How to properly initialize a COM object?

I'm trying to get the quota state of a volume in Windows using the win32 api through IDiskQuotaControl interface

The problem that I got into seems to be related the initialization. Please see the code below.

//go:build windows && amd64
package main
import (
"flag"
"fmt"
"runtime"
"sync"
"syscall"
"unsafe"
"golang.org/x/sys/windows"
)
// START OF basic stuff
const COINIT_APARTMENTTHREADED = 0x2
const CLSCTX_INPROC_SERVER = 0x1
const CLSCTX_INPROC_HANDLER = 0x2
const CLSCTX_LOCAL_SERVER = 0x4
var (
modole32 = syscall.NewLazyDLL("ole32.dll")
procCoInitializeEx   = modole32.NewProc("CoInitializeEx")
procCoUninitialize   = modole32.NewProc("CoUninitialize")
procCoCreateInstance = modole32.NewProc("CoCreateInstance")
)
func CoInitializeEx(pvReserved uintptr, dwCoInit uint32) (r1, r2 uintptr, lastErr error) {
r1, r2, lastErr = procCoInitializeEx.Call(
uintptr(pvReserved),
uintptr(dwCoInit),
)
return
}
func CoUninitialize() (r1, r2 uintptr, lastErr error) {
r1, r2, lastErr = procCoUninitialize.Call()
return
}
func CoCreateInstance(rclsid *GUID, pUnkOuter *byte, dwClsContext uint32, riid *GUID, ppv *uintptr) (r1, r2 uintptr, lastErr error) {
r1, r2, lastErr = procCoCreateInstance.Call(
uintptr(unsafe.Pointer(rclsid)),
uintptr(unsafe.Pointer(pUnkOuter)),
uintptr(dwClsContext),
uintptr(unsafe.Pointer(riid)),
uintptr(unsafe.Pointer(ppv)),
)
return
}
type GUID struct {
Data1 uint32
Data2 uint16
Data3 uint16
Data4 [8]byte
}
type IDiskQuotaControl struct {
lpVtbl *IDiskQuotaControlVtbl
}
type IDiskQuotaControlVtbl struct {
QueryInterface uintptr
AddRef         uintptr
Release        uintptr
Initialize                     uintptr
SetQuotaState                  uintptr
GetQuotaState                  uintptr
SetQuotaLogFlags               uintptr
GetQuotaLogFlags               uintptr
SetDefaultQuotaThreshold       uintptr
GetDefaultQuotaThreshold       uintptr
GetDefaultQuotaThresholdText   uintptr
SetDefaultQuotaLimit           uintptr
GetDefaultQuotaLimit           uintptr
GetDefaultQuotaLimitText       uintptr
AddUserSid                     uintptr
AddUserName                    uintptr
DeleteUser                     uintptr
FindUserSid                    uintptr
FindUserName                   uintptr
CreateEnumUsers                uintptr
CreateUserBatch                uintptr
InvalidateSidNameCache         uintptr
GiveUserNameResolutionPriority uintptr
ShutdownNameResolution         uintptr
}
func (x *IDiskQuotaControl) AddRef() (r1, r2 uintptr, lastErr error) {
r1, r2, lastErr = syscall.SyscallN(
x.lpVtbl.AddRef,
uintptr(unsafe.Pointer(x)),
)
return
}
func (x *IDiskQuotaControl) Release() (r1, r2 uintptr, lastErr error) {
r1, r2, lastErr = syscall.SyscallN(
x.lpVtbl.Release,
uintptr(unsafe.Pointer(x)),
)
return
}
func (x *IDiskQuotaControl) Initialize(pszPath *uint16, bReadWrite int32) (r1, r2 uintptr, lastErr error) {
r1, r2, lastErr = syscall.SyscallN(
x.lpVtbl.Initialize,
uintptr(unsafe.Pointer(x)),
uintptr(unsafe.Pointer(pszPath)),
uintptr(bReadWrite),
)
return
}
func (x *IDiskQuotaControl) GetQuotaState(pdwState *uint32) (r1, r2 uintptr, lastErr error) {
r1, r2, lastErr = syscall.SyscallN(
x.lpVtbl.GetQuotaState,
uintptr(unsafe.Pointer(x)),
uintptr(unsafe.Pointer(pdwState)),
)
return
}
func (x *IDiskQuotaControl) GetDefaultQuotaLimit(pllLimit *int64) (r1, r2 uintptr, lastErr error) {
r1, r2, lastErr = syscall.SyscallN(
x.lpVtbl.GetDefaultQuotaLimit,
uintptr(unsafe.Pointer(x)),
uintptr(unsafe.Pointer(pllLimit)),
)
return
}
var CLSID_DiskQuotaControl = &GUID{0x7988b571, 0xec89, 0x11cf, [8]byte{0x9c, 0x0, 0x0, 0xaa, 0x0, 0xa1, 0x4f, 0x56}}
var IID_IDiskQuotaControl = &GUID{0x7988b572, 0xec89, 0x11cf, [8]byte{0x9c, 0x0, 0x0, 0xaa, 0x0, 0xa1, 0x4f, 0x56}}
// END OF basic stuff
func getVolumeQuota(wg *sync.WaitGroup, volume string) {
defer wg.Done()
runtime.LockOSThread()
defer runtime.UnlockOSThread()
// Init COM
r1, r2, lastErr := CoInitializeEx(
0, // must be NULL
COINIT_APARTMENTTHREADED,
)
defer CoUninitialize()
fmt.Println("CoInitializeEx", r1, r2, lastErr) // Print results for debug
// Create a COM instance
var ppv uintptr
r1, r2, lastErr = CoCreateInstance(
CLSID_DiskQuotaControl,
nil,
CLSCTX_INPROC_SERVER,
IID_IDiskQuotaControl,
&ppv,
)
fmt.Println("CoCreateInstance", r1, r2, lastErr)
diskQuotaControl := (*IDiskQuotaControl)(unsafe.Pointer(ppv))
defer diskQuotaControl.Release()
pszPath, err := windows.UTF16PtrFromString(volume)
if err != nil {
panic(err)
}
// Initialize seems to fail even the return is a success
r1, r2, lastErr = diskQuotaControl.Initialize(
pszPath,
0, // false => read only
)
fmt.Println("Initialize", r1, r2, lastErr)
var pdwState uint32
r1, r2, lastErr = diskQuotaControl.GetQuotaState(
&pdwState,
)
fmt.Println("GetQuotaState", r1, r2, lastErr)
fmt.Println(pdwState)
var pllLimit int64
r1, r2, lastErr = diskQuotaControl.GetDefaultQuotaLimit(
&pllLimit,
)
fmt.Println("GetDefaultQuotaLimit", r1, r2, lastErr)
fmt.Println(pllLimit)
}
func main() {
volume := flag.String("v", `C:\`, "volume")
flag.Parse()
fmt.Println("Volume", *volume)
var wg sync.WaitGroup
wg.Add(1)
go getVolumeQuota(&wg, *volume)
wg.Wait()
}

