NRE in CSharpUseImplicitTypeHelper.AssignmentSupportsStylePreference

[mavasani]

Got the following NRE while editing some broken code in Roslyn.sln, I don't have repro steps:

System.NullReferenceException : Object reference not set to an instance of an object.
   at Microsoft.CodeAnalysis.CSharp.Utilities.CSharpUseImplicitTypeHelper.AssignmentSupportsStylePreference(SyntaxToken identifier,TypeSyntax typeName,ExpressionSyntax initializer,SemanticModel semanticModel,OptionSet optionSet,CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.CSharp.Utilities.CSharpUseImplicitTypeHelper.TryAnalyzeVariableDeclaration(TypeSyntax typeName,SemanticModel semanticModel,OptionSet optionSet,CancellationToken cancellationToken)
   at async Microsoft.CodeAnalysis.CSharp.CodeRefactorings.UseType.AbstractUseTypeCodeRefactoringProvider.ComputeRefactoringsAsync(<Unknown Parameters>)
   at async Microsoft.CodeAnalysis.CodeRefactorings.CodeRefactoringService.GetRefactoringFromProviderAsync(<Unknown Parameters>)
   at Microsoft.VisualStudio.Telemetry.WindowsErrorReporting.WatsonReport.GetClrWatsonExceptionInfo(Exception exceptionObject)

[mavasani]

Tagging @CyrusNajmabadi

[jinujoseph]

dupe of #26735 ?

[CyrusNajmabadi]

Hard to make anything from that callstack. Do we have any watsons that have dumps that point to a more specific location? An NRE could effectively be on pretty much any line of that function.

[CyrusNajmabadi]

@jinujoseph Could likely be a duple. From looking at the method my best guess is that it's:

            var declaredType = semanticModel.GetTypeInfo(typeName, cancellationToken).Type;
            if (declaredType != null && declaredType.TypeKind == TypeKind.Dynamic)
            {
                return false;
            }
//...

                return declaredType.Equals(initializerType);

It seems quite possible that declaredType could be null. Then, we would NRE several lines lower.

Note: i really didn't do anything with this metohd. It was just a move from here:
https://github.com/dotnet/roslyn/pull/25865/files#diff-843b2b95dec3ec4b6e5786077c4a455bL197

I'm happy to assist with it, but it's been effectively the same code for a while.

[jinujoseph]

@ivanbasov Can you check for any reference in Watson for this

[jcouv]

Found a repro. I'll go ahead with a fix.

Repro:

class C1
{
    ref int N() => throw null;
    public void M()
    {

        ref var x = N(); // poke at the "var"
    }
}

[ivanbasov]

I could not find any related Watson for this issue.