Relate Array.MaxLength to int.MaxValue

[GrabYourPitchforks]

Summary

Document the guarantee that Array.MaxLength < int.MaxValue. Typical for loops are subject to failure without this guarantee. Example:

for (int i = 0; i < someArray.Length; i++)
{
    _ = someArray[i]; // if MaxLength = int.MaxValue, 'i' could overflow to int.MinValue, resulting in illegal access
}

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-runtime

[tannergooding]

@GrabYourPitchforks, the code sample you gave isn't prone to the error.

Given:

for (int i = 0; i < someArray.Length; i++)
{
    _ = someArray[i]; // if MaxLength = int.MaxValue, 'i' could overflow to int.MinValue, resulting in illegal access
}

Then even if someArray.Length was int.MaxValue (such as it could be for Span<T>) then i itself can never be int.MaxValue. At worst you have i = 0; i = 1; ...; i = 2147483646 all of which are valid indices. Then i++ on the next iteration makes it i = 2147483647, the comparison i < someArray.Length occurs which evaluates as 2147483647 < 2147483647 which is always false, and the loop exits.

This is only really an issue with i <= someArray.Length, where it can be prone to an infinite loop, but would still fail on access due to the bounds check which is done strictly as i >= 0 && i < Length

[tannergooding]

Which is to say, in safe code such as the above, there is no risk for an illegal access. There is only risk of an infinite loop and prevention of an access resulting in IndexOutOfRangeException (the access is not actually attempted, as that would result in AccessViolationException or data that shouldn’t be returned getting returned if the location wasn’t protected)

[learn-build-service-prod]

Learn Build status updates of commit 5139c88:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System/Array.xml	✅Succeeded	View

For more details, please refer to the build report.

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[GrabYourPitchforks]

Thanks @tannergooding for the check! I'll correct the wording and resubmit after researching a few things.