As Chris Shain's great answer illuminated: you can not depend on the results being the same on different platforms or even across Major versions of .NET
We ran into this exact problem at @BlueLineGames because we wrote a game using MonoGame which ran in .NET on Windows, but then produced different values when it ran on Mono in OSX.
As Steve suggested, writing your own PRNG is a great solution. In the hopes that it saves somebody else some time, here is a class that you can drop in place of System.Random and some quick usage info.
/// <summary>
/// This class is a drop-in replacement to use instead of Random(). Its
/// main purpose is to work identically across multiple platforms for a given
/// seed value - whereas the Random class is not guaranteed to give the
/// same results on different platforms even when the seed is the same.
///
/// This is an implementation of a Linear Congruential Generator which is
/// a very simple pseudorandom number generator. It is used instead of Random()
/// since Random() is implemented differently in Mono than .NET (and even
/// between different major-versions of .NET).
///
/// This is NOT recommended to be used for anything that should be secure such
/// as generating passwords or secret tokens. A "cryptogrpahically secure pseudorandom
/// number generator" (such as Mersenne Twister) should be used for that, instead.
///
/// More about Linear Congruential Generators can be found here:
/// http://en.wikipedia.org/wiki/Linear_congruential_generator
/// </summary>
public class CrossPlatformRandom : Random
{
// To start with, we'll be using the same values as Borland Delphi, Visual Pascal, etc.
// http://en.wikipedia.org/wiki/Linear_congruential_generator#Parameters_in_common_use
private const int LCG_MULTIPLIER = 134775813; // 0x08088405
private const int LCG_INCREMENT = 1;
private int _seed;
/// <summary>
/// Initializes a new instance of the CrossPlatformRandom class, using a time-dependent
/// default seed value.
///
/// Please note that this values generated from this are NOT guaranteed to be the same
/// cross-platform because there is no seed value. In cases where the caller requires
/// predictable or repeatable results, they MUST specify the seed.
/// </summary>
public CrossPlatformRandom()
{
// System.Random is time-dependent, so we will just use its first number to generate
// the seed.
Random rand = new Random();
this._seed = rand.Next();
}
/// <summary>
/// Initializes a new instance of the System.Random class, using the specified seed value.
/// </summary>
/// <param name="seed">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.</param>
public CrossPlatformRandom(int seed)
{
_seed = seed;
}
private int GetNext() // note: returns negative numbers too
{
_seed = (_seed * LCG_MULTIPLIER) + LCG_INCREMENT;
return _seed;
}
/// <summary>
// Returns a nonnegative random number.
/// </summary>
/// <returns>A 32-bit signed integer greater than or equal to zero and less than System.Int32.MaxValue.</returns>
public override int Next()
{
return this.Next(int.MaxValue);
}
/// <summary>
/// Returns a nonnegative random number less than the specified maximum.
/// </summary>
/// <param name="maxValue">The exclusive upper bound of the random number to be generated. maxValue must be greater than or equal to zero.</param>
/// <returns> A 32-bit signed integer greater than or equal to zero, and less than maxValue; that is, the range of return values ordinarily includes zero but not maxValue. However, if maxValue equals zero, maxValue is returned.</returns>
/// <exception cref="System.ArgumentOutOfRangeException">maxValue is less than zero.</exception>
public override int Next(int maxValue)
{
if (maxValue < 0)
{
throw new System.ArgumentOutOfRangeException("maxValue is less than zero.");
}
ulong result = (ulong)(uint)GetNext() * (ulong)(uint)maxValue;
return (int)(result >> 32);
}
/// <summary>
/// Returns a random number within a specified range.
/// </summary>
/// <param name="minValue">The inclusive lower bound of the random number returned.</param>
/// <param name="maxValue">The exclusive upper bound of the random number returned. maxValue must be greater than or equal to minValue.</param>
/// <returns>A 32-bit signed integer greater than or equal to minValue and less than maxValue; that is, the range of return values includes minValue but not maxValue. If minValue equals maxValue, minValue is returned.</returns>
/// <exception cref="System.ArgumentOutOfRangeException">minValue is greater than maxValue.</exception>
public override int Next(int minValue, int maxValue)
{
if (minValue > maxValue)
{
throw new System.ArgumentOutOfRangeException("minValue is greater than maxValue.");
}
return minValue + this.Next(maxValue - minValue);
}
/// <summary>
/// Fills the elements of a specified array of bytes with random numbers.
/// </summary>
/// <param name="buffer">An array of bytes to contain random numbers.</param>
/// <exception cref="System.ArgumentNullException">buffer is null.</exception>
public override void NextBytes(byte[] buffer)
{
if (buffer == null)
{
throw new System.ArgumentNullException("buffer is null.");
}
for (int index = 0; index < buffer.Length; index++)
{
buffer[index] = (byte)this.Next((int)byte.MaxValue);
}
}
/// <summary>
/// Returns a random number between 0.0 and 1.0.
/// </summary>
/// <returns>A double-precision floating point number greater than or equal to 0.0, and less than 1.0.</returns>
public override double NextDouble()
{
return this.Sample();
}
/// <summary>
/// Returns a random number between 0.0 and 1.0.
///
/// Since System.Random no longer uses this as the basis for all of the other random methods,
/// this method isn't used widely by this class. It's here for completeness, primarily in case Random
/// adds new entry points and we are lucky enough that they base them on .Sample() or one of the other existing methods.
/// </summary>
/// <returns>A double-precision floating point number greater than or equal to 0.0, and less than 1.0.</returns>
protected override double Sample()
{
return ((double)this.Next() / (double)int.MaxValue);
}
}
You should be able to safely drop this class in, in place of any use of Random() but you really only need to do it in places that you construct it using a seed (eg: new Random(seedValue)).
Some example usage:
int seed = 42;
CrossPlatformRandom r = new CrossPlatformRandom(seed);
for (int i = 0; i < 10; ++i)
Console.WriteLine(r.Next(100));
