Number operations - .NET 9 behavior change

[paul-barry-delongchamps]

This is a simplified expression of a complex code path we found in our code base while migrating to .NET 10:
(uint)(((double)random.Next() + (uint)random.Next()) * random.Next())

This always returns the same magic number in .NET 9 and later version whatever the Random instance used: 4294967295. It returns random results in .NET Framework or .NET Core until version 8.

We found that by chance and will workaround it on our own. I guess the same returned value is caused by a sequence of casts, but I was curious as to why the behavior changed in .NET 9. Please tell me if this is not the right place for this question.

[huoyaoyuan]

It's caused by the breaking change of floating point to integer conversion.

This always returns the same magic number in .NET 9 and later version whatever the Random instance used: 4294967295.

It's not really magic number, but just uint.MaxValue, because the result of the double expression is likely exceeding the range of uint.

It returns random results in .NET Framework or .NET Core until version 8.

The behavior is hardware dependent on older platforms.

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