dotnet/roslyn

Got a false CS8602 warnning when use Null-conditional operator and Null-coalescing opreator in expression

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#78,432 opened on May 3, 2025

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Area-CompilersBugFeature - Nullable Reference Typeshelp wanted

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Description

Version Used:

  • csc.dll: 4.13.0-3.25155.17+398aa319196d4684c751a3d7c75c847e519a393c
  • .NET SDK: 9.0.203-dc7acfa194

Steps to Reproduce:

  1. Compile follow code
  2. Got a false CS8602 warnning

Sharplab

using System;

AnotherTestClass? anotherTestClass = null;
if (!anotherTestClass?.IsTrue ?? false)
{
    // CS8602 at `anotherTestClass`
    Console.WriteLine(anotherTestClass.IsTrue
        ? "Cancellation requested"
        : "Cancellation not requested");
}
Console.WriteLine("Test method ended");

class AnotherTestClass
{
    public bool IsTrue = false;
}

Diagnostic Id:

CS8602: Dereference of a possibly null reference.

Expected Behavior:

No warnning

Actual Behavior:

Got a falseCS8602 warnning

Contributor guide

Research direction
Investigate the nullable flow analysis in the Roslyn compiler for expressions combining the null conditional operator (?.) and the null coalescing operator (??). Specifically, trace how the compiler handles the pattern `!anotherTestClass?.IsTrue ?? false` and why it incorrectly reports CS8602 despite the null coalescing operator providing a fallback. Look at the NullableWalker or related analysis passes.
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#nullable reference types
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60