New source generators mode does not generate working cocreateable classes (e.g. ShellLink)

[Marv51]

Actual behavior

The new source-generator based COM (useComSourceGenerators: true) generates a class with a (traditional) "ComImport" attribute for a cocreateable class like ShellLink. This class is incompatible with the interfaces generated with the same configuration.

Expected behavior

The class and the interface are compatible with each other, both using the same technique.

Repro steps

1. NativeMethods.txt content:

IPersistFile
IShellLinkW
ShellLink

2. NativeMethods.json content (if present):

{
  "$schema": "https://aka.ms/CsWin32.schema.json",
  "comInterop": {
    "useComSourceGenerators": true
  }
}

3. Any of your own code that should be shared?

var shellLink = new ShellLink();
var shellLinkW = (IShellLinkW)shellLink;

shellLinkW.SetPath("explorer.exe");
// some more calls
var persistFile = (IPersistFile)shellLink;
persistFile.Save(shortcut, true);

Both casts in that code snippet create this warning: SYSLIB1099 Casting between a ComImport type and a source-generated COM type is not supported

And the warning looks to be accurate the generated ShellLink:

[Guid("00021401-0000-0000-C000-000000000046"),ComImport()]
[global::System.CodeDom.Compiler.GeneratedCode("Microsoft.Windows.CsWin32", "0.3.217+533aa1bddf.RR")]
internal partial class ShellLink
{
}

the generated IShellLinkW:

[Guid("000214F9-0000-0000-C000-000000000046"),InterfaceType(ComInterfaceType.InterfaceIsIUnknown),GeneratedComInterface()]
[SupportedOSPlatform("windows5.1.2600")]
[global::System.CodeDom.Compiler.GeneratedCode("Microsoft.Windows.CsWin32", "0.3.217+533aa1bddf.RR")]
internal unsafe partial interface IShellLinkW
{

I believe that this line is responsible:

CsWin32/src/Microsoft.Windows.CsWin32/Generator.Com.cs

Line 1318 in 56ee24c

.AddAttributeLists(AttributeList().AddAttributes(GUID(guid), ComImportAttributeSyntax));

But I do not know how to fix it.

For further context, I believe this issue created these classes: #453

Context

• CsWin32 version: 0.3.217
• Win32Metadata version (if explicitly set by project): not set
• Target Framework: net9.0-windows10.0.26100.0
• LangVersion (if explicitly set by project): [e.g. 9] not set

[Marv51]

I reformatted the issue a bit after I submitted it, because I did not realize, that coming from the source code browser GitHub does not prefill the issue template. So I manually adjusted to closely resemble the bug-report-template.

[jevansaks]

Good find. I'll be honest, I didn't know cswin32 was generating these or that .NET natively supported them this way. :) But talking with @jkoritzinsky, this doesn't have an equivalent in the COM source generated world. So cswin32 is going to need to generate something a little different:

1. We can't make the magic new ShellLink() work exactly. We can either hide the constructor or mark it obsolete with a note that says what to call instead.
2. No ComImport attribute.
3. We need an alternate "Create" method or factory thing.

To that end, I think the easiest thing to do is the following. I propose generating the following code in COM generators mode:

[Guid("00021401-0000-0000-C000-000000000046")]
[global::System.CodeDom.Compiler.GeneratedCode("Microsoft.Windows.CsWin32", "0.3.217+533aa1bddf.RR")]
internal partial class ShellLink
{
    [Obsolete("Use CreateInstance<T> method instead.")]
    public ShellLink() { throw new NotSupportedException("Use CreateInstance<T> method instead."); }

    public static T CreateInstance<T>() where T : class
    {
        PInvoke.CoCreateInstance<T>(typeof(ShellLink).GUID, null, CLSCTX.CLSCTX_INPROC_SERVER, out T ret).ThrowOnFailure();
        return ret;
    }
}

We could even have cswin32 emit this for the source generators mode too so that folks can switch back and forth, I suppose?

So the consumer would write code like this instead:

// OLD:
// var shellLink = new ShellLink();
// var shellLinkW = (IShellLinkW)shellLink;
// NEW:
var shellLinkW = ShellLink.CreateInstance<IShellLinkW>();

[jevansaks]

CC @AArnott @manodasanW @Jeremy-Price @Sergio0694 @AaronRobinsonMSFT for their thoughts also.

[Marv51]

I can already say thank you, this works well already with the new source generator mode:

PInvoke.CoCreateInstance(typeof(ShellLink).GUID, null, CLSCTX.CLSCTX_INPROC_SERVER, out IShellLinkW shellLinkW).ThrowOnFailure();
shellLinkW.SetPath("explorer.exe");
// some more calls
var persistFile = (IPersistFile)shellLinkW;
persistFile.Save(shortcut, true);

I look forward to your PR making this nice and clean, but with this I can already fully migrate to the new mode.