dotnet/roslyn

IDE0059 is triggered for recurrent lambda initialization

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#48,622 opened on Oct 15, 2020

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Area-AnalyzersBugFeature - IDE0059help wanted

Description

Version Used: VS 16.9.0 Preview 1.0 [30609.169.main]

Steps to Reproduce:

using System;

namespace TestNamespace
{
    public class TestClass
    {
        public void TestMethod()
        {
            Func<string, string> func = null;
            func = s =>
            {
                // When initialization above is removed, this line results in an error:
                // Use of unassigned local variable 'func'
                return func(s);
            };
        }
    }
}

Expected Behavior: IDE0059 should not be triggered for func.

Actual Behavior: IDE0059 is triggered for func but when initialization to null is removed, compiler raises error:

Use of unassigned local variable 'func'

For the return func(s); line.

Contributor guide

Tech stack
csharp
Domain
developer experience
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.
blocked
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C#basic understanding of lambda expressionsfamiliarity with Roslyn analyzers
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50
Research direction
The issue is about IDE0059 (unused assignment) being incorrectly triggered for a variable that is initialized to null but used in a recursive lambda. The fix likely involves modifying the flow analysis logic in the Roslyn analyzer responsible for IDE0059. Look for the analyzer source file, possibly in `src/Features/CSharp/Diagnostics/Analyzers/` or similar. The analyzer needs to detect that the variable is actually read inside the lambda. Examine the control flow graph to ensure initialization is not flagged as unused when it is referenced in a recursive call.