Parse/TryParse ignore NumberStyles: "1.5" and "1e5" parse under NumberStyles.Integer, and "-5" parses under NumberStyles.None

[matt-edmondson]

What's wrong

PreciseNumber.Parse(ReadOnlySpan<char>, NumberStyles, IFormatProvider?) (PreciseNumber/PreciseNumber.cs, around lines 1169-1310) applies only some of the NumberStyles flags. Whitespace trimming and AllowThousands follow the style. These parts do not:

• The decimal separator is accepted whether or not AllowDecimalPoint is set.
• The exponent (e/E) is accepted whether or not AllowExponent is set. Its digits then go through int.Parse(..., NumberStyles.Integer), which allows whitespace inside the exponent.
• A leading - is always accepted (ReadLeadingSign, about line 1295). Only the positive sign is gated on AllowLeadingSign.

Reproduction (current main, net10.0, invariant culture)

Call	PreciseNumber	decimal.TryParse with the same style
Parse("1.5", NumberStyles.Integer, inv)	1.5	false
Parse("1e5", NumberStyles.Integer, inv)	100000	false
Parse("-5", NumberStyles.None, inv)	-5	false
Parse("1e 5", NumberStyles.Float, inv)	100000	false
TryParse("1.5", NumberStyles.Integer, inv, out r)	true, r = 1.5	false

Why it matters

PreciseNumber implements INumberBase<T>. Callers use T.TryParse(s, NumberStyles.Integer, ...) to check that input is a whole number, and NumberStyles.None to reject signs. With T = PreciseNumber those checks pass for fractional, exponent or negative text. The code then gets a value it had explicitly excluded, for example a count of -5 or 1.5, with no error. Every other BCL numeric type rejects these inputs.

Suggested fix / acceptance criteria

• Accept the decimal separator only when style.HasFlag(NumberStyles.AllowDecimalPoint).
• Accept e/E only when style.HasFlag(NumberStyles.AllowExponent). Parse the exponent digits with NumberStyles.AllowLeadingSign only, not Integer, so "1e 5" is rejected.
• Accept a leading negative sign only when style.HasFlag(NumberStyles.AllowLeadingSign), matching the positive-sign check.
• Add tests that compare against decimal.TryParse for the inputs in the table above. Include these cases, which must keep working: NumberStyles.Float / Any / the default Parse(string, provider) overload on "1.5", "-1.5e-3" and "1,234.5" (with Any).

[matt-edmondson]

Triage

• Category: Bug
• Priority: Medium. Parse and TryParse accept fractional, exponent and negative input that the caller's NumberStyles explicitly excluded, so input validation written against INumberBase<T> passes values it should reject. Nothing crashes. Input the style allows still parses correctly, so the risk is limited to callers that rely on the style to validate.
• Area / suggested owner: Parsing (PreciseNumber.cs, Parse / ReadLeadingSign, about lines 1169-1310)
• Duplicates / in progress: None found, and no open PR covers it.
• Notes: This is small and self-contained. Tightening the flags is a behavior change for callers who pass NumberStyles.Integer today and get fractions back, so call it out in the changelog. Comparing against decimal.TryParse over the table in the issue gives a good parity test.

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