dotnet/runtime

Math.Round(value, digits, mode) does not return the correct result

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#1,643 opened on Apr 18, 2019

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Cost:Marea-System.Numericsbreaking-changebughelp wanted

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Description

The Math.Round(value, digits, mode) code returns a close, but not correct result most of the time.

The root problem is that the algorithm tries to follow normal mathematical rules to produce the result. However, due to the variable precision in IEEE floating-point numbers, this ultimately causes it to produce a close, but generally incorrect result most of the time and causes it to not accept inputs above a certain range. For example, you can't round double.MaxValue and the larger the input is, the more inaccurate the rounding will be.

These functions should be rewritten to be IEEE compliant and to accept any valid finite floating-point input.

Contributor guide

Research direction
Examine the current Math.Round implementation in the .NET runtime source (likely in src/libraries/System.Private.CoreLib/src/System/Math.cs). Understand the algorithm that attempts to round using mathematical rules and identify where floating point precision causes inaccuracies. Review IEEE 754 rounding modes (round to nearest, ties to even, etc.) and finite input handling. Propose a rewrite that avoids precision loss for all finite doubles, including edge cases like double.MaxValue. Check existing discussions in the issue for proposed solutions.
Tech stack
csharp
Domain
backendapi
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
.NET internalsIEEE 754 floating point arithmetic
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50