The reason why I think the problem is in the Initialize, is that whatever input I put (using the flag -v) I always get a success return.

Results:

Volume C:\
CoInitializeEx 0 0 The operation completed successfully.
CoCreateInstance 0 0 The operation completed successfully.
Initialize 0 9298000 The operation completed successfully.
GetQuotaState 2147942403 9298000 The system cannot find the path specified.
0
GetDefaultQuotaLimit 2147942421 9298000 The operation completed successfully.
0 
Volume \\?\C:\
CoInitializeEx 0 0 The operation completed successfully.
CoCreateInstance 0 0 The operation completed successfully.
Initialize 0 9103168 The operation completed successfully.
GetQuotaState 2147942403 9103168 The system cannot find the path specified.
0
GetDefaultQuotaLimit 2147942421 9103168 The operation completed successfully.
0
Volume Invalid:Volume://+
CoInitializeEx 0 0 The operation completed successfully.
CoCreateInstance 0 0 The operation completed successfully.
Initialize 0 8709600 The operation completed successfully.
GetQuotaState 2147942403 8709600 The system cannot find the path specified.
0
GetDefaultQuotaLimit 2147942421 8709600 The operation completed successfully.
0

The error in GetDefaultQuotaLimit (2147942421 = 0x80070015) translates to "This object has not been initialized"

The IDiskQuotaControl interface documentation:

> The IDiskQuotaControl interface inherits from the IUnknown interface.

is wrong! IDiskQuotaControl derives from IConnectionPointContainer , not from IUnknown. You can see that in dskquota.h:

DECLARE_INTERFACE_IID_(IDiskQuotaControl, IConnectionPointContainer, "7988B572-EC89-11cf-9C00-00AA00A14F56")

So, your vtable definition is wrong, you must add two methods between your Release and Initialize methods.