Can I simplify method signatures I am not using in a ComImport?

Question

I am trying to call GetImageDlg on IWiaDevMgr2. There are a number of quite complicated methods (which I am not using) referencing a number of types (that I am also not using). As I cannot find a TLB or an IDL from which to automatically generate my ComImport, I would prefer to avoid having to manually translate all of the referenced types.

Can I "skip" methods and types by substituting from

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("79C07CF1-CBDD-41ee-8EC3-F00080CADA7A")]
public interface IWiaDevMgr2
{
    IEnumWIA_DEV_INFO EnumDeviceInfo(
        int lFlags);

    IWiaItem2 CreateDevice(
        int lFlags,
        [MarshalAs(UnmanagedType.BStr)] string bstrDeviceID);

    // ...snip five other method declarations...

    IWiaItem2 GetImageDlg(
        int lFlags,
        [MarshalAs(UnmanagedType.BStr)] string bstrDeviceID,
        IntPtr IntPtrParent,
        [MarshalAs(UnmanagedType.BStr)] string bstrFolderName,
        [MarshalAs(UnmanagedType.BStr)] string bstrFilename,
        /* [out] */ out int plNumFiles,
        /* [size_is][size_is][out] */ [MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 5, ArraySubType = UnmanagedType.BStr)] out string[] ppbstrFilePaths);

};

to

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("79C07CF1-CBDD-41ee-8EC3-F00080CADA7A")]
public interface IWiaDevMgr2_Fake
{
    void VTablePlaceholder0();
    void VTablePlaceholder1();
    void VTablePlaceholder2();
    void VTablePlaceholder3();
    void VTablePlaceholder4();
    void VTablePlaceholder5();
    void VTablePlaceholder6();

    [return: MarshalAs(UnmanagedType.Interface)]
    object GetImageDlg(
        int lFlags,
        [MarshalAs(UnmanagedType.BStr)] string bstrDeviceID,
        IntPtr IntPtrParent,
        [MarshalAs(UnmanagedType.BStr)] string bstrFolderName,
        [MarshalAs(UnmanagedType.BStr)] string bstrFilename,
        /* [out] */ out int plNumFiles,
        /* [size_is][size_is][out] */ [MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 5, ArraySubType = UnmanagedType.BStr)] out string[] ppbstrFilePaths);

};

Everything seems to work fine, and as long as I don't call any of the placeholders. Can I do this without consequences?

Answer 1

Sure you can do this. You're the only one who will be using this interface definition, so that's fine.

Of course, if this interface was used as a callback from native to managed, it would be a problem, but in this case you would be implementing it somehow. Implementing such dummy methods is a sign of future problems.

Note there's an official way of doing it. You can use a special _VtblGap{0}_{1} name for the placeholder, where 0 is an index, and 1 is the number of methods you want to skip.

See the implementation in Roslyn here (it's also in the Common Language Infrastructure specification): https://github.com/dotnet/roslyn/blob/master/src/Compilers/Core/Portable/MetadataReader/ModuleExtensions.cs

        // From IMetaDataEmit::DefineMethod documentation (http://msdn.microsoft.com/en-us/library/ms230861(VS.100).aspx)
        // ----------------------
        // In the case where one or more slots need to be skipped, such as to preserve parity with a COM interface layout, 
        // a dummy method is defined to take up the slot or slots in the v-table; set the dwMethodFlags to the mdRTSpecialName 
        // value of the CorMethodAttr enumeration and specify the name as:
        //
        // _VtblGap<SequenceNumber><_CountOfSlots>
        //
        // where SequenceNumber is the sequence number of the method and CountOfSlots is the number of slots to skip in the v-table. 
        // If CountOfSlots is omitted, 1 is assumed.
        // ----------------------
        //
        // From "Partition II Metadata.doc"
        // ----------------------
        // For COM Interop, an additional class of method names are permitted:
        // _VtblGap<SequenceNumber><_CountOfSlots>
        // where <SequenceNumber> and <CountOfSlots> are decimal numbers
        // ----------------------

So in your case, it would be:

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("79C07CF1-CBDD-41ee-8EC3-F00080CADA7A")]
public interface IWiaDevMgr2_Fake
{
    void _VtblGap1_7(); // skip seven methods

    ...
};