BigInteger.Parse creates invalid instances from hex strings

[adetunji-david]

Description

By design, the following expression creates an instance of BigInteger with the value of -2
var x = BigInteger.Parse("FFFFFFFFE", NumberStyles.HexNumber);
However, x is not equal to another instance with a value of -2.

Reproduction Steps

var x = BigInteger.Parse("FFFFFFFFE", NumberStyles.HexNumber);
var minusTwo = new BigInteger(-2);
Console.WriteLine(x);
Console.WriteLine(x == minusTwo);
Console.WriteLine(x == -2);

Expected behavior

Output is
-2
True
True

Actual behavior

Output is
-2
True
False

Regression?

No response

Known Workarounds

No response

Configuration

.Net version: 6.0.300
Operating System: Microsoft Windows 10 Pro
Architecture: x64

Other information

No response

[ghost]

Tagging subscribers to this area: @dotnet/area-system-numerics
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Issue Details

---

Description

By design, the following expression creates an instance of BigInteger with the value of -2
var x = BigInteger.Parse("FFFFFFFFE", NumberStyles.HexNumber);
However, x is not equal to another instance with a value of -2.

Reproduction Steps

var x = BigInteger.Parse("FFFFFFFFE", NumberStyles.HexNumber);
var minusTwo = new BigInteger(-2);
Console.WriteLine(x);
Console.WriteLine(x == minusTwo);
Console.WriteLine(x == -2);

Expected behavior

Output is
-2
True
True

Actual behavior

Output is
-2
True
False

Regression?

No response

Known Workarounds

No response

Configuration

.Net version: 6.0.300
Operating System: Microsoft Windows 10 Pro
Architecture: x64

Other information

No response

Author:	adetunji-david
Assignees:	-
Labels:	area-System.Numerics
Milestone:	-

[tannergooding]

This bug also repro's in .NET 6. Parse is creating a BigInteger with bits = { 0, 2 }, sign = -1

Where-as casting and implicit conversion creates a BigInteger with bits = null, sign = -2

[speshuric]

It seems I found the cause of the bug. I've fixed it locally, but it needs to be tested. I think I'll send PR in a couple of days.