我已经构建了一个随机字符串生成器，但是我遇到了一个问题，即如果我在Page_Load方法中多次调用该函数，则该函数会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
但如果我打电话给这样的话
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
我第二次打电话给它，它将是
3QM0 $
3QM0 $
我错过了什么？我希望每个随机字符串都是唯一的。
答案 0 :(得分：7)
这样做的原因是当你构造一个Random类的实例时，它会从时钟中播种出来，但是这个时钟的准确性不足以在每次调用时生成一个新的种子，如果你快速连续称呼它。
换句话说，这个：
Random r = new Random();
int i = r.Next(1000);
r = new Random();
int j = r.Next(1000);
很有可能在i和j中生成相同的值。
您需要做的是：
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，所以至少为每个线程保留一个缓存副本）这是一个示例程序，它为每个线程创建一个单独的Random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播种每个随机实例，但至少每个线程生成随机序列，您可以像这样简化它：
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彼此非常接近，以相同的值播种，因此需要权衡。
答案 1 :(得分：1)
为了防止使用相同的种子值，为了停止生成相同的随机序列，您可以通过使用如下函数将GUID（隐式随机化）煮沸为int值来创建随机种子：
Private Function GetNewSeed() As Integer
Dim arrBytes As Byte() = Guid.NewGuid().ToByteArray() '16 bytes
Dim seedNum As Integer = 0
' Boil GUID down 4 bytes at a time (size of int) and merge into Integer value
For i As Integer = 0 To arrBytes.Length - 1 Step 4
seedNum = seedNum Xor BitConverter.ToInt32(arrBytes, i)
Next
Return seedNum
End Function
使用返回的int为随机数生成器设定种子。
现在使用自定义函数GetNewSeed来解决问题。
Dim rnd1 As New Random( GetNewSeed )
这会处理问题的根源，即种子值。
答案 2 :(得分：1)
如果你想要没有序列的真正随机性，那么绝对不要使用System.Random函数。最好使用System.Security.Cryptography函数，理想情况下检查您的硬件是否支持.NET将自动使用的RNG（随机数生成）。
这是一个很好的例子： http://www.obviex.com/Samples/Password.aspx
答案 3 :(得分：1)
我使用以下方法创建一个独特的种子。
Session["seedRandom"] = 1;
在page_load创建了一个会话变量。 此会话变量将递增并添加到DateTime.Now.Ticks
private string getRandAlphaNum() { int seed_value = (int)DateTime.Now.Ticks; seed_value = seed_value + Int32.Parse(Session["seedRandom"].ToString()); //change the Session variable by incrementing its value to 1 after creating seed value. Session["seedRandom"] = Int32.Parse(Session["seedRandom"].ToString()) + 1; Random rand = new Random(seed_value); ..... ..... }
因此每次我都会获得种子 DateTimeNow.Ticks +更新的会话变量
答案 4 :(得分：0)
我做了一些改动，对我来说似乎很好。我不喜欢使用关键字作为变量名。请注意，我移动了随机语句：
Public Class Form1
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim myCS As New CustomStrings
Dim l As New List(Of String)
For x As Integer = 1 To 10
Dim s As String = myCS.GenerateRandom(5)
l.Add(s)
Next
For x As Integer = 0 To l.Count - 1
Debug.WriteLine(l(x))
Next
'Debug output
'YGXiV
'rfLmP
'OVUW9
'$uaMt
'^RsPz
'k&91k
'(n2uN
'ldbQQ
'zYlP!
'30kNt
End Sub
Public Class CustomStrings
Private myRnd As New Random()
Public Function GenerateRandom(ByVal n As Integer, _
Optional ByVal UseSpecial As Boolean = True, _
Optional ByVal SpecialOnly As Boolean = False) As String
Dim ichars As Integer = 74 'number of characters to use out of the chars string'
Dim schars As Integer = 0 ' number of characters to skip out of the characters string'
Dim chars() As Char = New Char() {"A"c, "B"c, "C"c, "D"c, "E"c, "F"c, _
"G"c, "H"c, "I"c, "J"c, "K"c, "L"c, _
"M"c, "N"c, "O"c, "P"c, "Q"c, "R"c, _
"S"c, "T"c, "U"c, "V"c, "W"c, "X"c, _
"Y"c, "Z"c, "0"c, "1"c, "2"c, "3"c, _
"4"c, "5"c, "6"c, "7"c, "8"c, "9"c, _
"a"c, "b"c, "c"c, "d"c, "e"c, "f"c, _
"g"c, "h"c, "i"c, "j"c, "k"c, "l"c, _
"m"c, "n"c, "o"c, "p"c, "q"c, "r"c, _
"s"c, "t"c, "u"c, "v"c, "w"c, "x"c, _
"y"c, "z"c, "!"c, "@"c, "#"c, "$"c, _
"%"c, "^"c, "&"c, "*"c, "("c, ")"c, _
"-"c, "+"c}
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 rndStr As String = String.Empty
Dim i As Integer = 0
While i < n
rndStr += chars(Me.myRnd.Next(schars, ichars))
System.Math.Max(System.Threading.Interlocked.Increment(i), i - 1)
End While
Return rndStr
End Function
End Class
End Class