Allow generating structs with inner struct properties staying .NET references instead of pointers (optimally only for some methods)

[drauch]

Describe the solution you'd like
I'd like to write code like the following:

var myStruct = CreateStruct();
PInvoke.MyMethod(..., myStruct, ...);
[...]
OUTER_STRUCT CreateStruct ()
{
    INNER_STRUCT is;
    // fill is

    OUTER_STRUCT os;
    // fill os
    os.Inner = &is; // unfortunately pointer type instead of a C# reference type

    return os;
}

That doesn't work though, as the address &is is invalid once I return from CreateStruct.

Is your feature request related to a problem? Please describe.
I'm frustrated when I can't write it like that but instead have to write code like the following as a workaround:

var disposers = new List<Action>();
try
{
    var myStruct = CreateStruct(disposers);
    PInvoke.MyMethod(..., myStruct, ...);
}
finally
{
    foreach(var disposer in disposers)
        disposer();
}

[...]
OUTER_STRUCT CreateStruct (List<Action> disposers)
{
    INNER_STRUCT is;
    // fill is
    INNER_STRUCT* isPinned = GCHandle.Alloc(is, GCHandleType.Pinned);
    disposers.Add(() => isPinned.Free());

    OUTER_STRUCT os;
    // fill os
    os.Inner = (INNER_STRUCT*) is.AddrOfPinnedObject();

    return os;
}

Describe alternatives you've considered
None so far.

Additional context
See also discussion around #1002 (comment)

Best regards,
D.R.

[AArnott]

Can you give a concrete example of a pair of structs that are problematic for you so we can test the solution?

[drauch]

Here are some examples from my current SignerSignEx2 call:

• SIGNER_SUBJECT_INFO & SIGNER_FILE_INFO
• SIGNER_CERT & SIGNER_CERT_STORE_INFO
• SIGNER_SIGNATURE_INFO & SIGNER_ATTR_AUTHCODE

Best regards,
D.R.

[AArnott]

Oh, those all use explicit layout unions, so either C# or the runtime will likely not allow what you're asking for. Can you try declaring the structs yourself with the proposed modifications and see if it works? You can start by copying out the CsWin32 generated ones into your own code. If you use the same namespace and type names, CsWin32 will gracefully stop emitting the types that you yourself define.