dotnet/roslyn

Strange behaviour in async method

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#32,276 opened on Jan 9, 2019

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Area-InteractiveBugDeveloper CommunityInteractive-Debugginghelp wanted

Description

I have a solution targeting NetCoreApp2.0.

Lately I've seen that when running an async method, the debugger shows me variables that are non allocated (out of scope).

If this variable is an IQueryable<string> it will show show up in the 'Locals window' as NullReferenceException.

If I manually set the 'next statement' to first statement in the block containing this IQueryable<string>, a NullReferenceException will be thrown when accessing the variable.

This issue has been moved from https://developercommunity.visualstudio.com/content/problem/238451/strange-behaviour-in-async-method.html VSTS ticketId: 604787 These are the original issue comments: (no comments) These are the original issue solutions: (no solutions)

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.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
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.
needs investigation
Prerequisites
C# async/awaitRoslyn compiler internalsDebugging symbol generation
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
15
Research direction
Investigate the async state machine transformation in Roslyn, specifically in the AsyncRewriter class. Look at how local variables are mapped to closures and whether the debugger scoping information is correctly emitted. Check relevant tests for async debugging. No linked PRs found; this issue may require deep analysis of the compiler's code generation for async methods.