dotnet/roslyn

Confusing error when assigning result of ref readonly-returning method to a ref local

Open

#28,653 opened on Jul 18, 2018

View on GitHub
 (0 comments) (0 reactions) (0 assignees)C# (20,414 stars) (4,257 forks)batch import
Area-CompilersBugConcept-Diagnostic Clarityhelp wanted

Description

Version Used: 2.8.2; master from 14 July 2018 (c4189de08b943f87d9f03d45ef01e615800e359d)

Steps to Reproduce:

Try to assign the result of a ref readonly-returning method to a non-readonly ref local:

class C
{
    void M()
    {
        ref int x = ref Bar();
    }
    
    ref readonly int Bar() => throw null;
}

Actual Behavior:

error CS8329: Cannot use method 'C.Bar()' as a ref or out value because it is a readonly variable

I think this is not a good error message because:

  1. It's not correct: method is not a variable.
  2. It does not explain well what the error is and how to fix it.

Contributor guide

Tech stack
csharp
Domain
developer experiencetooling
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.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C# ref readonly featureRoslyn error message infrastructure
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
The issue reports a confusing error message CS8329 when assigning a ref readonly method return to a ref local. The fix involves locating the error message in the Roslyn source code, likely in ErrorFacts.cs or related error reporting files, and rewording it to be more accurate. The new message should clarify that the issue is using a ref readonly method result as a ref or out value. Review existing tests for CS8329 to ensure they pass after the change. Consider whether a new error code is needed or if the existing one should be updated.