Display Class used when not required + related flow analysis

[CortiWins]

Version Used:
VisualStudio.17.Release/17.12.3+35527.113
Microsoft .NET Framework
Version 4.8.09037
C# Tools 4.12.0-3.24572.7+dfa7fc6bdea31a858a402168384192b633c811fa

There are two (related?) issues

1. unexpected created code for lambda containing this
2. unexpected result of SemanticModel.AnalyzeDataFlow

Description
I was wondering why c# creates a display class to capture this when it could just not do it.
My proposal for that (dotnet/csharplang#8914) was met with a comment claiming it already does what i wished for. That turned out to be true, sometimes. The compiler uses a display class for a lambda that does not need it, if the same scope also contains a lambda that needs it. That is issue 1).Example code is provided below as sharplab.io-links.

I encountered issue 2) when i tried to update a ClrHeap-Analyzer (https://github.com/sonnemaf/RoslynClrHeapAllocationAnalyzer) to not show warnings for cases when no display class is created. The source of the warnings from that analyzer is, that SemanticModel.AnalyzeDataFlow claims that a ParenthesizedLambdaExpressionSyntax with no local capture requires one even if it does not.
An example is provided as an uploaded solution.

Issue 1: Code Examples Display Class Creation

Example 1: if both the lamba and the local function only require the capture of this, they are created as member method and no display_class is created.

https://sharplab.io/#v2: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

Example 2: If one lambda requires capture of a stack variable through a display_class, even the cases where the display_class is not required are created as members of the display_class

https://sharplab.io/#v2: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

Issue 2: Project that shows result of SemanticModel.AnalyzeDataFlow
RoslynFlowCaptureTest.zip

Sourcecode:

// Built 27.12.2024
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;

string code = @"using System;

private sealed class ThisCaptureTwo
{
    void TestFunction(int parameter)
    {
        // Make sure the lambda that does not require capturing is in its own scope
        {
            var action1 = new System.Action(() =>
            {
                this.TestFunction(66);
            });
        }

        // Make sure the lambda that DOES require capturing is in its own scope
        {
            int localVariable = 5;
            var action2 = new System.Action(() =>
            {
                this.TestFunction(localVariable);
            });
        }
    }
}
";

SyntaxTree tree = CSharpSyntaxTree.ParseText(code);

// Create a Compilation
var compilation = CSharpCompilation.Create("MyCompilation",
    syntaxTrees: [tree],
    references: [MetadataReference.CreateFromFile(typeof(object).Assembly.Location)]);

// Get the Semantic Model
var semanticModel = compilation.GetSemanticModel(tree);

SyntaxNode root = tree.GetRoot();
var lambdaExpressionSyntaxes = root.DescendantNodes().OfType<ParenthesizedLambdaExpressionSyntax>().ToList();

ParenthesizedLambdaExpressionSyntax lambdaexpNoVariable = lambdaExpressionSyntaxes[0];
var dataFlowAnalysis1 = semanticModel.AnalyzeDataFlow(lambdaexpNoVariable);

Console.WriteLine("-- CODE -------------------------------------------------");
Console.WriteLine(code);

Console.WriteLine("-- LAMBDA -------------------------------------------------");
Console.WriteLine(lambdaexpNoVariable.ToFullString());
Console.WriteLine("DataFlow Analysis Captured: " + dataFlowAnalysis1.Captured.Length);
for (int i = 0; i < dataFlowAnalysis1.Captured.Length; i++)
{
    Console.WriteLine($"{i} {dataFlowAnalysis1.Captured[i].Name}");
}

ParenthesizedLambdaExpressionSyntax lambdaexpWithLocalVariable = lambdaExpressionSyntaxes[1];
var dataFlowAnalysis2 = semanticModel.AnalyzeDataFlow(lambdaexpWithLocalVariable);

Console.WriteLine("-- LAMBDA -------------------------------------------------");
Console.WriteLine(lambdaexpWithLocalVariable.ToFullString());
Console.WriteLine("DataFlow Analysis Captured: " + dataFlowAnalysis2.Captured.Length);
for (int i = 0; i < dataFlowAnalysis2.Captured.Length; i++)
{
    Console.WriteLine($"{i} {dataFlowAnalysis2.Captured[i].Name}");
}

Console.ReadKey(true);

Console Output:

-- LAMBDA -------------------------------------------------
() =>
            {
                this.TestFunction(66);
            }
DataFlow Analysis Captured: 2
0 this
1 localVariable
-- LAMBDA -------------------------------------------------
() =>
            {
                this.TestFunction(localVariable);
            }
DataFlow Analysis Captured: 2
0 this
1 localVariable

[CyrusNajmabadi]

I believe @jaredpar has covered this in the past. Precise tracking has pros/cons. So we've opted for a simpler model which results in simpler display class layout and allocations, at the cost of this sort of thing happening in some cases.