Source generator doesn't see DefaultMemberAttribute

[twsouthwick]

I'm exploring using source generators for sgen and need to get the default member for part of that. However, when the source generator runs, the INamedTypeSymbol has no custom attributes so it can't be found.

Version Used:
Visual Studio 17.12.4

Steps to Reproduce:

Here's a repro with an isolated example: https://github.com/twsouthwick/XmlSerializerGenerator/tree/no-default-repro

Go to the test-generator project. It is hooked up to run the source generator. It will attempt to grab the custom attributes from the type supplied to the MyAttr attribute. At runtime, there are 3 attributes (2 nullable contexts and the DefaultMemberAttribute); the source generator does not see any.

The source generator is here. It gets a type from an attribute and enumerates the custom attributes.

Note: duplicate of #73229 but that one has been closed so opening a new one.

[twsouthwick]

In case it matters, I don't actually need the attribute - I just need the name that is used for the attribute.

[jaredpar]

@333fred PTAL

[333fred]

This is expected behavior. For attributes that mean specific things in C# (including NullableAttribute, NullableContextAttribute, and DefaultMemberAttribute), they are filtered out, in favor of producing the actual C# things that are expected. So, for example, types are annotated with the correct nullability, and when the class has an indexer, that indexer is exposed, instead of DefaultMemberAttribute. If the class does not have an indexer, only then is the attribute exposed. Here's an example of this (click to expand):

Example Roslyn program

using System.Diagnostics;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;

var source = """
namespace Test;

public class C1
{
    public int this[int i]
    {
        get => i;
        set {}
    }
}

[System.Reflection.DefaultMember("Age")]
public class C2
{
    public int Age
    {
        get => 1;
        set {}
    }
}
""";

var tree = CSharpSyntaxTree.ParseText(source);

var comp1 = CSharpCompilation.Create("test", options: new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary))
    .AddReferences(Basic.Reference.Assemblies.Net80.References.All)
    .AddSyntaxTrees(tree);

Console.WriteLine("In Source");

printC1AndC2Attributes(comp1);

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

var referenceStream = new MemoryStream();
var emitResult = comp1.Emit(referenceStream);
Debug.Assert(emitResult.Success);

referenceStream.Position = 0;
var comp2 = CSharpCompilation.Create("test", options: new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary))
    .AddReferences([MetadataReference.CreateFromStream(referenceStream), .. Basic.Reference.Assemblies.Net80.References.All])
    .AddSyntaxTrees(tree);

Console.WriteLine("In Metadata");
printC1AndC2Attributes(comp2);

static void printC1AndC2Attributes(CSharpCompilation comp2)
{
    var c1 = comp2.GetTypeByMetadataName("Test.C1");
    var c2 = comp2.GetTypeByMetadataName("Test.C2");

    Console.WriteLine("C1 attributes:");
    foreach (var attr in c1!.GetAttributes())
    {
        Console.WriteLine(attr.AttributeClass!.Name);
    }

    Console.WriteLine("C2 attributes:");
    foreach (var attr in c2!.GetAttributes())
    {
        Console.WriteLine(attr.AttributeClass!.Name);
    }
}

Output:

In Source
C1 attributes:
C2 attributes:
DefaultMemberAttribute
---------
In Metadata
C1 attributes:
C2 attributes:
DefaultMemberAttribute

Your generator will need to handle this difference, and presumably use the indexer directly when defined, rather than using a member by name.

[twsouthwick]

Ok - I went with the manual checking of the indexer already so I'll just keep that. Seemed a bit unintuitive, but thanks for the response.