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IDispatchEx Interface

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IDispatchEx, an extension of the IDispatch interface, supports features appropriate for dynamic languages such as scripting languages. This section describes the IDispatchEx interface itself, the differences between IDispatch and IDispatchEx, and the rationale for the extensions. It is expected that readers are familiar with IDispatch and have access to the IDispatch documentation.

Remarks

IDispatch was developed essentially for Microsoft Visual Basic. The primary limitation of IDispatch is that it assumes that objects are static. In other words, since objects do not change during run time, type information can fully describe them at compile time. Dynamic run-time models that are found in scripting languages such as Visual Basic Scripting Edition (VBScript) and JavaScript and object models such as Dynamic HTML require a more flexible interface.

IDispatchEx was developed to provide all the services of IDispatch as well as some extensions that are appropriate for more dynamic late-bound languages such as scripting languages. The additional features of IDispatchEx beyond those provided by IDispatch are:

  • Add new members to an object ("expando")—use GetDispID with the fdexNameEnsure flag.

  • Delete members of an object—use DeleteMemberByName or DeleteMemberByDispID.

  • Case-sensitive dispatch operations—use fdexNameCaseSensitive or fdexNameCaseInsensitive.

  • Search for member with implicit name—use fdexNameImplicit.

  • Enumerate DISPIDs of an object—use GetNextDispID.

  • Map from DISPID to element name—use GetMemberName.

  • Obtain properties of object members—use GetMemberProperties.

  • Method invocation with this pointer—use InvokeEx with DISPATCH_METHOD.

  • Allow browsers that support the concept of name spaces to obtain the name space parent of an object—use GetNameSpaceParent.

    Objects that support IDispatchEx might also support IDispatch for backward compatibility. The dynamic nature of objects that support IDispatchEx has a few implications for the IDispatch interface of those objects. For example, IDispatch makes the following assumption:

  • The member and parameter DISPIDs must remain constant for the lifetime of the object. This allows a client to obtain DISPIDs once and cache them for later use.

    Since IDispatchEx allows the addition and deletion of members, the set of valid DISPIDs does not remain constant. However, IDispatchEx requires that the mapping between DISPID and member name remain constant. This means that if a member is deleted:

  • The DISPID cannot be reused until a member with the same name is created.

  • The DISPID must remain valid for GetNextDispID.

  • The DISPID must be accepted gracefully by any of the IDispatch or IDispatchEx methods—they must recognize the member as deleted and return an appropriate error code (usually DISP_E_MEMBERNOTFOUND or S_FALSE).

Examples

This JavaScript code in the function test() does the following:

  • Creates a new object by calling the Object constructor and assigns a pointer to the new object to the variable Obj.

  • Creates a new element named Elem in the object and assigns to this element a pointer to the function cat.

  • Calls this function. Since it is being called as a method, the this pointer refers to the object Obj. The function adds a new element, Bar, to the object.

    The full HTML code is:

<html>  
<body>  
<script type="text/javascript">  
function cat()  
{  
   // Create new element and assign the value 10  
   this.Bar = 10;  
}  
  
function test()  
{  
   // Construct new object  
   Obj = new Object();  
  
   // Create new element and assign function pointer  
   Obj.Elem = cat;  
  
   // Call Elem method ("this" == Obj)  
   Obj.Elem();  
  
   // Obj.Bar now exists  
}  
test();  
</script>  
</body>  
</html>

A control placed on this same Web page could obtain a dispatch pointer to the script engines from the browser. The control could then implement the function test():

<html>  
<body>  
<script type="text/javascript">  
function cat()  
{  
   // Create new element and assign the value 10  
   this.Bar = 10;  
}  
</script>  
<object id="test" <CLASSID="CLSID:9417DB5D-FA2A-11D0-8CB3-00C04FC2B085">>  
</object>  
</body>  
</html>

Code from the control, test, does the same thing as the JavaScript function test(). Note that these dispatch calls are made into the running JavaScript engine and change the state of the engine:

  • Obtains the dispatch pointer to the cat function using GetDispID().

  • Obtains the dispatch pointer to the Object function using GetDispID().

  • Constructs an object by calling the Object function with InvokeEx() and obtains a dispatch pointer to the newly constructed object.

  • Creates a new element, Elem, in the object using GetDispID() with the fdexNameEnsure flag.

  • Puts the dispatch pointer to cat in the element using InvokeEx().

  • Calls the dispatch pointer to cat as a method by calling InvokeEx() and passing in the dispatch pointer to the constructed object as the this pointer.

  • The cat method creates a new element, Bar, on the current this object.

