dotnet/roslyn

CS8618 suggests using `required` on events.

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#81679 opened on Dec 13, 2025

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Area-CompilersConcept-Diagnostic Clarityhelp wanted

Description

Version Used: 5.3.0-2.25603.1

Similar to #78152, but for events, both static and non-static.

Steps to Reproduce:

.NET Lab

using System;

public class C {
    public event Action OnEvent;
}

Diagnostic Id:

CS8618 Non-nullable event 'OnEvent' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the event as nullable.

Expected Behavior:

The error message is:

CS8618 Non-nullable event 'OnEvent' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the event as nullable.

Actual Behavior:

The error message gives wrong advice. Adding the required modifier results in error CS0106 The modifier 'required' is not valid for this item.

Contributor guide

Tech stack
csharp
Domain
tooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C# basicsfamiliarity with Roslyn codebase
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
75
Research direction
Search the Roslyn source code for the CS8618 diagnostic message string that mentions 'required'. The message is likely in a resource file or a string constant. Modify it to remove the suggestion of adding the 'required' modifier for events, as it is invalid for events. Reference the similar fix in issue #78152 for guidance. After making the change, build and run the relevant tests to confirm the fix.