c# - Did Microsoft change Random default seed?

Monday, May 28, 2018

c# - Did Microsoft change Random default seed?

Today, I was doing some tests in .NET Core, and I have come across some interesting thing.

Before (~ .NET Framework 4), Random used Environment.TickCount, but now I believe this has changed.

Consider the following code:

while (true)
{
    Random random = new Random();
    Console.WriteLine(random.Next(0, 10000));
}

In older .NET Framework versions, the new Random() empty constructor would use Environment.TickCount, which would lead to repetition of pseudo-random values.

So you could expect results like:

so on and so fourth.

On the latest .NET Core version using the latest compiler, I have received the following result:

Which is definitely improved.

Other S.O questions demonstrating this behaviour in older versions:

How do I generate a random int number?

generate random numbers with no repeat in c#

Non-repetitive random number

Is C# Random Number Generator thread safe?

My setup: .NET Core 2.2 / latest C# compiler.

The actual question

So my question is, has the PRNG really improved or they just changed constructor to use another default seeds, and, if so, what they're using as a seed? Is it safer now for cryptography (if they actually changed the implementation)?

Answer

In the latest version of dotnet core, the Random default constructor assigns its seed from a hidden private instance of Random. The private instance uses Interop.GetRandomBytes for its seed. New instances use the private instances's Next() result as its seed.

This basically makes it 'safe' to create several random instances in a loop.

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Monday, May 28, 2018