How does CreateStdDispatch know which method to call?

Question

I came across the implementation of the IDispatch interface. There are four methods, and, fortunately, 3 of them are easy:

 function TIEEventsSink.GetTypeInfoCount(...): HResult; { Result := E_NOTIMPL; } function TIEEventsSink.GetTypeInfo(...): HResult; { Result := E_NOTIMPL; } function TIEEventsSink.GetIDsOfNames(...): HResult; { Result := E_NOTIMPL; } 

This is the last method, Invoke , which is complicated. Here I am faced with the need to actually use DispID and call the appropriate method; unmarhsalling parameters from an array variant.

 function Invoke( dispIdMember: DISPID; riid: REFIID; lcid: LCID; wFlags: WORD; var pDispParams: DISPPARAMS; var pVarResult: VARIANT; var pExcepInfo: EXCEPINFO; var puArgErr: DWORD ): HRESULT; 

Not wanting to write all the tedious code of the template, I will probably get errors, I went googling - instead of doing any work.

I found this snippet on the MSDN IDispatch.Invoke documentation :

Generally, you should not implement Invoke directly.

Excellent! I still did not want to implement it! Continuing reading:

Instead, use the submit interface to create the CreateStdDispatch and DispInvoke functions . For more information, see CreateStdDispatch , DispInvoke , Creating an IDispatch Interface and Exposing ActiveX Objects .

Creating an IDispatch interface says:

You can implement IDispatch in any of the following ways:

• [incision]
• Call the CreateStdDispatch function. This approach is the simplest, but it does not provide rich error handling or several national languages.
• [incision]

Great, CreateStdDispatch :

Creates a standard implementation of the IDispatch interface through a single function call. This makes it easy to expose objects through Automation.

 HRESULT CreateStdDispatch( IUnknown FAR* punkOuter, void FAR* pvThis, ITypeInfo FAR* ptinfo, IUnknown FAR* FAR* ppunkStdDisp ); 

I was going to call him like:

 CreateStdDispatch( myUnk, //Pointer to the object IUnknown implementation. anotherObject, //Pointer to the object to expose. nil //Pointer to the type information that describes the exposed object (i has no type info) dispInterface //the IUnknown of the object that implements IDispatch for me ); 

What I cannot understand is how the Windows API implementation of CreateStdDispatch knows which methods to call on my object, especially since CreateStdDispatch does not know which object-oriented language I use, or its calling conventions.

How CreateStdDispatch knows

• what method to call for this dispid ?
• calling convention of my language?
• How to handle exceptions from the language in which an object-oriented object is written?

Note : I have no choice but to implement dispinterface ; I did not define the interface . I wish this was a simple early border of IUnknown , but it is not.

Answer 1

Is the ITypeInfo parameter passed to CreateStdDispatch all the information about the method?

So, you must create the type information first calling CreateDispTypeInfo and pass it through CreateStdDispatch , which can then use the type information to determine which method to call with CreateDispTypeInfo requires INTERFACEDATA , which contains all this information

I could be wrong because I do not have time for this, but it makes sense to me. I will investigate this later and update the answer.