Recipe and best practice for accessing COM

[newToCOM]

Still struggling..

test.d:
-----------------------------------
( ... )  /* Other imports */
import win32.directx.d2d1;

alias win32.directx.d2d1.IID IID;
IID IID_ID2D1Factory = { 0x06152247, 0x6F50, 0x465A, [0x92, 0x45, 
0x11, 0x8B, 0xFD, 0x3B, 0x60, 0x07] };

extern (Windows)
int WinMain( ... )
{
     ( ... ) /* Boilerplate code */
}

int myWinMain( ... )
{
     ( ... ) /* Register class, Create window, show window, 
message loop */
}

LRESULT WndProc( ... )
{
     switch (message)
     {
         ( ... ) /* Default and case WM_DESTROY */

         case WM_CREATE:

             // arg2
             REFIID pIID_ID2D1Factory;
             pIID_ID2D1Factory = cast(REFIID)&IID_ID2D1Factory;

             // arg3
             D2D1_FACTORY_OPTIONS factoryOptions;
                 factoryOptions.debugLevel = 
D2D1_DEBUG_LEVEL.INFORMATION;
             const D2D1_FACTORY_OPTIONS* pfactoryOptions = 
&factoryOptions;

             // arg4
             void* pID2D1Factory;


             HRESULT hr = D2D1CreateFactory(
                     D2D1_FACTORY_TYPE.SINGLE_THREADED,
                     pIID_ID2D1Factory,
                     pfactoryOptions,
                     pID2D1Factory
                     );

             writeln(hr);               // Prints 0
             writeln(pID2D1Factory);    // Prints a non-null 
pointer

             // Trying to use the interface
             float dpiX, dpiY;
             pID2D1Factory.GetDesktopDpi(dpiX, dpiY);
             // !!! Error: undefined identifier 'GetDesktopDpi'
             // I thought I had a pointer to the interface and 
could call
             // its methods...

             return 0;

------------------------------

* Coffimplib was used to convert the d2d1.lib from Microsoft 
DirectX SDK (June 2010). The converted d2d1.dll was placed in 
C:\dcode\dlls
* With test.d in C:\dcode, the file was compiled and linked in 
C:\dcode with:
dmd test.d %cd%\dlls\d2d1.lib -I%cd%\dlls\ 
-version=DXSDK_JUNE_2010


- Why is the method identifier undefined?
- Are there any code examples showing the right use of the 
bindings?