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Explicitly cast double values down to float in Row.GetFloat(). #706

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merged 2 commits into from Sep 25, 2019

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@lauxjpn lauxjpn commented Sep 25, 2019

Support casting to float for Pomelo.EntityFramworkCore.MySql for MySQL < 8.0.17.
See PomeloFoundation/Pomelo.EntityFrameworkCore.MySql#827 (comment)

Signed-off-by: Laurents Meyer laucomm@gmail.com

Supports casting to float in Pomelo.EntityFramworkCore.MySql for MySQL < 8.0.17.

Signed-off-by: Laurents Meyer <laucomm@gmail.com>
@lauxjpn lauxjpn force-pushed the float_conversion branch 3 times, most recently from 9697e5d to 5881931 Compare Sep 25, 2019
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lauxjpn commented Sep 25, 2019

If you are concerned, that the explicit guard against information loss could lead to an InvalidCastException in cases, where the double value is even only a tiny bit larger/smaller than the float.MinValue/float.MaxValue, it could be dropped entirely.
But it would also mean, that a way higher or lower double value would silently be converted to a float.MinValue/float.MaxValue, which might be unexpected.

@lauxjpn lauxjpn closed this Sep 25, 2019
@lauxjpn lauxjpn reopened this Sep 25, 2019
…ks, to ensure no significant loss of information, when casting from double to float.

Signed-off-by: Laurents Meyer <laucomm@gmail.com>
// Use explicit range checks to guard against that.
return value switch
{
double doubleValue => (doubleValue >= float.MinValue && doubleValue <= float.MaxValue ? (float) doubleValue : throw new InvalidCastException("The value cannot be safely cast to Single.")),
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@bgrainger bgrainger Sep 25, 2019

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This test could be if doubleValue is contained in any of the following ranges: [float.MinValue, -float.Epsilon], [0, 0], [float.Epsilon, float.MaxValue]; what do you think?

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So the difference would be, that double values between -/+ float.Epsilon and 0 would throw.

The question is, if it is expected that a double between float.Epsilon and 0 is being rounded to float.Epsilon or 0. I think it is, because it's just a result of precision-loss.

(But there could probably be made a case against it as well, which would expect those values to end up as float.Epsilon. I think that is imprecise though, and they need to be rounded. And I would assume, that C#/the processor is doing exactly that in the first place.)

I would not throw in any of these cases, because this would be unexpected and a bit random.

The only potential problem I see with the current test is, that multiple arithmetic operations with a double near the float's min or max threshold, could end up pushing the value over the threshold due to precision error and thus end up throwing an InvalidCastException.
But this is an unlikely corner case and I think guarding against an accidental call of GetFloat on a double column, that then leads to a loss of information, is a much more common and severe case and is worth guarding against.

@bgrainger bgrainger merged commit 93e0d96 into mysql-net:master Sep 25, 2019
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@lauxjpn lauxjpn deleted the float_conversion branch Sep 25, 2019
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bgrainger commented Sep 25, 2019

Fixed in 0.59.0.

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