How to not use builtin com interop to make trimming work in .NET

Question

In C# .NET 8 given a pointer (IntPtr) to a native object implementing a COM-compatible interface and a C# interface definition, how do I convert the pointer to an object implementing such interface without using built-in COM to enable trimming?

Background:

I have a small C# app (.NET 8) which uses a COM library via autogenerated bindings from tlb. I would like to publish a standalone exe and use .NET's trimming option <PublishTrimmed> to make the exe's size reasonable.

The .NET built-in com interop does not support trimming. I have found a subset of dlls to mark not trimmable to make the app run. However, that still generates a lot of warnings and is I believe technically unsupported by .NET and can break in later .NET versions.

The COM DLL doesn't actually require full COM, it just uses COM-compatible API. All I need is to call a P/Invoke method which returns a pointer to a COM-like object, which matches the autogenerated tlb interface. How would I convert that pointer to an object implementing a particular C# COM-compatible interface using only features supported by trimming?

// auto generated from tlb
[ComImport]
[Guid(<guid>)]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface ISomeInterface
{
    [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
    void method([In][MarshalAs(UnmanagedType.LPWStr)] string input, out int pbData);
    ...
}

// my (simplified) current code using built in com interop
void GetInstance([MarshalAs(UnmanagedType.Interface)] out object createdObject);

GetInstance(out object comObject);
return comObject as ISomeInterface ?? throw new InvalidOperationException("Failed to create ISomeInterface instance.");

To simplify, there are only 2 ways to define COM interface in .NET Core (speaking only version 8+): 1) use ComImport/InterfaceType which uses built-in interop and doesn't support trimming/AOT or 2) use GeneratedComInterface+DisableRuntimeMarshalling which uses the new source generator. That's about it. You can't have trimming/AOT and keep old auto-generated code from a TLB. — Simon Mourier
– Simon Mourier, Commented Feb 21 at 18:24
So what you must do is take your ISomeInterface interface and follow this learn.microsoft.com/en-us/dotnet/standard/native-interop/… to convert it to the new GeneratedComInterface way manually. AFAIK there's currently to TLB to GeneratedComInterface tool available. This "dscom" tool is able to generate TLB for .NET Core with built-in marshaling but it looks they won't go further github.com/dspace-group/dscom/issues/212 — Simon Mourier
– Simon Mourier, Commented Feb 21 at 18:24

Palo · Accepted Answer · 2025-03-15 20:17:02Z

1

Simon Mourier's comments is the answer I used. I have decompiled the generated tlb dll back into C# using ILSpy . Then pseudo-manually and with help of AI to speed it up I converted attributes on the interfaces to using GeneratedComInterface instead of ComImport. Fortunately, the COM interface I am using is relatively basic, so this wasn't that difficult with no custom marshalling required.

Simon's comment provides a link with documentation for the basic cases I needed: https://learn.microsoft.com/en-us/dotnet/standard/native-interop/comwrappers-source-generation

edited Mar 15 at 20:17

answered Feb 27 at 1:10

Palo

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