IDispatch Interface - OLE Programmer's Reference

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IDispatch Interface

Implemented by
Used by
Header filename
Applications
that expose
programmable objects.
Applications
that access
programmable objects.
OLEAUTO.H (32-bit systems)
DISPATCH.H (16-bit systems)

OLE Automation objects may implement the IDispatch interface for access by OLE Automation controllers, such as Visual Basic. The object's properties and methods can be accessed using IDispatch::GetIDsOfNames and IDispatch::Invoke.

The following examples show how to access an OLE Automation object through the IDispatch interface. Note that the code is abbreviated and omits error handling.

// Declarations of variables used

DEFINE_GUID(CLSID_Hello, //...portions omitted for brevity...

HRESULT hresult;

IUnknown * punk;

IDispatch * pdisp;

OLECHAR FAR* szMember = "SayHello";

DISPID dispid;

DISPPARAMS dispparamsNoArgs = {NULL, NULL, 0, 0};

EXCEPINFO excepinfo;

UINT nArgErr;

In the following lines, OleInitialize loads the OLE DLLs and CoCreateInstance initializes the OLE Automation object's class factory. For more information on these two functions, see the OLE 2 Programmer's Reference, Volume 1.

// Initialize OLE DLLs

hresult = OleInitialize(NULL);

// OLE function CoCreateInstance starts application using GUID

hresult = CoCreateInstance(CLSID_Hello, &punk);

QueryInterface checks whether the object supports IDispatch. (As with any call to QueryInterface, the returned pointer must be released when it is no longer needed.)

// QueryInterface to see if it supports IDispatch.

hresult = punk->QueryInterface(IID_IDispatch, &pdisp);

GetIDsOfNames retrieves the dispatch identifier (DISPID) for the indicated method or property, in this case, szMember.

// Retrieve the dispatch identifier for the SayHello method.

// Use defaults where possible.

hresult = pdisp->GetIDsOfNames(

IID_NULL,

&szMember,

LOCALE_SYSTEM_DEFAULT,

&dispid);

In the following call to Invoke, the dispatch identifier (dispid) indicates the property or method to invoke. The SayHello method takes no parameters, so the fifth argument (&dispparamsNoArgs), contains NULL and 0, as initialized at declaration. To invoke a property or method that requires parameters, you would need to supply the parameters in the DISPPARAMS structure.

// Invoke the method. Use defaults where possible.

hresult = pdisp->Invoke(

dispid,

IID_NULL,

LOCALE_SYSTEM_DEFAULT,

DISPATCH_METHOD,

&dispparamsNoArgs,

NULL,

NULL);

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