CODEWITHSUNDEEP
c#cominteropmarshalling
TheCodeJunkie
TheCodeJunkie

Reputation: 9616

COM interface declarations

When creating COM interface declarations in C# are there any "rules" you have to stick to? I think there are and would like to get some info on it. For example I'm toying around with the ITaskbarList, ITaskbarList2 and ITaskbarList3 interfaces and it seems to me that I

For example the following appears to work just fine

[ComImport]
[Guid("56FDF342-FD6D-11D0-958A-006097C9A090")] 
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface ITaskbarList
{
    void HrInit();
    void AddTab([In] IntPtr hwnd);
    void DeleteTab([In] IntPtr hwnd);
    void ActivateTab([In] IntPtr hwnd);
    void SetActiveAlt([In] IntPtr hwnd);
}

While reordering the members breaks the functionality

[ComImport]
[Guid("56FDF342-FD6D-11D0-958A-006097C9A090")] 
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface ITaskbarList
{
    void DeleteTab([In] IntPtr hwnd);
    void HrInit();
    void AddTab([In] IntPtr hwnd);
    void SetActiveAlt([In] IntPtr hwnd);
    void ActivateTab([In] IntPtr hwnd);
}

For example, this doesn't appear to work

[ComImport]
[Guid("56FDF342-FD6D-11D0-958A-006097C9A090")]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface ITaskbarList
{
    void HrInit();
    void AddTab([In] IntPtr hwnd);
    void DeleteTab([In] IntPtr hwnd);
    void ActivateTab([In] IntPtr hwnd);
    void SetActiveAlt([In] IntPtr hwnd);
}

[ComImport]
[Guid("602D4995-B13A-429B-A66E-1935E44F4317")]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface ITaskbarList2
    : ITaskbarList
{
    void MarkFullscreenWindow(
        [In] IntPtr hwnd,
        [In, MarshalAs(UnmanagedType.Bool)] bool fullscreen);
}

Instead I'm forced to do the following

[ComImport]
[Guid("602D4995-B13A-429B-A66E-1935E44F4317")]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface ITaskbarList2
{
    void HrInit();
    void AddTab([In] IntPtr hwnd);
    void DeleteTab([In] IntPtr hwnd);
    void ActivateTab([In] IntPtr hwnd);
    void SetActiveAlt([In] IntPtr hwnd);
    void MarkFullscreenWindow(
        [In] IntPtr hwnd,
        [In, MarshalAs(UnmanagedType.Bool)] bool fullscreen);
}

I.e declare it in a single interface while still taking the member order into consideration.

So what are the guidelines for declaring managed interfaces for their unmanaged counterparts? Is there anyway to achieve what I want, it interface inheritance on the managed side + declare the members in any order I want (I really just want to sort them alphabetically)

Upvotes: 2

Views: 1403

Answers (1)

i_am_jorf
i_am_jorf

Reputation: 54600

Your discoveries are caused by the same thing: vtables. You can't change the vtable order, and interfaces that inherit from other interfaces always have all the base entries in the vtable as well. So, the rules you're looking for is "make sure the vtable is the same."

Upvotes: 3

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