Inheritance, two COM interfaces

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Hi,
I have a DLL Project with IDL interfaces.
I want to have two interfaces in my dll so that one of them can be derived from another.
I created two interfaces with ATL Simple Object Wizard.

C++

[
    object,
    uuid(7359AF6C-6E90-4372-991F-556602CB3977),
    dual,
    nonextensible,
    pointer_default(unique)
]
interface IZInterface : IDispatch{
    [id(1)] HRESULT ZGetStr([out,retval] BSTR* str);
    [id(2)] HRESULT GetSize([in,out] LONG* nSize);
};
[
	object,
	uuid(8CA6DBF2-E402-464D-96AE-3D6642D91E14),
	pointer_default(unique)
]
interface IBClass : IUnknown{
	[] HRESULT Method11([out,retval] LONG* l);
};
library DllStandardLib
{
	importlib("stdole2.tlb");
	[
		uuid(491DF659-012F-4C20-90AA-0CBC5BDE5A68)		
	]
	coclass ZInterface
	{
		[default] interface IZInterface;
	};
        [
		uuid(43CE897F-17F2-4D45-9098-26B7AEE6EC23)		
	]
	coclass BClass
	{
		[default] interface IBClass;
	};
};

Now, I right-click on the CZInterface in the Class View, then I select "Impelement Interface IBClass".

But in a container that is a C# project:

C#

DllStandardLib.ZInterface dd = new DllStandardLib.ZInterface();
dd.Method11();//---> Error: DllStandardLib.ZInterface' does not contain a definition for 'Method11' and no extension method 'Method11' accepting a first argument of type ...

What is the problem with my project? I want that the second (derived) interface would know all the methods and properties of the base interface.

Please, help me!

Posted 17-Feb-15 20:09pm

Zon-cpp

Updated 17-Feb-15 22:22pm

Vladimir Svyatski

v2

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Vladimir Svyatski · Answer 1 · 2015-02-17T22:31:00

The problem is that your IZInterface is not derived (according to your code) from IBClass, therefore it doesn't have the method Method1. Your code should look something like

C++

[
	object,
	uuid(8CA6DBF2-E402-464D-96AE-3D6642D91E14),
	pointer_default(unique)
]
interface IBClass : IDispatch{
	[] HRESULT Method11([out,retval] LONG* l);
};
[
    object,
    uuid(7359AF6C-6E90-4372-991F-556602CB3977),
    dual,
    nonextensible,
    pointer_default(unique)
]
interface IZInterface : IBClass{//it's derived from IBClass, see?
    [id(1)] HRESULT ZGetStr([out,retval] BSTR* str);
    [id(2)] HRESULT GetSize([in,out] LONG* nSize);
};
library DllStandardLib
{
	importlib("stdole2.tlb");
	[
		uuid(491DF659-012F-4C20-90AA-0CBC5BDE5A68)		
	]
	coclass ZInterface
	{
		[default] interface IZInterface;
	};
        [
		uuid(43CE897F-17F2-4D45-9098-26B7AEE6EC23)		
	]
	coclass BClass
	{
		[default] interface IBClass;
	};
};

Thanks to Microsoft "The current implementation of MIDL does not handle function overloading or multiple inheritance". So your IBClass can be inherited from IDispatch rather than from IUnknown. But you can try to emulate multiple inheritance by writing some of your classes in C++. For example, here is the definition of IDispatch from OAIdl.h

C++

MIDL_INTERFACE("00020400-0000-0000-C000-000000000046")
IDispatch : public IUnknown
{
public:
    virtual HRESULT STDMETHODCALLTYPE GetTypeInfoCount(
        /* [out] */ __RPC__out UINT *pctinfo) = 0;

    virtual HRESULT STDMETHODCALLTYPE GetTypeInfo(
        /* [in] */ UINT iTInfo,
        /* [in] */ LCID lcid,
        /* [out] */ __RPC__deref_out_opt ITypeInfo **ppTInfo) = 0;

    virtual HRESULT STDMETHODCALLTYPE GetIDsOfNames(
        /* [in] */ __RPC__in REFIID riid,
        /* [size_is][in] */ __RPC__in_ecount_full(cNames) LPOLESTR *rgszNames,
        /* [range][in] */ __RPC__in_range(0,16384) UINT cNames,
        /* [in] */ LCID lcid,
        /* [size_is][out] */ __RPC__out_ecount_full(cNames) DISPID *rgDispId) = 0;

    virtual /* [local] */ HRESULT STDMETHODCALLTYPE Invoke(
        /* [annotation][in] */
        _In_  DISPID dispIdMember,
        /* [annotation][in] */
        _In_  REFIID riid,
        /* [annotation][in] */
        _In_  LCID lcid,
        /* [annotation][in] */
        _In_  WORD wFlags,
        /* [annotation][out][in] */
        _In_  DISPPARAMS *pDispParams,
        /* [annotation][out] */
        _Out_opt_  VARIANT *pVarResult,
        /* [annotation][out] */
        _Out_opt_  EXCEPINFO *pExcepInfo,
        /* [annotation][out] */
        _Out_opt_  UINT *puArgErr) = 0;

};

Then just include your header in your IDL.