Disadvantages of System.Random in C#

In this lesson, we introduce the System.Random in C# and highlight some of the problems with it.

Introducing System.Random

The C# design team tries hard to make the language a pit of success, where the natural way to write programs is also the correct, elegant and performant way. And then System.Random comes along; it is almost always wrong, and it is seldom easy to make it right.

Let’s start with the obvious problem: the natural way to use it is also the wrong way.

The naive developer thinks “I need a new random dice roll”

void M()
  Random r = new Random();
  int x = r.Next(1, 6);

Let’s highlight some of the issues with this code.

Problem with System.Random

Firstly, every time M() is called, we create a new Random, but in most of the implementations of Random available for the last couple decades, by default, it seeds itself with the current time, and the granularity of the timer is only a few milliseconds. Computers run on the nanosecond scale, and so the likelihood that we’ll get two Randoms with the same seed is very high, and therefore it is very likely that successive calls to M() produce runs of the same number. You never want to make a new Random if you can avoid it; you want to make one once, and then re-use it. But it’s a bad idea to stash it away in a static because it’s not threading safe!

This problem has been fixed in some versions of the CLR. In those versions, a new Random() now seeds itself randomly, rather than based on the current time.

Second, the arguments to Next are the minimum value produced, inclusive, and the maximum value produced, exclusive! This program produces random numbers drawn from 1,2,3,4,51, 2, 3, 4, 5. The correct way to get numbers from 11 to 66 is, of course, Next(1, 7).

The fundamental problem here is that we’re working at too low a level of abstraction. It is not the 1970s anymore when rand() was good enough. We have sophisticated problems in statistical modeling and the attendant probabilistic reasoning to solve in modern programming languages. We need to up our game by writing code in the “business domain” of probabilities, not the “mechanism domain” of calls to methods that return random numbers.


Let’s have a look at the basic Random class in C#:

Try running the code again and again to notice a change in output.

using System;
using System.Threading;
namespace Probability
static class RandomIsAwful
static Random shared = new Random();
static string s = "";
public static void DoIt()
Console.WriteLine("If this code runs in older versions of C#, the output will be all or mostly the same number; if it is run in newer versions of C# then the numbers will be randomly chosen from 1 through 5. In no case will we get a six.");
for (int i = 0; i < 100; ++i)
Random random = new Random();
Console.Write(random.Next(1, 6) + " ");
Console.WriteLine("Similarly, in earlier days this would eventually print all zeros; Random is not thread safe, and its common failure mode is to get into a state where it can only produce zero. This bug has also been fixed, though this is still a bad idea");
for (int i = 0; i < 100; ++i)
new Thread(() => s += shared.Next(1, 6) + " ").Start();
// Yeah we should wait for those to finish.
Console.WriteLine("The real problem though is that this interface is not strong enough to do all the interesting stuff we want to do with stochastic programming in the modern era. That's what we'll be exploring in this course.");

In the next chapter, we will start by simply improving the existing implementation of Random, but from that humble beginning, we’ll develop a new class library that makes programming with probabilities much more readable, powerful and efficient in C#.