  • Verifies that the new element, Bar, was created in the constructed object by enumerating through the elements using GetNextDispID().

    The code for the test control:

   BOOL test(IDispatchEx *pdexScript)  
   {  
      HRESULT hr;  
      VARIANT var;  
      DISPID dispid, putid;  
      BOOL retval = FALSE;  
      BSTR bstrName = NULL;  
      IDispatch   *pdispObj = NULL, *pdispCat = NULL;  
      IDispatchEx *pdexObj = NULL;  
      DISPPARAMS dispparams, dispparamsNoArgs = {NULL, NULL, 0, 0};  
  
      // Get dispatch pointer for "cat"  
      bstrName = SysAllocString(OLESTR("cat"));  
         if (bstrName == NULL) goto LDone;  
      hr = pdexScript->GetDispID(bstrName, 0, &dispid);  
         if (FAILED(hr)) goto LDone;  
      SysFreeString(bstrName);  
         bstrName = NULL;  
      hr = pdexScript->InvokeEx(dispid, LOCALE_USER_DEFAULT,   
         DISPATCH_PROPERTYGET, &dispparamsNoArgs,   
         &var, NULL, NULL);  
         if (FAILED(hr)) goto LDone;  
      pdispCat = var.pdispVal;  
  
      // Create object by calling "Object" constructor  
      bstrName = SysAllocString(OLESTR("Object"));  
         if (NULL == bstrName) goto LDone;  
      hr = pdexScript->GetDispID(bstrName, 0, &dispid);  
         if (FAILED(hr)) goto LDone;  
      SysFreeString(bstrName);  
         bstrName = NULL;  
      hr = pdexScript->InvokeEx(dispid, LOCALE_USER_DEFAULT,   
         DISPATCH_CONSTRUCT, &dispparamsNoArgs,   
         &var, NULL, NULL);  
         if (FAILED(hr)) goto LDone;  
      pdispObj = var.pdispVal;  
      hr = pdispObj->QueryInterface(IID_IDispatchEx, (void **)&pdexObj);  
         if (FAILED(hr)) goto LDone;  
  
      // Create new element in object  
      bstrName = SysAllocString(OLESTR("Elem"));  
         if (NULL == bstrName) goto LDone;  
      hr = pdexObj->GetDispID(bstrName, fdexNameEnsure, &dispid);  
         if (FAILED(hr)) goto LDone;  
      SysFreeString(bstrName);  
         bstrName = NULL;  
  
      // Assign "cat" dispatch pointer to element  
      putid = DISPID_PROPERTYPUT;  
      var.vt = VT_DISPATCH;  
      var.pdispVal = pdispCat;  
      dispparams.rgvarg = &var;  
      dispparams.rgdispidNamedArgs = &putid;  
      dispparams.cArgs = 1;  
      dispparams.cNamedArgs = 1;  
      hr = pdexObj->InvokeEx(dispid, LOCALE_USER_DEFAULT,   
         DISPATCH_PROPERTYPUTREF, &dispparams,  
         NULL, NULL, NULL);  
         if (FAILED(hr)) goto LDone;  
  
      // Invoke method with "this" pointer  
      putid = DISPID_THIS;  
      var.vt = VT_DISPATCH;  
      var.pdispVal = pdispObj;  
      dispparams.rgvarg = &var;  
      dispparams.rgdispidNamedArgs = &putid;  
      dispparams.cArgs = 1;  
      dispparams.cNamedArgs = 1;  
      hr = pdexObj->InvokeEx(dispid, LOCALE_USER_DEFAULT,  
         DISPATCH_METHOD, &dispparams,  
            NULL, NULL, NULL);  
         if (FAILED(hr)) goto LDone;  
  
      // Confirm that new element "Bar" is in object  
      hr = pdexObj->GetNextDispID(fdexEnumAll, DISPID_STARTENUM, &dispid);  
      while (hr == NOERROR)  
      {  
            hr = pdexObj->GetMemberName(dispid, &bstrName);  
            if (FAILED(hr)) goto LDone;  
            retval = !wcscmp(bstrName, OLESTR("Bar"));  
            SysFreeString(bstrName);  
            bstrName = NULL;  
            if (retval) goto LDone;  
         hr = pdexObj->GetNextDispID(fdexEnumAll, dispid, &dispid);  
      }  
LDone:  
   SysFreeString(bstrName);  
   if (pdispCat != NULL)  
      pdispCat->Release();  
   if (pdispObj != NULL)  
      pdispObj->Release();  
   if (pdexObj != NULL)  
      pdexObj->Release();  
  
   return retval;  
   }

Methods

IDispatchEx Methods