dotnet/roslyn

Nullable check ignores NotNullIfNotNullAttribute when the argument is method group

Open

#77,475 opened on Mar 9, 2025

View on GitHub
 (5 comments) (0 reactions) (0 assignees)C# (4,257 forks)batch import
Area-CompilersBugFeature - Nullable Reference Typeshelp wanted

Repository metrics

Stars
 (20,414 stars)
PR merge metrics
 (Avg merge 6d 17h) (256 merged PRs in 30d)

Description

.NET SDK 9.0.200:

Steps to Reproduce:

#nullable enable
using System.Linq; // Enumerable.Select

string[] a = ["1", "a"]; // element not null

_ = a.Select(Foo) // returns string?[]
    .First().ToString(); // CS8602, unexpected

_ = a.Select(t => Foo(t)) // returns string[]
    .First().ToString(); // correct, expected

[return: NotNullIfNotNull(nameof(x))]
string? Foo(string? x) => x;

Diagnostic Id:

CS8602

Expected Behavior:

Nullable check Does not ignore NotNullIfNotNullAttribute when the argument is method group

Actual Behavior:

Nullable check ignores NotNullIfNotNullAttribute when the argument is method group

Contributor guide

Research direction
Examine the Roslyn nullable analysis code for method group conversions and how NotNullIfNotNullAttribute is applied. Look at the handling of Select and method group arguments.
Tech stack
csharp
Domain
compilers
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.
1-3 hours
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
C#
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40