Export (0) Print
Expand All
38 out of 49 rated this helpful - Rate this topic

Random Class

Updated: March 2011

Represents a pseudo-random number generator, a device that produces a sequence of numbers that meet certain statistical requirements for randomness.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)
[SerializableAttribute]
[ComVisibleAttribute(true)]
public class Random

Pseudo-random numbers are chosen with equal probability from a finite set of numbers. The chosen numbers are not completely random because a definite mathematical algorithm is used to select them, but they are sufficiently random for practical purposes. The current implementation of the Random class is based on a modified version of Donald E. Knuth's subtractive random number generator algorithm. For more information, see D. E. Knuth. "The Art of Computer Programming, volume 2: Seminumerical Algorithms". Addison-Wesley, Reading, MA, second edition, 1981.

The random number generation starts from a seed value. If the same seed is used repeatedly, the same series of numbers is generated. One way to produce different sequences is to make the seed value time-dependent, thereby producing a different series with each new instance of Random. By default, the parameterless constructor of the Random class uses the system clock to generate its seed value, while its parameterized constructor can take an Int32 value based on the number of ticks in the current time. However, because the clock has finite resolution, using the parameterless constructor to create different Random objects in close succession creates random number generators that produce identical sequences of random numbers. The following example illustrates that two Random objects that are instantiated in close succession generate an identical series of random numbers.

byte[] bytes1 = new byte[100];
byte[] bytes2 = new byte[100];
Random rnd1 = new Random();
Random rnd2 = new Random();

rnd1.NextBytes(bytes1);
rnd2.NextBytes(bytes2);

Console.WriteLine("First Series:");
for (int ctr = bytes1.GetLowerBound(0); 
     ctr <= bytes1.GetUpperBound(0); 
     ctr++) { 
   Console.Write("{0, 5}", bytes1[ctr]);
   if ((ctr + 1) % 10 == 0) Console.WriteLine();
} 
Console.WriteLine();
Console.WriteLine("Second Series:");        
for (int ctr = bytes2.GetLowerBound(0);
     ctr <= bytes2.GetUpperBound(0);
     ctr++) {
   Console.Write("{0, 5}", bytes2[ctr]);
   if ((ctr + 1) % 10 == 0) Console.WriteLine();
}   
// The example displays the following output to the console: 
//       First Series: 
//          97  129  149   54   22  208  120  105   68  177 
//         113  214   30  172   74  218  116  230   89   18 
//          12  112  130  105  116  180  190  200  187  120 
//           7  198  233  158   58   51   50  170   98   23 
//          21    1  113   74  146  245   34  255   96   24 
//         232  255   23    9  167  240  255   44  194   98 
//          18  175  173  204  169  171  236  127  114   23 
//         167  202  132   65  253   11  254   56  214  127 
//         145  191  104  163  143    7  174  224  247   73 
//          52    6  231  255    5  101   83  165  160  231 
//        
//       Second Series: 
//          97  129  149   54   22  208  120  105   68  177 
//         113  214   30  172   74  218  116  230   89   18 
//          12  112  130  105  116  180  190  200  187  120 
//           7  198  233  158   58   51   50  170   98   23 
//          21    1  113   74  146  245   34  255   96   24 
//         232  255   23    9  167  240  255   44  194   98 
//          18  175  173  204  169  171  236  127  114   23 
//         167  202  132   65  253   11  254   56  214  127 
//         145  191  104  163  143    7  174  224  247   73 
//          52    6  231  255    5  101   83  165  160  231        

This problem can be avoided by creating a single Random object rather than multiple ones.

To improve performance, create one Random object to generate many random numbers over time, instead of repeatedly creating a new Random objects to generate one random number.

To generate a cryptographically secure random number suitable for creating a random password, for example, use a class derived from System.Security.Cryptography.RandomNumberGenerator such as System.Security.Cryptography.RNGCryptoServiceProvider.

Notes to Callers:

The implementation of the random number generator in the Random class is not guaranteed to remain the same across major versions of the .NET Framework. As a result, your application code should not assume that the same seed will result in the same pseudo-random sequence in different versions of the .NET Framework.

Notes to Inheritors:

In the .NET Framework versions 1.0 and 1.1, a minimum implementation of a class derived from Random required overriding the Sample method to define a new or modified algorithm for generating random numbers. The derived class could then rely on the base class implementation of the Random.Next(), Random.Next(Int32), Random.Next(Int32, Int32), NextBytes, and NextDouble methods to call the derived class implementation of the Sample method.

In the .NET Framework version 2.0 and later, the behavior of the Random.Next(), Random.Next(Int32, Int32), and NextBytes methods have changed so that these methods do not necessarily call the derived class implementation of the Sample method. As a result, classes derived from Random that target the .NET Framework 2.0 and later should also override these three methods.

The following example creates a single random number generator and calls its NextBytes, Next, and NextDouble methods to generate sequences of random numbers within different ranges.

// Instantiate random number generator using system-supplied value as seed.
Random rand = new Random();
// Generate and display 5 random byte (integer) values.
byte[] bytes = new byte[4];
rand.NextBytes(bytes);
Console.WriteLine("Five random byte values:");
foreach (byte byteValue in bytes)
   Console.Write("{0, 5}", byteValue);
Console.WriteLine();   
// Generate and display 5 random integers.
Console.WriteLine("Five random integer values:");
for (int ctr = 0; ctr <= 4; ctr++)
   Console.Write("{0,15:N0}", rand.Next());
Console.WriteLine();
// Generate and display 5 random integers between 0 and 100.//
Console.WriteLine("Five random integers between 0 and 100:");
for (int ctr = 0; ctr <= 4; ctr++)
   Console.Write("{0,8:N0}", rand.Next(101));
Console.WriteLine();
// Generate and display 5 random integers from 50 to 100.
Console.WriteLine("Five random integers between 50 and 100:");
for (int ctr = 0; ctr <= 4; ctr++)
   Console.Write("{0,8:N0}", rand.Next(50, 101));
Console.WriteLine();
// Generate and display 5 random floating point values from 0 to 1.
Console.WriteLine("Five Doubles.");
for (int ctr = 0; ctr <= 4; ctr++)
   Console.Write("{0,8:N3}", rand.NextDouble());
Console.WriteLine();
// Generate and display 5 random floating point values from 0 to 5.
Console.WriteLine("Five Doubles between 0 and 5.");
for (int ctr = 0; ctr <= 4; ctr++)
   Console.Write("{0,8:N3}", rand.NextDouble() * 5);
// Sample console output might appear as follows: 
//    Five random byte values: 
//      194  185  239   54  116 
//    Five random integer values: 
//        507,353,531  1,509,532,693  2,125,074,958  1,409,512,757    652,767,128 
//    Five random integers between 0 and 100: 
//          16      78      94      79      52 
//    Five random integers between 50 and 100: 
//          56      66      96      60      65 
//    Five Doubles. 
//       0.943   0.108   0.744   0.563   0.415 
//    Five Doubles between 0 and 5. 
//       2.934   3.130   0.292   1.432   4.369      
Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0

Date

History

Reason

March 2011

Noted that the class uses a modified version of the subtractive random number generator algorithm, and added the Notes to Callers section.

Content bug fix.

Did you find this helpful?
(1500 characters remaining)
Thank you for your feedback

Community Additions

ADD
Show:
© 2014 Microsoft. All rights reserved.