How Can I Access COM Functions from Go Using the `DllGetClassObject` Function?

How to Leverage COM (Component Object Model) in Go

Introduction:
Accessing COM (Component Object Model) functions from within Go can present a challenge. This article aims to address this issue by guiding you through the process of calling COM methods from Go, using a specific example to illustrate the technique.

Background:
The provided code attempts to invoke the ConnectServer method of a Windows DLL through the DllGetClassObject function. However, the compiler raises an error due to the incorrect usage of the syscall.LazyProc.

Solution:
To resolve the error, it is necessary to use the Call method of syscall.LazyProc to pass the appropriate arguments, converted to uintptrs, as per the DllGetClassObject's signature. The code below outlines the modification:

<code class="go">ret, _, _ := getClassObject.Call(rclsid, riid, ppv)</code>

COM Object Wrapping:
Once the COM object has been obtained, Go wrapper types can be created to enable interaction with its methods. This involves defining a custom type with a vtbl (virtual table) pointer and introducing methods that correspond to the COM object's functions.

Example:
For the hypothetical XA_Session object with the ConnectServer and DisconnectServer methods, a Go wrapper might look like this:

<code class="go">type XASession struct {
    vtbl *xaSessionVtbl
}

func (obj *XASession) ConnectServer(id int) int {
    ret, _, _ := syscall.Syscall(
        obj.vtbl.ConnectServer, // function address
        2, // number of parameters to this function
        uintptr(unsafe.Pointer(obj)), // always pass the COM object address first
        uintptr(id), // then all function parameters follow
        0,
    )
    return int(ret)
}

func (obj *XASession) DisconnectServer() {
    syscall.Syscall(
        obj.vtbl.DisconnectServer,
        1,
        uintptr(unsafe.Pointer(obj)),
        0,
        0,
    )
}</code>

By utilizing this approach, it becomes possible to interact with COM objects and seamlessly access their methods from within Go.

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