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Add note on BigInteger bin/hex formatting of positive values #46473

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Add note on BigInteger bin/hex formatting of positive values
SolalPirelli May 29, 2025
2505458
Update parsing-numeric.md
SolalPirelli Jun 18, 2025
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8 changes: 8 additions & 0 deletions docs/standard/base-types/parsing-numeric.md
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Expand Up @@ -61,6 +61,7 @@ All numeric types have two static parsing methods, `Parse` and `TryParse`, that
|<xref:System.Globalization.NumberStyles.AllowThousands?displayProperty=nameWithType>|The group separator is permitted. The group separator character is determined by the <xref:System.Globalization.NumberFormatInfo.NumberGroupSeparator%2A?displayProperty=nameWithType> or <xref:System.Globalization.NumberFormatInfo.CurrencyGroupSeparator%2A?displayProperty=nameWithType> property.|
|<xref:System.Globalization.NumberStyles.AllowCurrencySymbol?displayProperty=nameWithType>|The currency symbol is permitted. The currency symbol is defined by the <xref:System.Globalization.NumberFormatInfo.CurrencySymbol%2A?displayProperty=nameWithType> property.|
|<xref:System.Globalization.NumberStyles.AllowHexSpecifier?displayProperty=nameWithType>|The string to be parsed is interpreted as a hexadecimal number. It can include the hexadecimal digits 0-9, A-F, and a-f. This flag can be used only to parse integer values.|
|<xref:System.Globalization.NumberStyles.AllowBinarySpecifier?displayProperty=nameWithType>|The string to be parsed is interpreted as a binary number. It can include the binary digits 0 and 1. This flag can be used only to parse integer values.|

In addition, the <xref:System.Globalization.NumberStyles> enumeration provides the following composite styles, which include multiple <xref:System.Globalization.NumberStyles> flags.

Expand All @@ -72,6 +73,13 @@ All numeric types have two static parsing methods, `Parse` and `TryParse`, that
|<xref:System.Globalization.NumberStyles.Currency?displayProperty=nameWithType>|Includes all styles except <xref:System.Globalization.NumberStyles.AllowExponent?displayProperty=nameWithType> and <xref:System.Globalization.NumberStyles.AllowHexSpecifier?displayProperty=nameWithType>.|
|<xref:System.Globalization.NumberStyles.Any?displayProperty=nameWithType>|Includes all styles except <xref:System.Globalization.NumberStyles.AllowHexSpecifier?displayProperty=nameWithType>.|
|<xref:System.Globalization.NumberStyles.HexNumber?displayProperty=nameWithType>|Includes the <xref:System.Globalization.NumberStyles.AllowLeadingWhite?displayProperty=nameWithType>, <xref:System.Globalization.NumberStyles.AllowTrailingWhite?displayProperty=nameWithType>, and <xref:System.Globalization.NumberStyles.AllowHexSpecifier?displayProperty=nameWithType> styles.|
|<xref:System.Globalization.NumberStyles.BinaryNumber?displayProperty=nameWithType>|Includes the <xref:System.Globalization.NumberStyles.AllowLeadingWhite?displayProperty=nameWithType>, <xref:System.Globalization.NumberStyles.AllowTrailingWhite?displayProperty=nameWithType>, and <xref:System.Globalization.NumberStyles.AllowBinarySpecifier?displayProperty=nameWithType> styles.|

## Parsing binary and hexadecimal BigIntegers

When parsing <xref:System.Numerics.BigInteger> with the <xref:System.Globalization.NumberStyles.AllowHexSpecifier> or <xref:System.Globalization.NumberStyles.AllowBinarySpecifier> flags, the input string is interpreted as a hexadecimal/binary number of exactly the length the string has.
For instance, parsing `"11"` as a binary BigInteger yields `-1`, because that is the interpretation of `11` as a signed two's complement value with exactly 2 digits.
If you want a positive result, add a leading `0`, such as `"011"` which is parsed as `3`.

## Parsing and Unicode Digits

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6 changes: 6 additions & 0 deletions docs/standard/base-types/standard-numeric-format-strings.md
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Expand Up @@ -98,6 +98,9 @@ The binary ("B") format specifier converts a number to a string of binary digits

The precision specifier indicates the minimum number of digits desired in the resulting string. If required, the number is padded with zeros to its left to produce the number of digits given by the precision specifier.

For <xref:System.Numerics.BigInteger>, positive values always have a leading zero to distinguish them from negative values. This ensures the output round-trips to the original value when parsed.
For instance, the number `3` converted with the format specifier `"B2"` is `011` because the binary number `11` represents the negative value `-1`.
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It might be worth clarifying that 11 is interpreted as a signed two's complement value that is 2-bits wide. Thus 1, 11, 111, 1111, etc are all interpreted as -1. This allows BigInteger to always use the shortest roundtrippable string when formatting and to support inputs from any smaller signed type without loss of data.

This is different from int.Parse and similar, where:

sbyte x = -1;
int y = int.Parse(x.ToString("B")); // 255

Which is due to them having a fixed number of bits and so interpreting any unspecified bits as being 0

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@tannergooding Thank you for breaking my brain. How does this sound?

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For instance, the number `3` converted with the format specifier `"B2"` is `011` because the binary number `11` represents the negative value `-1`.
For instance, the number `3` converted with the format specifier `"B2"` is `011` because the binary number `11` represents the negative value `-1`. Negative numbers are interpreted as a signed two's complement value that's 2-bits wide. This allows <xref:System.Numerics.BigInteger> to always use the shortest round-trippable string when formatting, especially with smaller negative numbers.

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Given that the 2 here is just an example, maybe:

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For instance, the number `3` converted with the format specifier `"B2"` is `011` because the binary number `11` represents the negative value `-1`.
For instance, the number `3` converted with the format specifier `"B2"` is `011`. That's because the binary number `11` represents the negative value `-1`, as it is interpreted as a number with exactly `2` bits due to the `"B2"` format.

(I've been thinking about something of the form "it's like intN.Parse if you had an intN type", but adding one extra concept C# doesn't actually have to understand BigIntege is probably not a great idea)


The result string is not affected by the formatting information of the current <xref:System.Globalization.NumberFormatInfo> object.

<a name="CFormatString"></a>
Expand Down Expand Up @@ -309,6 +312,9 @@ The hexadecimal ("X") format specifier converts a number to a string of hexadeci

The precision specifier indicates the minimum number of digits desired in the resulting string. If required, the number is padded with zeros to its left to produce the number of digits given by the precision specifier.

For <xref:System.Numerics.BigInteger>, positive values always have a leading zero to distinguish them from negative values. This ensures the output round-trips to the original value when parsed.
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It might be worth adding a similar note to NumberStyles.BinaryNumber/HexNumber for parsing.

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Done. I also noticed the page didn't have table rows for the binary NumberStyles, presumably since they're new, so I added those based on the hex versions.

For instance, the number `F` converted with the format specifier `"X1"` is `0F` because the hexadecimal number `F` represents the negative value `-1`.

The result string is not affected by the formatting information of the current <xref:System.Globalization.NumberFormatInfo> object.

The following example formats <xref:System.Int32> values with the hexadecimal format specifier.
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