BigInteger incorrectly parses large integer hex string

[alexhelms]

Description

The following expression results in the wrong parsed value:

string s = BigInteger.Parse("FFFFFFFFFFFFFFFF01", NumberStyles.HexNumber, null).ToString("X")
Debug.Assert(s == "F01");

I would expect the string to be the same as the input -- FFFFFFFFFFFFFFFF01. Further, any number of Fs results in the same F01 result.

Trying another huge number results in the correct parsing:

string s = BigInteger.Parse("abcdefabcdefabcdefabcdefabcdef", NumberStyles.HexNumber, null).ToString("X")
Debug.Assert(s == "ABCDEFABCDEFABCDEFABCDEFABCDEF");

Configuration

.NET SDK (reflecting any global.json):
 Version:   5.0.201
 Commit:    a09bd5c86c

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.18362
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\5.0.201\

Host (useful for support):
  Version: 5.0.4
  Commit:  f27d337295

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Issue Details

---

Description

The following expression results in the wrong parsed value:

string s = BigInteger.Parse("FFFFFFFFFFFFFFFF01", NumberStyles.HexNumber, null).ToString("X")
Debug.Assert(s == "F01");

I would expect the string to be the same as the input -- FFFFFFFFFFFFFFFF01. Further, any number of Fs results in the same F01 result.

Trying another huge number results in the correct parsing:

string s = BigInteger.Parse("abcdefabcdefabcdefabcdefabcdef", NumberStyles.HexNumber, null).ToString("X")
Debug.Assert(s == "ABCDEFABCDEFABCDEFABCDEFABCDEF");

Configuration

.NET SDK (reflecting any global.json):
 Version:   5.0.201
 Commit:    a09bd5c86c

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.18362
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\5.0.201\

Host (useful for support):
  Version: 5.0.4
  Commit:  f27d337295

Author:	alexhelms
Assignees:	-
Labels:	area-System.Numerics, untriaged
Milestone:	-

[tannergooding]

This is different from how other integral types, like int32 parse hex values like this.
Int32 and others treat FF01 as a positive value, while BigInteger is treating it as a signed value and therefore produces -255.

However, this is a long time difference and changing it will be breaking.
It would require a bar check on how breaking it would be and if the change is ultimately worthwhile.

[alexhelms]

Got it. I understand this is because of how two's complement works and the assumptions BigInteger makes. I supposed the documentation for Parse could indicate that it assumes signed integers? I don't see anything in the docs for Parse that says it assumes signed integers. Longer term, perhaps an additional flag to indicate signed or unsigned?

[tannergooding]

Closing this as "by design".

We are happy to take a PR updating the documentation or a new issue proposing (https://github.com/dotnet/runtime/issues/new?assignees=&labels=api-suggestion&template=02_api_proposal.md&title=) an API that allows you to explicitly parse as signed vs unsigned.