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Add information about pitfalls and usage of derived COM interfaces across assembly boundaries #42051

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Aug 8, 2024
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Add information about pitfalls and usage of derived COM interfaces across assembly boundaries #42051

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Add information about pitfalls and usage of derived COM interfaces ac…
jkoritzinsky Aug 6, 2024
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14 changes: 14 additions & 0 deletions docs/standard/native-interop/comwrappers-source-generation.md
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Expand Up @@ -186,6 +186,20 @@ interface IDerived : IBase

Note that an interface with the `GeneratedComInterface` attribute can only inherit from one base interface that has the `GeneratedComInterface` attribute.

#### Derived interfaces across assembly boundaries

In .NET 8, it isn't supported to define an interface with the <xref:System.Runtime.InteropServices.Marshalling.GeneratedComInterfaceAttribute> attribute that derives from a `GeneratedComInterface`-attributed interface that's defined in another assembly.

In .NET 9 and later versions, this scenario is supported with the following restrictions:

- The base interface type must be compiled targeting the same target framework as the derived type.
- The base interface type must not shadow any members of its base interface, if it has one.

Additionally, any changes to any generated virtual method offsets in the base interface chain defined in another assembly won't be accounted for in the derived interfaces until the project is rebuilt.

> [!NOTE]
> In .NET 9 and later versions, a warning is emitted when inheriting generated COM interfaces across assembly boundaries to inform you about the restrictions and pitfalls of using this feature. You can disable this warning to acknowledge the limitations and inherit across assembly boundaries.

### Marshal APIs

Some APIs in <xref:System.Runtime.InteropServices.Marshal> are not compatible with source-generated COM. Replace these methods with their corresponding methods on a `ComWrappers` implementation.
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