英語で読む 編集

次の方法で共有


Random Constructors

Definition

Initializes a new instance of the Random class.

Overloads

Random()

Initializes a new instance of the Random class using a default seed value.

Random(Int32)

Initializes a new instance of the Random class, using the specified seed value.

Random()

Source:
Random.cs
Source:
Random.cs
Source:
Random.cs

Initializes a new instance of the Random class using a default seed value.

public Random ();

Examples

The following example uses the parameterless constructor to instantiate three Random objects and displays a sequence of five random integers for each. If it is run on .NET Framework, because the first two Random objects are created in close succession, they are instantiated using identical seed values based on the system clock and, therefore, they produce an identical sequence of random numbers. On the other hand, the parameterless constructor of the third Random object is called after a two-second delay caused by calling the Thread.Sleep method. Because this produces a different seed value for the third Random object, it produces a different sequence of random numbers.

using System;
using System.Threading;

public class RandomNumbers
{
   public static void Main()
   {
      Random rand1 = new Random();
      Random rand2 = new Random();
      Thread.Sleep(2000);
      Random rand3 = new Random();
      ShowRandomNumbers(rand1);
      ShowRandomNumbers(rand2);
      ShowRandomNumbers(rand3);
   }

   private static void ShowRandomNumbers(Random rand)
   {
      Console.WriteLine();
      byte[] values = new byte[5];
      rand.NextBytes(values);
      foreach (byte value in values)
         Console.Write("{0, 5}", value);
      Console.WriteLine();
   }
}
// The example displays an output similar to the following:
//       28   35  133  224   58
//
//       28   35  133  224   58
//
//       32  222   43  251   49

Remarks

In .NET Framework, the default seed value is derived from the system clock, which has finite resolution. As a result, different Random objects that are created in close succession by a call to the parameterless constructor have identical default seed values and, therefore, produce identical sets of random numbers. You can avoid this problem by using a single Random object to generate all random numbers. You can also work around it by generating your own random seed value and passing it to the Random(Int32) constructor. For more information, see the Random(Int32) constructor.

In .NET Core, the default seed value is produced by the thread-static, pseudo-random number generator, so the previously described limitation does not apply. Different Random objects created in close succession produce different sets of random numbers in .NET Core.

Call this constructor if you want your random number generator to generate a random sequence of numbers. To generate a fixed sequence of random numbers that will be the same for different random number generators, call the Random(Int32) constructor with a fixed seed value. This Random constructor overload is frequently used when testing apps that use random numbers.

Once you've instantiated the random number generator, you call individual Random methods, such as Next() or NextDouble(), to generate random numbers.

Applies to

.NET 9 およびその他のバージョン
製品 バージョン
.NET Core 1.0, Core 1.1, Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7, 8, 9
.NET Framework 1.1, 2.0, 3.0, 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8, 4.8.1
.NET Standard 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 2.0, 2.1
UWP 10.0

Random(Int32)

Source:
Random.cs
Source:
Random.cs
Source:
Random.cs

Initializes a new instance of the Random class, using the specified seed value.

public Random (int Seed);

Parameters

Seed
Int32

A number used to calculate a starting value for the pseudo-random number sequence. If a negative number is specified, the absolute value of the number is used.

Examples

The following example creates Random objects with the class constructor that takes a seed parameter and generates a sequence of random integers and doubles. The example illustrates that the same sequence is generated when the Random object is created again with the constructor and seed parameter.

// Example of the Random class constructors and Random.NextDouble()
// method.
using System;
using System.Threading;

public class RandomObjectDemo
{
    // Generate random numbers from the specified Random object.
    static void RunIntNDoubleRandoms(Random randObj)
    {
        // Generate the first six random integers.
        for(int j = 0; j < 6; j++)
            Console.Write(" {0,10} ", randObj.Next());
        Console.WriteLine();

        // Generate the first six random doubles.
        for(int j = 0; j < 6; j++)
            Console.Write(" {0:F8} ", randObj.NextDouble());
        Console.WriteLine();
    }

    // Create a Random object with the specified seed.
    static void FixedSeedRandoms(int seed)
    {
        Console.WriteLine(
            "\nRandom numbers from a Random object with " +
            "seed = {0}:", seed);
        Random fixRand = new Random(seed);

        RunIntNDoubleRandoms(fixRand);
    }

    // Create a random object with a timer-generated seed.
    static void AutoSeedRandoms()
    {
        // Wait to allow the timer to advance.
        Thread.Sleep(1);

