c# - 随机列表<T>(Randomize a List<T>)

Question

What is the best way to randomize the order of a generic list in C#?

(在C＃中随机化泛型列表顺序的最佳方法是什么？)

I've got a finite set of 75 numbers in a list I would like to assign a random order to, in order to draw them for a lottery type application.

(为了给彩票类型的应用程序绘制一个数字，我想分配一个随机顺序的列表中有75个数字的有限集合。)

  ask by mirezus translate from so

Answer 1

Shuffle any (I)List with an extension method based on the Fisher-Yates shuffle :

(使用基于Fisher-Yates shuffle的扩展方法对(I)List进行随机排序 ：)

private static Random rng = new Random();  

public static void Shuffle<T>(this IList<T> list)  
{  
    int n = list.Count;  
    while (n > 1) {  
        n--;  
        int k = rng.Next(n + 1);  
        T value = list[k];  
        list[k] = list[n];  
        list[n] = value;  
    }  
}

Usage:

(用法：)

List<Product> products = GetProducts();
products.Shuffle();

The code above uses the much criticised System.Random method to select swap candidates.

(上面的代码使用备受批评的System.Random方法来选择交换候选。)

It's fast but not as random as it should be.

(它速度很快，但不如应有的随机。)

If you need a better quality of randomness in your shuffles use the random number generator in System.Security.Cryptography like so:

(如果您需要更好的随机性，请使用System.Security.Cryptography中的随机数生成器，如下所示：)

using System.Security.Cryptography;
...
public static void Shuffle<T>(this IList<T> list)
{
    RNGCryptoServiceProvider provider = new RNGCryptoServiceProvider();
    int n = list.Count;
    while (n > 1)
    {
        byte[] box = new byte[1];
        do provider.GetBytes(box);
        while (!(box[0] < n * (Byte.MaxValue / n)));
        int k = (box[0] % n);
        n--;
        T value = list[k];
        list[k] = list[n];
        list[n] = value;
    }
}

A simple comparison is available at this blog (WayBack Machine).

(在此博客 （WayBack Machine）上可以进行简单的比较。)

Edit: Since writing this answer a couple years back, many people have commented or written to me, to point out the big silly flaw in my comparison.

(编辑：自从几年前写下这个答案以来，许多人向我发表评论或写信给我，指出我的比较中的一个愚蠢的缺陷。)

They are of course right.

(他们当然是对的。)

There's nothing wrong with System.Random if it's used in the way it was intended.

(如果System.Random按预期方式使用，则没有任何问题。)

In my first example above, I instantiate the rng variable inside of the Shuffle method, which is asking for trouble if the method is going to be called repeatedly.

(在上面的第一个示例中，我在Shuffle方法内部实例化了rng变量，该变量询问是否要重复调用该方法。)

Below is a fixed, full example based on a really useful comment received today from @weston here on SO.

(以下是一个固定的完整示例，该示例基于今天从@weston收到的关于SO的非常有用的评论。)

Program.cs:

(Program.cs：)

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

namespace SimpleLottery
{
  class Program
  {
    private static void Main(string[] args)
    {
      var numbers = new List<int>(Enumerable.Range(1, 75));
      numbers.Shuffle();
      Console.WriteLine("The winning numbers are: {0}", string.Join(",  ", numbers.GetRange(0, 5)));
    }
  }

  public static class ThreadSafeRandom
  {
      [ThreadStatic] private static Random Local;

      public static Random ThisThreadsRandom
      {
          get { return Local ?? (Local = new Random(unchecked(Environment.TickCount * 31 + Thread.CurrentThread.ManagedThreadId))); }
      }
  }

  static class MyExtensions
  {
    public static void Shuffle<T>(this IList<T> list)
    {
      int n = list.Count;
      while (n > 1)
      {
        n--;
        int k = ThreadSafeRandom.ThisThreadsRandom.Next(n + 1);
        T value = list[k];
        list[k] = list[n];
        list[n] = value;
      }
    }
  }
}