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GenFragmentationPercent(Gen) sometimes throws a null ref exception #2012

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mrsharm opened this issue Mar 21, 2024 · 1 comment · Fixed by #2211
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GenFragmentationPercent(Gen) sometimes throws a null ref exception #2012

mrsharm opened this issue Mar 21, 2024 · 1 comment · Fixed by #2211
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@mrsharm
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mrsharm commented Mar 21, 2024

The call to GenSizeAfterMB in the GenFragmentationPercent call is what causes the null ref.

    public double GenFragmentationPercent(Gens gen)
    {
        return GenFragmentationMB(gen) * 100.0 / GenSizeAfterMB(gen);
    }

That GenSizeAfterMB call fails if the HeapStats are somehow null:

    public double GenSizeAfterMB(Gens gen)
    {
        return gen switch
        {
            Gens.GenPinObj => (double)HeapStats.GenerationSize4 / 1000000.0,
            Gens.GenLargeObj => (double)HeapStats.GenerationSize3 / 1000000.0,
            Gens.Gen2 => (double)HeapStats.GenerationSize2 / 1000000.0,
            Gens.Gen1 => (double)HeapStats.GenerationSize1 / 1000000.0,
            Gens.Gen0 => (double)HeapStats.GenerationSize0 / 1000000.0,
            _ => double.NaN,
        };
    }

This should be guarded against.
@brianrob
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If HeapStats is null, then GenSizeAfterMB should return NaN, and NaN should be propagated out of GenFragmentationPercent.

@Copilot Copilot AI mentioned this issue May 23, 2025
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Fix null reference exception in GenFragmentationPercent method microsoft/perfview
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@brianrob @mrsharm