        Console.WriteLine(
            "\nRandom numbers from a Random object " +
            "with an auto-generated seed:");
        Random autoRand = new Random();

        RunIntNDoubleRandoms(autoRand);
    }

    static void Main()
    {	
        Console.WriteLine(
            "This example of the Random class constructors and " +
            "Random.NextDouble() \n" +
            "generates the following output.\n");
        Console.WriteLine(
            "Create Random objects, and then generate and " +
            "display six integers and \nsix doubles from each.");

        FixedSeedRandoms(123);
        FixedSeedRandoms(123);

        FixedSeedRandoms(456);
        FixedSeedRandoms(456);

        AutoSeedRandoms();
        AutoSeedRandoms();
        AutoSeedRandoms();
    }
}

/*
This example of the Random class constructors and Random.NextDouble()
generates an output similar to the following:

Create Random objects, and then generate and display six integers and
six doubles from each.

Random numbers from a Random object with seed = 123:
 2114319875  1949518561  1596751841  1742987178  1586516133   103755708
 0.01700087  0.14935942  0.19470390  0.63008947  0.90976122  0.49519146

Random numbers from a Random object with seed = 123:
 2114319875  1949518561  1596751841  1742987178  1586516133   103755708
 0.01700087  0.14935942  0.19470390  0.63008947  0.90976122  0.49519146

Random numbers from a Random object with seed = 456:
 2044805024  1323311594  1087799997  1907260840   179380355   120870348
 0.21988117  0.21026556  0.39236514  0.42420498  0.24102703  0.47310170

Random numbers from a Random object with seed = 456:
 2044805024  1323311594  1087799997  1907260840   179380355   120870348
 0.21988117  0.21026556  0.39236514  0.42420498  0.24102703  0.47310170

Random numbers from a Random object with an auto-generated seed:
  380213349   127379247  1969091178  1983029819  1963098450  1648433124
 0.08824121  0.41249688  0.36445811  0.05637512  0.62702451  0.49595560

Random numbers from a Random object with an auto-generated seed:
  861793304  2133528783  1947358439   124230908   921262645  1087892791
 0.56880819  0.42934091  0.60162512  0.74388610  0.99432979  0.30310005

Random numbers from a Random object with an auto-generated seed:
 1343373259  1992194672  1925625700   412915644  2026910487   527352458
 0.04937517  0.44618494  0.83879212  0.43139707  0.36163507  0.11024451
*/

Remarks

Providing an identical seed value to different Random objects causes each instance to produce identical sequences of random numbers. This is often done when testing apps that rely on random number generators.

If your application requires different random number sequences, invoke this constructor repeatedly with different seed values. One way to produce a unique seed value is to make it time-dependent. For example, derive the seed value from the system clock, as the Random() overload does. However, the system clock might not have sufficient resolution to provide different invocations of this constructor with a different seed value. On the .NET Framework, this results in random number generators that generate identical sequences of pseudo-random numbers, as illustrated by the first two Random objects in the following example. To prevent this, apply an algorithm to differentiate the seed value in each invocation, or call the Thread.Sleep method to ensure that you provide each constructor with a different seed value.

using System;
using System.Threading;

public class Example
{
   public static void Main()
   {
      Random rand1 = new Random((int) DateTime.Now.Ticks & 0x0000FFFF);
      Random rand2 = new Random((int) DateTime.Now.Ticks & 0x0000FFFF);
      Thread.Sleep(20);
      Random rand3 = new Random((int) DateTime.Now.Ticks & 0x0000FFFF);
      ShowRandomNumbers(rand1);
      ShowRandomNumbers(rand2);
      ShowRandomNumbers(rand3);
   }

   private static void ShowRandomNumbers(Random rand)
   {
      Console.WriteLine();
      byte[] values = new byte[4];
      rand.NextBytes(values);
      foreach (var value in values)
         Console.Write("{0, 5}", value);

      Console.WriteLine();
   }
}
// The example displays output similar to the following:
//   145  214  177  134  173
//
//   145  214  177  134  173
//
//   126  185  175  249  157

Another option is to instantiate a single Random object that you use to generate all the random numbers in your application. This yields slightly better performance, since instantiating a random number generator is fairly expensive.

Applies to

.NET 9 およびその他のバージョン
製品 バージョン
.NET Core 1.0, Core 1.1, Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7, 8, 9
.NET Framework 1.1, 2.0, 3.0, 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8, 4.8.1
.NET Standard 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 2.0, 2.1
UWP 10.0