Add note on BigInteger bin/hex formatting of positive values #46473
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See dotnet/runtime#115618. It's somewhat unexpected that "print `3` with 2 binary digits" returns a string of length 3 (`"011"`), but also makes sense given the round-trip requirement and the historical context of not using `-` for negative bin/hex numbers. I think documenting this behavior would be useful.
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| @@ -309,6 +312,9 @@ The hexadecimal ("X") format specifier converts a number to a string of hexadeci | |||
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| 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. | |||
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| 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.
| @@ -98,6 +98,9 @@ The binary ("B") format specifier converts a number to a string of binary digits | |||
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| 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. | |||
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| 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")); // 255Which is due to them having a fixed number of bits and so interpreting any unspecified bits as being 0
tannergooding
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May 30, 2025
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Summary
Fixes dotnet/runtime#115618.
It's somewhat unexpected that "print
3with 2 binary digits" returns a string of length 3 ("011"), but also makes sense given the round-trip requirement and the historical context of not using-for negative bin/hex numbers.I think documenting this behavior would be useful.
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