DebuggerDisplay attribute displays incorrectly under certain (extremely rare) conditions

[f2bo]

Description

The text displayed by the debugger for a variable containing a reference to a class is incorrect only when the following three conditions are satisfied. The namespace of the class must be net9_preview, the class must be named TextSegment, and it must contain a property named Segments.

To be honest, the conditions seem to be so rare that I don't expect many people will be hitting this issue, if any. I only stumbled upon this problem in a console app where I quickly test out new ideas and that I keep around since the .NET 9 preview days. Its default namespace, which I never bothered to change is net9_preview, which I certainly don't need to preserve. So I'm really reporting this more out of curiosity than any expectation that this will be fixed.

Reproduction Steps

1. Create a new console application that targets .NET 9 and replace Program.cs with the following code.

using System.Diagnostics;
using net9_preview;

TextSegment segment = new();

return;

namespace net9_preview
{
    [DebuggerDisplay($"I'm a {nameof(TextSegment)}")]
    internal class TextSegment
    {
        public int Segments => 42;
    }
}

2. Execute in the debugger and examine the text shown for the segment variable after executing the line that initializes it.

Expected behavior

The debugger tooltip should display the text "I'm a TextSegment".

Actual behavior

The debugger tooltip displays the text "Segments = 42" .

Regression?

No response

Known Workarounds

Changing any single one of the following for the TextSegment class restores the behavior expected from the applied DebuggerDisplay attribute.

• namespace (e.g. change to net10_preview),
• class name (e.g. change to TextChunk),
• Segments property name (e.g. change to Items)

Configuration

.NET 9.0.1
OS Name: Windows
OS Version: 10.0.19045
OS Platform: Windows
RID: win-x64

Other information

No response

[stephentoub]

I can't repro this.

[f2bo]

@stephentoub Thanks for taking a look.

I tested again in a new console app and I can still reproduce it. I don't know what could be different. It's quite weird, particularly because it seems to be so specific and only happens with these names. As soon as I change the namespace, or the class name, or the property name it starts to behave normally. When I revert the change, the problem is back.

As I said, this is not critical to me and I only wanted to satisfy my curiosity. If anyone has any theories...

Thanks!

[KalleOlaviNiemitalo]

It could almost be caused by a Directory.Build.props file in some outer directory:

<Project>
  <ItemGroup>
    <AssemblyAttribute Include="System.Diagnostics.DebuggerDisplayAttribute">
      <_Parameter1>Segments = {Segments}</_Parameter1>
      <TargetTypeName>net9_preview.TextSegment</TargetTypeName>
    </AssemblyAttribute>
  </ItemGroup>
</Project>

However, you wrote that you get the expected behavior if you rename the Segments property. If there were such an assembly-level DebuggerDisplayAttribute, then renaming the property would cause the debugger to display an error:

Segments = error CS0103: The name 'Segments' does not exist in the current context

So that's not it. An empty <_Parameter1/> wouldn't work either, as MSBuild wouldn't distinguish that from omitting the parameter and wouldn't be able to find the correct constructor.

Perhaps it's instead something you have added to the Visual Studio installation. Like the autoexp.dll mentioned in https://learn.microsoft.com/en-us/visualstudio/debugger/using-the-debuggerdisplay-attribute?view=vs-2022.

[f2bo]

@KalleOlaviNiemitalo thanks for sharing these ideas.

It could almost be caused by a Directory.Build.props file in some outer directory:

Just to make sure that no .props file is involved, I copied the project to the drive's root. No changes.

Perhaps it's instead something you have added to the Visual Studio installation. Like the autoexp.dll mentioned in https://learn.microsoft.com/en-us/visualstudio/debugger/using-the-debuggerdisplay-attribute?view=vs-2022.

I didn't find any mention of the text Segment anywhere in the autoexp.cs file and the autoexp.dll in the same directory has the same timestamp so, presumably, it's what was originally shipped with Visual Studio.

I can conceive some unusual condition that was never anticipated that triggers this. Perhaps a naming collision with TextSegments or Segments somewhere, or the use of a digit and an underscore or the text preview in the namespace, but that all these conditions must occur together and that I can successfully use net8_preview or net10_preview but not net9_preview?!

[dotnet-policy-service]

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[tommcdon]

Hi @f2bo! The .NET runtime does not provide expression evaluation control over the debugger for debugger display attributes, as that functionality comes from the Visual Studio debugger. I suggest opening a Visual Studio feedback item referencing this github issue, directions can be found here. We will keep this issue open a while to help ensure that the feedback item is routed to the correct team.

[f2bo]

Hi @tommcdon. I reported the problem here.

https://developercommunity.visualstudio.com/t/DebuggerDisplay-attribute-displays-incor/10841087

Thanks!