Bug in System.Random constructor?

Question

The System.Threading.ConcurrentQueue.TryDequeue method threw an exception the other day that took me totally by surprise. Here's the stack trace:

System.OverflowException: Negating the minimum value of a twos complement number is invalid.
   at System.Math.AbsHelper(Int32 value)
   at System.Random..ctor(Int32 Seed)
   at System.Threading.Collections.ConcurrentQueue`1.TryDequeueCore(T& result)
   at System.Threading.Collections.ConcurrentQueue`1.TryDequeue(T& result)
   at MyProgram.ThreadProc() in c:\MyProgram\Main.cs:line 118
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()

At first I thought the problem was that TryDequeueCore called the Random constructor with a bad value. But further investigation reveals that TryDequeueCore calls the default constructor. It looks to me like the error is in the Random constructor:

.method public hidebysig specialname rtspecialname 
        instance void  .ctor() cil managed
{
  // Code size       12 (0xc)
  .maxstack  8
  IL_0000:  ldarg.0
  IL_0001:  call       int32 System.Environment::get_TickCount()
  IL_0006:  call       instance void System.Random::.ctor(int32)
  IL_000b:  ret
} // end of method Random::.ctor

As the documentation for the System.Environment.TickCount property says:

The value of this property is derived from the system timer and is stored as a 32-bit signed integer. Consequently, if the system runs continuously, TickCount will increment from zero to Int32..::.MaxValue for approximately 24.9 days, then jump to Int32..::.MinValue, which is a negative number, then increment back to zero during the next 24.9 days.

So, if you call the Random constructor during that one-millisecond period (after the system has been up for int.MaxValue milliseconds), it's going to throw this exception.

Does anybody have a workaround? For my own code, I can make a CreateRandom method that gets the TickCount value and checks it for int.MinValue. But what to do about code that I have no control over?

I hope the RTL team fixes this in .NET 4.0.

Update 2009/07/22: The BCL Team responded to the bug and said that it has been resolved for the next release.

Answer 1

try/catch and retry a millisecond later seems just about the only thing you can do until this bug does get fixed.