C# COM assembly implementing C++ interface

Question

Does anybody know whether the following implementation of a C++ interface in C# is correct, particularly with regards to the marshaling of the GUID type.

The original C++ we are implementing:

[
    object,
    pointer_default(unique),
    uuid(Z703B6E9-A050-4C3C-A050-6A5F4EE32767)
]
interface IThing: IUnknown
{
    [propget] HRESULT Prop1([out, retval] GUID* prop1); 
    [propget] HRESULT Prop2([out, retval] EnumProp2* prop2);
    [propget] HRESULT Prop3([out, retval] HSTRING* prop3);
};

The converted interface in our C# COM assembly:

[ComVisible(true), Guid("Z703B6E9-A050-4C3C-A050-6A5F4EE32767"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface IThing
{
    Guid Prop1 { get; }
    EnumProp2 Prop2 { get; }
    string Prop3 { get; }
}

The concrete class:

public class Thing : IThing
{    
    public Thing(Guid prop1, EnumProp2 prop2, string prop3)
    {
        Prop1 = prop1;
        Prop2 = prop2;
        Prop3 = prop3;
    }

    public Guid Prop1 { get; }
    public EnumProp2 Prop2 { get; }
    public string Prop3 { get; }
}

the Guid structures of C# and the <windows.h> definition of GUID have a different grouping, but they are "binary compatible" as they're both 16 byte long and bytes are ordered consistently: stackoverflow.com/a/14891293/1132334 so it should work. do you have any reason to believe your ComVisible interface would be wrong? can you just test it? — Cee McSharpface
– Cee McSharpface, Commented Mar 12, 2017 at 18:53
ok looks like it should by a byte[16] which you can get using Guid.ToByteArray. reference source shows that the Guid struct is a complex type and does not expose the raw bits. — Cee McSharpface
– Cee McSharpface, Commented Mar 12, 2017 at 19:05

James Harcourt · Accepted Answer · 2017-03-13 15:44:03Z

1

It turns out the issue was actually not marshaling the Guid structure, but the string.

It seems that when moving a string from the managed CLR heap to the unmanaged heap, the default marshaling type is UnmanagedType.BStr (unicode character string, length-prefixed) - and the host app in this case required a different marshal type:

public string Name { [return: MarshalAs(UnmanagedType.HString)]get; }

answered Mar 13, 2017 at 15:44

James Harcourt

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