[ComInterfaceGenerator] Custom enum return type in base interface unrecognized when processing derived interface

[lordmilko]

Description

The ComInterfaceGenerator throws an error when attempting to parse a derived interface whose base interface contains an unqualified custom enum type as the return type of a method.

For example, attempting to compile the following code

public enum HRESULT
{
}

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
[Guid("4BBA36C0-20BB-4DFF-9241-71992D498B39")]
[GeneratedComInterface]
public partial interface Base
{
    // Base will generate its wrapper successfully, but Derived will complain Base.M1's HRESULT type is unknown
    HRESULT M1();
}

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
[Guid("8BCFEC21-589D-45BC-908C-E038252F3453")]
[GeneratedComInterface]
public partial interface Derived : Base
{
    HRESULT M2();
}

generates the following errors

error CS0246: The type or namespace name 'HRESULT' could not be found (are you missing a using directive or an assembly reference?)
error CS0539: 'InterfaceImplementation.M1()' in explicit interface declaration is not found among members of the interface that can be implemented

• If Derived is modified to not derive from Base the application successfully compiles
• Changing the return type of M1 to a runtime type like int successfully compiles
• Fully qualifying the return type of M1 (e.g. ConsoleApp1.HRESULT M1()) also successfully compiles
• Specifying an argument of type HRESULT to M1 with return type void successfully compiles - the issue only occurs when our custom type is used as a return type

This behavior occurs regardless of whether PreserveSigAttribute is specified

Reproduction Steps

public enum HRESULT
{
}

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
[Guid("4BBA36C0-20BB-4DFF-9241-71992D498B39")]
[GeneratedComInterface]
public partial interface Base
{
    HRESULT M1();
}

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
[Guid("8BCFEC21-589D-45BC-908C-E038252F3453")]
[GeneratedComInterface]
public partial interface Derived : Base
{
    HRESULT M2();
}

Expected behavior

Custom return types are properly recognized without needing to fully qualify their namespaces

Actual behavior

error CS0246: The type or namespace name 'HRESULT' could not be found (are you missing a using directive or an assembly reference?)
error CS0539: 'InterfaceImplementation.M1()' in explicit interface declaration is not found among members of the interface that can be implemented

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response

[ghost]

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Issue Details

---

Description

The ComInterfaceGenerator throws an error when attempting to parse a derived interface whose base interface contains an unqualified custom enum type as the return type of a method.

For example, attempting to compile the following code

public enum HRESULT
{
}

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
[Guid("4BBA36C0-20BB-4DFF-9241-71992D498B39")]
[GeneratedComInterface]
public partial interface Base
{
    // Base will generate its wrapper successfully, but Derived will complain Base.M1's HRESULT type is unknown
    HRESULT M1();
}

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
[Guid("8BCFEC21-589D-45BC-908C-E038252F3453")]
[GeneratedComInterface]
public partial interface Derived : Base
{
    HRESULT M2();
}

generates the following errors

error CS0246: The type or namespace name 'HRESULT' could not be found (are you missing a using directive or an assembly reference?)
error CS0539: 'InterfaceImplementation.M1()' in explicit interface declaration is not found among members of the interface that can be implemented

• If Derived is modified to not derive from Base the application successfully compiles
• Changing the return type of M1 to a runtime type like int successfully compiles
• Fully qualifying the return type of M1 (e.g. ConsoleApp1.HRESULT M1()) also successfully compiles
• Specifying an argument of type HRESULT to M1 with return type void successfully compiles - the issue only occurs when our custom type is used as a return type

This behavior occurs regardless of whether PreserveSigAttribute is specified

Reproduction Steps

public enum HRESULT
{
}

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
[Guid("4BBA36C0-20BB-4DFF-9241-71992D498B39")]
[GeneratedComInterface]
public partial interface Base
{
    HRESULT M1();
}

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
[Guid("8BCFEC21-589D-45BC-908C-E038252F3453")]
[GeneratedComInterface]
public partial interface Derived : Base
{
    HRESULT M2();
}

Expected behavior

Custom return types are properly recognized without needing to fully qualify their namespaces

Actual behavior

error CS0246: The type or namespace name 'HRESULT' could not be found (are you missing a using directive or an assembly reference?)
error CS0539: 'InterfaceImplementation.M1()' in explicit interface declaration is not found among members of the interface that can be implemented

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response

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

[jkoritzinsky]

I believe this has been fixed by #87661

[AaronRobinsonMSFT]

@lordmilko We'll close this for now as I agree with @jkoritzinsky. Please let us know if this wasn't addressed in the next preview.