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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 #46473

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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


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.

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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