param name="SpecialOnly">should the method include only the special characters and excludes alpha numeric</param>'
    ''' <returns>a random string n characters long</returns>'
    Public Function GenerateRandom(ByVal n As Integer, Optional ByVal UseSpecial As Boolean = True, Optional ByVal SpecialOnly As Boolean = False) As String

        Dim chars As String() ' a character array to use when generating a random string'
        Dim ichars As Integer = 74 'number of characters to use out of the chars string'
        Dim schars As Integer = 0 ' number of characters to skip out of the characters string'

        chars = { _
         "A", "B", "C", "D", "E", "F", _
         "G", "H", "I", "J", "K", "L", _
         "M", "N", "O", "P", "Q", "R", _
         "S", "T", "U", "V", "W", "X", _
         "Y", "Z", "0", "1", "2", "3", _
         "4", "5", "6", "7", "8", "9", _
         "a", "b", "c", "d", "e", "f", _
         "g", "h", "i", "j", "k", "l", _
         "m", "n", "o", "p", "q", "r", _
         "s", "t", "u", "v", "w", "x", _
         "y", "z", "!", "@", "#", "$", _
         "%", "^", "&", "*", "(", ")", _
         "-", "+"}


        If Not UseSpecial Then ichars = 62 ' only use the alpha numeric characters out of "char"'
        If SpecialOnly Then schars = 62 : ichars = 74 ' skip the alpha numeric characters out of "char"'

        Dim rnd As New Random()
        Dim random As String = String.Empty
        Dim i As Integer = 0
        While i < n
            random += chars(rnd.[Next](schars, ichars))
            System.Math.Max(System.Threading.Interlocked.Increment(i), i - 1)
        End While
        rnd = Nothing
        Return random
    End Function
End Class

但如果我这样称呼

    Dim rnd1 As New CustomStrings
    Dim rnd2 As New CustomStrings

    Dim str1 As String = rnd1.GenerateRandom(5) 
    Dim str2 As String = rnd2.GenerateRandom(5) 

    rnd1 = Nothing
    rnd2 = Nothing

响应将是这样的

g*3Jq
g*3Jq

我第二次调用它时，它将是

3QM0$
3QM0$

我错过了什么？我希望每个随机字符串都生成为唯一的。

Answer 1

这样做的原因是，当您构造 Random 的实例时类，它会从时钟中播种自己，但是如果您快速连续地调用它，则该时钟的准确性不足以在每次调用时产生新的种子。

换句话说，这:

Random r = new Random();
int i = r.Next(1000);
r = new Random();
int j = r.Next(1000);

在 i 中产生相同值的概率非常高和 j .

您需要做的是:

创建并缓存 Random实例，以便它与每次调用使用的实例相同(但不幸的是，该类不是线程安全的，因此至少为每个线程保留一个缓存副本)

用每次调用都会改变的东西来播种(这有点难，因为用顺序值播种会产生可预测的随机数)

这是一个示例程序，它创建了一个单独的 Random每个线程的实例，并从全局随机对象中播种这些实例。同样，这可能会产生可预测的序列。

using System;
using System.Collections.Generic;
using System.Threading.Tasks;

namespace SO2755146
{
    public class Program
    {
        public static void Main()
        {
            List<Task> tasks = new List<Task>();
            for (int index = 0; index < 1000; index++)
                tasks.Add(Task.Factory.StartNew(() => Console.Out.WriteLine(RNG.Instance.Next(1000))));
            Task.WaitAll(tasks.ToArray());
        }
    }

    public static class RNG
    {
        private static Random _GlobalSeed = new Random();
        private static object _GlobalSeedLock = new object();

        [ThreadStatic]
        private static Random _Instance;

        public static Random Instance
        {
            get
            {
                if (_Instance == null)
                {
                    lock (_GlobalSeedLock)
                    {
                        _Instance = new Random(_GlobalSeed.Next());
                    }
                }
                return _Instance;
            }
        }
    }
}

如果您只想从时钟中为每个随机实例播种，但至少为每个线程生成随机序列，则可以将其简化为:

using System;
using System.Collections.Generic;
using System.Threading.Tasks;

namespace SO2755146
{
    public class Program
    {
        public static void Main()
        {
            List<Task> tasks = new List<Task>();
            for (int index = 0; index < 1000; index++)
                tasks.Add(Task.Factory.StartNew(() => Console.Out.WriteLine(RNG.Instance.Next(1000))));
            Task.WaitAll(tasks.ToArray());
        }
    }

    public static class RNG
    {
        [ThreadStatic]
        private static Random _Instance;

        public static Random Instance
        {
            get
            {
                if (_Instance == null)
                    _Instance = new Random();

                return _Instance;
            }
        }
    }
}

这可能会使两个线程开始时彼此非常接近，以使用相同的值进行播种，因此需要权衡。