[ComInterfaceGenerator] Trailing trivia after GeneratedComInterfaceAttribute included in generated files

[lordmilko]

Consider the following declaration of an interface designed to be multi-targeted against .NET 8 and earlier versions of .NET

    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    [Guid("7DAC8207-D3AE-4C75-9B67-92801A497D44")]
#if !GENERATED_MARSHALLING
    [ComImport]
#else
    [GeneratedComInterface]
#endif
    public partial interface IFoo
    {
        void Foo();
    }

Attempting to compile this results in the following error:

error CS1028: Unexpected preprocessor directive

In the generated source code, we can see the #ifdef directive has erroneously been carried over to two locations

// MyProject.IFoo.cs
namespace MyProject
{
    [System.Runtime.InteropServices.Marshalling.IUnknownDerivedAttribute<InterfaceInformation, InterfaceImplementation>]
#endif
    public partial interface IFoo
    {
    }
}

namespace MyProject
{
#endif
    public partial interface IFoo
    {
    }
}

If we flip our #if directive around to the following

#if GENERATED_MARSHALLING
    [GeneratedComInterface]
#else
    [ComImport]
#endif
    public partial interface IFoo

then the #else, [ComImport] and #endif lines are all copied over to the generated source file.

You can "work around" this by moving the interface header into the #if statement, as follows

#if !GENERATED_MARSHALLING
    [ComImport]
    public partial interface IFoo
#else
    [GeneratedComInterface]
    public partial interface IFoo
#endif

However I would present that users attempting to multi-target by applying an #if around which attribute to use is a reasonable scenario; I would imagine upon reading the GeneratedComInterfaceAttribute AttributeSyntax the source generator should just trim all trailing trivia

[ghost]

Tagging subscribers to this area: @dotnet/interop-contrib
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Consider the following declaration of an interface designed to be multi-targeted against .NET 8 and earlier versions of .NET

    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    [Guid("7DAC8207-D3AE-4C75-9B67-92801A497D44")]
#if !GENERATED_MARSHALLING
    [ComImport]
#else
    [GeneratedComInterface]
#endif
    public partial interface IFoo
    {
        void Foo();
    }

Attempting to compile this results in the following error:

error CS1028: Unexpected preprocessor directive

In the generated source code, we can see the #ifdef directive has erroneously been carried over to two locations

// MyProject.IFoo.cs
namespace MyProject
{
    [System.Runtime.InteropServices.Marshalling.IUnknownDerivedAttribute<InterfaceInformation, InterfaceImplementation>]
#endif
    public partial interface IFoo
    {
    }
}

namespace MyProject
{
#endif
    public partial interface IFoo
    {
    }
}

If we flip our #if directive around to the following

#if GENERATED_MARSHALLING
    [GeneratedComInterface]
#else
    [ComImport]
#endif
    public partial interface IFoo

then the #else, [ComImport] and #endif lines are all copied over to the generated source file.

You can "work around" this by moving the interface header into the #if statement, as follows

#if !GENERATED_MARSHALLING
    [ComImport]
    public partial interface IFoo
#else
    [GeneratedComInterface]
    public partial interface IFoo
#endif

However I would present that users attempting to multi-target by applying an #if around which attribute to use is a reasonable scenario; I would imagine upon reading the GeneratedComInterfaceAttribute AttributeSyntax the source generator should just trim all trailing trivia

Author:	lordmilko
Assignees:	-
Labels:	area-System.Runtime.InteropServices, untriaged
Milestone:	-