How to create instance for `IDesktopWallpaper` COM interface

[seayxu]

Is your feature request related to a problem? Please describe.

• Target:IDesktopWallpaper
• Built-in and source-generated COM interop

[Guid("B92B56A9-8B55-4E14-9A89-0199BBB6F93B"),InterfaceType(ComInterfaceType.InterfaceIsIUnknown),ComImport()]
[SupportedOSPlatform("windows8.0")]
[global::System.CodeDom.Compiler.GeneratedCode("Microsoft.Windows.CsWin32", "0.3.49-beta+91f5c15987")]
internal interface IDesktopWallpaper
{
}

The code not gen GeneratedComInterface,so not gen implement in below.

[GeneratedComClass]
internal partial class DesktopWallpaper : IDesktopWallpaper
{
}

• https://github.com/microsoft/CsWinRT/issues/1337
• https://github.com/microsoft/CsWinRT/issues/1533

[AArnott]

The [GeneratedComClass] attribute is unrelated to this source generator. CsWin32 generates all the code necessary for interop of the APIs it emits, as far as I'm aware. I'm not too familiar with that attribute but from the docs you linked to (thank you!) it seems that that triggers a source generator to fill in more. But as a source generator itself, CsWin32 cannot depend on the compiler running any other source generator after itself, so it must be self-sufficient.

Further, I see that both IDesktopWallpaper and DesktopWallpaper are in the metadata, so I expect both can be emitted by CsWin32.

Can you clarify what it generates and how that falls short to support your scenario?

[dongle-the-gadget]

CsWin32 currently uses the built-in COM interop system via [ComImport] (when runtime marshalling is enabled), which isn't trimming or AOT compatible. [GeneratedComClass] and [GeneratedComInterface] are two attributes introduced in .NET 8 to support the new source-generated COM wrappers, which are trimming and AOT compatible.

As far as I'm aware, when an attribute is marked with [GeneratedComInterface], a source generator then provides the [IUnknownDerivedAttribute<T,TImpl>] (source code) attribute for it, which provides metadata for the StrategyBasedComWrappers class to initiate RCWs and CCWs for the interface.

[AArnott]

when runtime marshalling is enabled

I think you've already qualified your statement appropriately here. If you don't want to rely on the built-in COM interop system, turn off allowMarshaling in the NativeMethods.json file. This is what WinForms does in order to get trim-friendly COM interop from CsWin32.